Textbook: Cytology, embryology, general histology. Histology. Concept of fabrics. Types of fabrics. Structure and functions of epithelial tissue Classification of connective tissue
Histology (from the Greek ίστίομ - tissue and the Greek Λόγος - knowledge, word, science) is a branch of biology that studies the structure of tissues of living organisms. This is usually done by cutting the tissue into thin layers using a microtome. Unlike anatomy, histology studies the structure of the body at the tissue level. Human histology is a branch of medicine that studies the structure of human tissues. Histopathology is the branch of microscopic examination of diseased tissue and is an important tool in pathology (pathological anatomy), since accurate diagnosis of cancer and other diseases usually requires histopathological examination of specimens. Forensic histology – section forensic medicine, studying the characteristics of damage at the tissue level.
Histology originated long before the invention of the microscope. The first descriptions of fabrics are found in the works of Aristotle, Galen, Avicenna, Vesalius. In 1665, R. Hooke introduced the concept of a cell and observed the cellular structure of some tissues through a microscope. Histological studies were carried out by M. Malpighi, A. Leeuwenhoek, J. Swammerdam, N. Grew and others. A new stage in the development of science is associated with the names of K. Wolf and K. Baer, the founders of embryology.
In the 19th century, histology was a full-fledged academic discipline. In the middle of the 19th century, A. Kölliker, Leiding and others created the foundations of the modern doctrine of fabrics. R. Virchow laid the foundation for the development of cellular and tissue pathology. Discoveries in cytology and creation cell theory stimulated the development of histology. The works of I. I. Mechnikov and L. Pasteur, who formulated the basic ideas about the immune system, had a great influence on the development of science.
The 1906 Nobel Prize in Physiology or Medicine was awarded to two histologists, Camillo Golgi and Santiago Ramon y Cajal. They had mutually opposing views on the nervous structure of the brain in different examinations of the same photographs.
In the 20th century, the improvement of methodology continued, which led to the formation of histology in its current form. Modern histology is closely related to cytology, embryology, medicine and other sciences. Histology deals with issues such as patterns of development and differentiation of cells and tissues, adaptation at the cellular and tissue levels, problems of tissue and organ regeneration, etc. The achievements of pathological histology are widely used in medicine, making it possible to understand the mechanism of development of diseases and propose methods for their treatment.
Research methods in histology include the preparation of histological preparations and their subsequent study using a light or electron microscope. Histological preparations are smears, prints of organs, thin sections of pieces of organs, possibly stained with a special dye, placed on a microscope slide, enclosed in a preservative medium and covered with a coverslip.
Tissue histology
Tissue is a phylogenetically formed system of cells and non-cellular structures that have a common structure, often origin, and are specialized to perform specific specific functions. The tissue is formed during embryogenesis from the germ layers. The ectoderm forms the epithelium of the skin (epidermis), the epithelium of the anterior and posterior sections of the digestive canal (including the epithelium of the respiratory tract), the epithelium of the vagina and urinary tract, the parenchyma of the major salivary glands, the outer epithelium of the cornea and nervous tissue.
Mesenchyme and its derivatives are formed from the mesoderm. These are all types of connective tissue, including blood, lymph, smooth muscle tissue, as well as skeletal and cardiac muscle tissue, nephrogenic tissue and mesothelium (serous membranes). From the endoderm - the epithelium of the middle part of the digestive canal and the parenchyma of the digestive glands (liver and pancreas). Tissues contain cells and intercellular substance. At the beginning, stem cells are formed - these are poorly differentiated cells capable of dividing (proliferation), they gradually differentiate, i.e. acquire the features of mature cells, lose the ability to divide and become differentiated and specialized, i.e. capable of performing specific functions.
The direction of development (cell differentiation) is determined genetically - determination. This direction is ensured by the microenvironment, the function of which is performed by the stroma of organs. A set of cells that are formed from one type of stem cell - differon. Tissues form organs. The organs are divided into stroma, formed by connective tissues, and parenchyma. All tissues regenerate. A distinction is made between physiological regeneration, which constantly occurs under normal conditions, and reparative regeneration, which occurs in response to irritation of tissue cells. The regeneration mechanisms are the same, only reparative regeneration is several times faster. Regeneration is at the heart of recovery.
Regeneration mechanisms:
Through cell division. It is especially developed in the earliest tissues: epithelial and connective; they contain many stem cells, the proliferation of which ensures regeneration.
Intracellular regeneration - it is inherent in all cells, but is the leading mechanism of regeneration in highly specialized cells. This mechanism is based on the strengthening of intracellular metabolic processes, which lead to restoration of the cell structure, and with further strengthening of individual processes
hypertrophy and hyperplasia of intracellular organelles occurs. which leads to compensatory hypertrophy of cells capable of performing a greater function.
Origin of fabrics
The development of an embryo from a fertilized egg occurs in higher animals as a result of repeated cell divisions (cleavage); The resulting cells are gradually distributed to their places in different parts future embryo. Initially, embryonic cells are similar to each other, but as their number increases, they begin to change, acquiring characteristic features and the ability to perform certain specific functions. This process, called differentiation, ultimately leads to the formation of different tissues. All tissues of any animal come from three original germ layers: 1) the outer layer, or ectoderm; 2) the innermost layer, or endoderm; and 3) the middle layer, or mesoderm. For example, muscles and blood are derivatives of mesoderm, the lining of the intestinal tract develops from endoderm, and ectoderm forms integumentary tissues and the nervous system.
Tissues have developed in evolution. There are 4 groups of tissues. The classification is based on two principles: histogenetic, which are based on origin, and morphofunctional. According to this classification, the structure is determined by the function of the tissue. The first to emerge were epithelial or integumentary tissues whose most important functions were protective and trophic. They have a high content of stem cells and regenerate through proliferation and differentiation.
Then connective tissues or supporting-trophic tissues of the internal environment appeared. Leading functions: trophic, supporting, protective and homeostatic - maintaining a constant internal environment. They are characterized by a high content of stem cells and regenerate through proliferation and differentiation. This tissue is divided into an independent subgroup - blood and lymph - liquid tissues.
The next ones are muscle (contractile) tissues. The main property - contractility - determines the motor activity of organs and the body. There are smooth muscle tissue - a moderate ability to regenerate through the proliferation and differentiation of stem cells, and striated (cross-striped) muscle tissue. These include cardiac tissue - intracellular regeneration, and skeletal tissue - regenerates due to the proliferation and differentiation of stem cells. The main recovery mechanism is intracellular regeneration.
Then nervous tissue arose. Contains glial cells, they are able to proliferate. but the nerve cells (neurons) themselves are highly differentiated cells. They react to stimuli, form a nerve impulse and transmit this impulse along the processes. Nerve cells have intracellular regeneration. As the tissue differentiates, the leading method of regeneration changes - from cellular to intracellular.
Main types of fabrics
Histologists usually distinguish four main tissues in humans and higher animals: epithelial, muscle, connective (including blood) and nervous. In some tissues, the cells have approximately the same shape and size and fit one another so tightly that there is no or almost no intercellular space left between them; such tissues cover the outer surface of the body and line its internal cavities. In other tissues (bone, cartilage), the cells are not so densely located and are surrounded by the intercellular substance (matrix) that they produce. The cells of the nervous tissue (neurons) that form the brain and spinal cord have long processes that end very far from the cell body, for example, at points of contact with muscle cells. Thus, each tissue can be distinguished from others by the nature of the arrangement of cells. Some tissues have a syncytial structure, in which the cytoplasmic processes of one cell transform into similar processes of neighboring cells; this structure is observed in embryonic mesenchyme, loose connective tissue, reticular tissue, and can also occur in some diseases.
Many organs are composed of several types of tissue, which can be recognized by their characteristic microscopic structure. Below is a description of the main types of tissue found in all vertebrates. Invertebrates, with the exception of sponges and coelenterates, also have specialized tissues similar to the epithelial, muscle, connective and nervous tissues of vertebrates.
Epithelial tissue. The epithelium may consist of very flat (scaly), cubic or cylindrical cells. Sometimes it is multi-layered, i.e. consisting of several layers of cells; such epithelium forms, for example, the outer layer of human skin. In other parts of the body, for example in the gastrointestinal tract, the epithelium is single-layered, i.e. all its cells are connected to the underlying basement membrane. In some cases, a single-layer epithelium may appear stratified: if the long axes of its cells are not parallel to each other, then the cells appear to be at different levels, although in fact they lie on the same basement membrane. Such epithelium is called multirow. The free edge of epithelial cells is covered with cilia, i.e. thin hair-like outgrowths of protoplasm (such ciliated epithelium lines, for example, the trachea), or ends with a “brush border” (epithelium lining the small intestine); this border consists of ultramicroscopic finger-like projections (so-called microvilli) on the surface of the cell. In addition to its protective functions, the epithelium serves as a living membrane through which gases and dissolved substances are absorbed by cells and released to the outside. In addition, the epithelium forms specialized structures, such as glands, that produce substances necessary for the body. Sometimes secretory cells are scattered among other epithelial cells; examples include mucus-producing goblet cells in the superficial layer of skin in fish or in the lining of the intestines in mammals.
Muscle. Muscle tissue differs from others in its ability to contract. This property is due to the internal organization of muscle cells containing a large number of submicroscopic contractile structures. There are three types of muscles: skeletal, also called striated or voluntary; smooth, or involuntary; cardiac muscle, which is striated but involuntary. Smooth muscle tissue consists of spindle-shaped mononuclear cells. Striated muscles are formed from multinucleated elongated contractile units with characteristic transverse striations, i.e. alternating light and dark stripes perpendicular to the long axis. Cardiac muscle consists of mononuclear cells connected end to end and has transverse striations; at the same time, the contractile structures of neighboring cells are connected by numerous anastomoses, forming a continuous network.
Connective tissue. There are different types of connective tissue. The most important supporting structures of vertebrates consist of two types of connective tissue - bone and cartilage. Cartilage cells (chondrocytes) secrete a dense elastic ground substance (matrix) around themselves. Bone cells (osteoclasts) are surrounded by a ground substance containing deposits of salts, mainly calcium phosphate. The consistency of each of these tissues is usually determined by the nature of the underlying substance. As the body ages, the content of mineral deposits in the underlying substance of the bone increases, and it becomes more brittle. In young children, the ground substance of bone, as well as cartilage, is rich in organic substances; due to this, they usually do not have real bone fractures, but so-called. fractures (greenstick fractures). Tendons are made of fibrous connective tissue; its fibers are formed from collagen, a protein secreted by fibrocytes (tendon cells). Adipose tissue can be located in different parts of the body; This is a peculiar type of connective tissue, consisting of cells in the center of which there is a large globule of fat.
Blood. Blood is a very special type of connective tissue; some histologists even distinguish it as a separate type. The blood of vertebrates consists of liquid plasma and formed elements: red blood cells, or erythrocytes, containing hemoglobin; a variety of white cells, or leukocytes (neutrophils, eosinophils, basophils, lymphocytes and monocytes), and blood platelets, or platelets. In mammals, mature red blood cells entering the bloodstream do not contain nuclei; in all other vertebrates (fish, amphibians, reptiles and birds), mature functioning red blood cells contain a nucleus. Leukocytes are divided into two groups - granular (granulocytes) and non-granular (agranulocytes) - depending on the presence or absence of granules in their cytoplasm; in addition, they are easy to differentiate using staining with a special mixture of dyes: with this staining, eosinophil granules acquire a bright pink color, the cytoplasm of monocytes and lymphocytes - a bluish tint, basophil granules - a purple tint, neutrophil granules - a faint purple tint. In the bloodstream, cells are surrounded by a clear liquid (plasma) in which various substances are dissolved. Blood delivers oxygen to tissues, removes carbon dioxide and metabolic products from them, and transports nutrients and secretion products, such as hormones, from one part of the body to another.
Nervous tissue. Nervous tissue consists of highly specialized cells - neurons, concentrated mainly in the gray matter of the brain and spinal cord. The long process of a neuron (axon) extends long distances from the place where the nerve cell body containing the nucleus is located. The axons of many neurons form bundles that we call nerves. Dendrites also extend from neurons - shorter processes, usually numerous and branched. Many axons are covered with a special myelin sheath, which consists of Schwann cells containing fat-like material. Adjacent Schwann cells are separated by small gaps called nodes of Ranvier; they form characteristic grooves on the axon. Nerve tissue is surrounded by a special type of supporting tissue known as neuroglia.
Tissue responses to abnormal conditions
When tissues are damaged, there may be some loss of their typical structure as a reaction to the disturbance.
Mechanical damage. In case of mechanical damage (cut or fracture), the tissue reaction is aimed at filling the resulting gap and reuniting the edges of the wound. Poorly differentiated tissue elements, in particular fibroblasts, rush to the site of rupture. Sometimes the wound is so large that the surgeon must insert pieces of tissue into it to stimulate the initial stages of the healing process; For this purpose, fragments or even whole pieces of bone obtained during amputation and stored in a “bone bank” are used. In cases where the skin surrounding a large wound (for example, with burns) cannot provide healing, transplants of healthy skin flaps taken from other parts of the body are resorted to. In some cases, such transplants do not take root, since the transplanted tissue does not always manage to form contact with those parts of the body to which it is transferred, and it dies or is rejected by the recipient.
Pressure. Calluses occur when there is constant mechanical damage to the skin as a result of pressure exerted on it. They appear in the form of familiar calluses and thickened skin on the soles of the feet, palms of the hands and other areas of the body that are under constant pressure. Removing these thickenings by excision does not help. As long as the pressure continues, the formation of calluses will not stop, and by cutting them off we only expose the sensitive underlying layers, which can lead to the formation of wounds and the development of infection.
Concept of fabrics.Types of fabrics.
Structure and functions
epithelial tissue.
Concept and types of fabrics
Tissue is a system of cells similar inorigin, structure and
functions and intercellular (tissue)
liquid.
The study of tissues is called
histology (Greek histos - tissue, logos
- teaching). Types of fabrics:
-epithelial
or cover
-connective
I (fabrics
internal
environment);
- muscular
- nervous
Epithelial tissue
Epithelial tissue (epithelium) istissue covering the surface of the skin
eye, as well as lining all cavities
body, inner surface
hollow digestive organs,
respiratory, genitourinary systems,
found in most glands
body. There are integumentary and
glandular epithelium.
Functions of the epithelium
PokrovnayaProtective
excretory
Provides mobility
internal organs in serous
cavities
Classification of epithelium:
Single layer:flat – endothelium (all vessels from the inside) and
mesothelium (all serous membranes)
cuboidal epithelium (renal tubules,
salivary gland ducts)
prismatic (stomach, intestines, uterus,
fallopian tubes, bile ducts)
cylindrical, ciliated and ciliated
(intestines, respiratory tract)
Ferrous (single or multilayer)
Classification of epithelium
Multilayer:flat
keratinizing (epidermis
skin) and non-keratinizing (mucous
membranes, cornea of the eye) - are
cover
transition
- in the urinary tract
structures: renal pelvis, ureters,
bladder, the walls of which
subject to strong stretching
Connective tissue. Features of the structure.
Connective tissue is made up of cells anda large amount of intercellular substance,
including the main amorphous substance and
Connective tissue.
fibers.
Featuresfabric
buildings.
Connective
is a fabric
internal environment, does not come into contact with the external
environment and internal body cavities.
Participates in the construction of all internal
organs.
Functions of connective tissue:
mechanical, supporting and shaping,makes up the body's support system: bones
skeleton, cartilage, ligaments, tendons, forming
capsule and stroma of organs;
protective, carried out by
mechanical protection (bones, cartilage, fascia),
phagocytosis and production of immune bodies;
trophic, associated with the regulation of nutrition,
metabolism and maintaining homeostasis;
plastic, expressed in active
participation in wound healing processes.
Classification of connective tissue:
Connective tissue itself:Loose fibrous connective tissue (surrounds
blood vessels, organ stroma)
Dense fibrous connective tissue can be shaped
(ligaments, tendons, fascia, periosteum) and unformed
(mesh layer of skin)
With special properties:
adipose - white (in adults) and brown (in newborns), lipocyte cells
reticular (CCM, lymph nodes, spleen),
reticular cells and fibers
pigmented (nipples, scrotum, around the anus,
iris, moles), cells - pigmentocytes Skeletal connective tissue:
Cartilaginous: chondroblasts, chondrocytes, collagen and
elastic fibers
hyaline (articular cartilages, costal, thyroid
cartilage, larynx, bronchi)
elastic (epiglottis, auricle, auditory
passage)
fibrous (intervertebral discs, pubic
symphysis, menisci, mandibular joint, sternoclavicular joint)
Bone:
coarse fibrous (in the embryo, in the sutures of the adult’s skull)
lamellar (all human bones)
Muscle
Striated muscle tissue - all skeletalmuscles. It consists of long multi-core
cylindrical threads capable of contraction, and their ends
end with tendons. SFE – muscle fiber
Smooth muscle tissue - found in the walls of hollow
organs, blood and lymphatic vessels, in the skin and
choroid of the eyeball. Cut smooth
muscle tissue is not subject to our will.
Cardiac striated muscle tissue
cardiomyocytes are small in size, have one or two nuclei,
abundance of mitochondria, do not end with tendons, have
special contacts - nexuses for transmitting impulses. Not
regenerate
Nervous tissue
The main functional propertynervous tissue is excitability and
conductivity (impulse transmission). She
able to perceive irritations from
external and internal environment and transmit
them along their fibers to other tissues and
organs of the body. Nervous tissue consists of
neurons and supporting cells –
neuroglia. Neurons are
polygonal cells with
processes along which they are carried out
impulses. Neurons extend from the cell body
two types of shoots. The longest of
them (the only one), conducting
irritation from the neuron body - axon.
Short branching shoots
by which impulses are conducted along
direction towards the neuron body are called
dendrites (Greek dendron - tree).
Types of neurons by number of processes
unipolar – with one axon, rarelymeet
pseudounipolar - the axon and dendrite of which
begin from the general growth of the cell body with
subsequent T-shaped division
bipolar - with two processes (axon and
dendrite).
multipolar – more than 2 processes
Types of neurons by function:
afferent (sensitive) neurons- carry impulses from receptors to reflex
center.
intercalary neurons
- carry out communication between neurons.
efferent (motor) neurons transmit impulses from the central nervous system to effectors
(executive bodies).
Neuroglia
Neuroglia from everyonesides surrounds
neurons and makes up
stroma of the central nervous system. Cells
neuroglia 10 times
more than
neurons, they can
share. Neuroglia
is about 80%
brain mass. She
performs in nervous
support tissue,
secretory,
trophic and
protective functions.
Nerve fibers
these are processes (axons) of nerve cells, usually coveredshell. A nerve is a collection of nerve fibers
enclosed in a common connective tissue membrane.
The main functional property of nerve fibers
is conductivity. Depending on the structure
Nerve fibers are divided into myelin (pulp) and
nonmyelinated (pulpless). At regular intervals
the myelin sheath is interrupted by nodes of Ranvier.
This affects the speed of excitation along
nerve fiber. Excitation in myelin fibers
transmitted spasmodically from one interception to another with
high speed, reaching 120 m/s. IN
non-myelinated fibers, rate of excitation transmission
does not exceed 10 m/s.
Synapse
From (Greek synaps - connection, connection) - connection betweenpresynaptic axon terminal and membrane
postsynaptic cell. In any synapse there are three
main parts: presynaptic membrane, synaptic
cleft and postsynaptic membrane.
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Ministry Agriculture and food of the Republic of Belarus
Vitebsk Order of the Badge of Honor
State Academy of Veterinary Medicine"
Department of Pathological Anatomy and Histology
DIPLOMAMY JOB
on the topic: “Studying issues of cytology, histology and embryology”
Vitebsk 2011
1. Histology as a science, its relationship with other disciplines, its role in the formation and practical work of a doctor of veterinary medicine
2. Definition of the concept “cell”. Its structural organization
3. Composition and purpose of the cytoplasm
4. Cell organelles (definition, classification, characteristics of the structure and functions of mitochondria, lamellar complex, lysosomes, endoplasmic reticulum)
5. Structure and functions of the nucleus
6. Types of cell division
8. The structure of sperm and their biological properties
9. Spermatogenesis
10. Structure and classification of eggs
11. Stages of embryo development
12. Features embryonic development mammals (formation of trophoblast and membranes)
13. Placenta (structure, functions, classifications)
14. Morphological classification and a brief description of main types of epithelium
15. general characteristics blood as a tissue of the internal environment of the body
16. Structure and functional significance of granulocytes
17. Structure and functional significance of agranulocytes
18. Morphofunctional characteristics of loose connective tissue
19. General characteristics of nervous tissue (composition, classification of neurocytes and neuroglia)
20. Structure and functions of the thymus
21. Structure and functions of lymph nodes
22. Structure and functions
23. Structure and functions of the monochamber stomach. Characteristics of its sinewy apparatus
24. Structure and functions of the small intestine
25. Structure and functions of the liver
26. Structure and functions of the lung
27. Structure and functions of the kidney
28. Structure and functions of the testes
29. Structure and functions of the uterus
30. Composition and purpose of the endocrine system
31. Cellular structure of the cerebral cortex
1. G histology as a science, its relationship with other disciplines, its role in the formation and practical work of a doctor of veterinary medicine
Histology (histos - tissue, logos - study, science) is the science of the microscopic structure, development and vital activity of cells, tissues and organs of animals and humans. The body is a single integral system built from many parts. These parts are closely interconnected, and the body itself constantly interacts with the external environment. In the process of evolution, the animal body acquired a multi-level nature of its organization:
Molecular.
Subcellular.
Cellular.
Fabric.
Organ.
Systemic.
Organic.
This allows, when studying the structure of animals, to divide their organisms into separate parts, to apply various research methods and to distinguish the following sections in histology as separate branches of knowledge:
1. Cytology - studies the structure and functions of body cells;
2. Embryology - studies the patterns of embryonic development of the organism:
a) General embryology - the science of the earliest stages of embryo development, including the period of the appearance of organs that characterize the belonging of individuals to a certain type and class of the animal kingdom;
b) Particular embryology - a system of knowledge about the development of all organs and tissues of the embryo;
3. General histology - the study of the structure and functional properties of body tissues;
4. Particular histology is the most extensive and important section of the discipline, including the entirety of knowledge about the structural features and functional functions of organs that form certain systems of the body.
Histology belongs to the morphological sciences and is one of the fundamental biological disciplines. It is closely related to other general biological ones (biochemistry, anatomy, genetics, physiology, immunomorphology, molecular biology), disciplines of the livestock complex, as well as veterinary medicine (pathoanatomy, veterinary examination, obstetrics, therapy, etc.). Together they form the theoretical basis for the study of veterinary medicine. Histology is also important practical significance: many histological research methods are widely used in medical practice.
Objectives and significance of histology.
1. Together with other sciences, it forms medical thinking.
2. Histology creates the biological basis for the development of veterinary medicine and animal husbandry.
3. Histological methods are widely used in the diagnosis of animal diseases.
4. Histology provides quality control and effectiveness of the use of feed additives and preventive agents.
5. Using histological research methods, the therapeutic effectiveness of veterinary drugs is monitored.
6. Provides assessment of the quality of breeding work with animals and herd reproduction.
7. Any targeted intervention in the animal body can be monitored by histological methods.
2. Definition of the concept “cell”. Its structural organization
A cell is the basic structural and functional unit that underlies the structure, development and life of animal and plant organisms. It consists of 2 inextricably linked parts: cytoplasm and nucleus. Cytoplasm includes 4 components:
Cell membrane (plasmolemma).
Hyaloplasma
Organelles (organelles)
Cellular inclusions
The core also consists of 4 parts:
Nuclear membrane, or karyolemma
Nuclear juice, or karyoplasm
Chromatina
The plasmalemma is the outer membrane of the cell. It is constructed of a biological membrane, a supra-membrane complex and a sub-membrane apparatus. Retains cellular contents, protects the cell and ensures its interaction with the pericellular environment, other cells and tissue elements.
Hyaloplasm is a colloidal environment of the cytoplasm. Serves to accommodate organelles, inclusions, and their interaction.
Organelles are permanent structures of the cytoplasm that perform certain functions in it.
Inclusions are substances that enter the cell for nutritional purposes or are formed in it as a result of vital processes.
The nuclear envelope consists of two biological membranes, separating the contents of the nucleus from the cytoplasm and at the same time ensuring their close interaction.
Nuclear juice is the colloidal environment of the nucleus.
Chromatin is the form of existence of chromosomes. Consists of DNA, histone and non-histone proteins, RNA.
The nucleolus is a complex of DNA nucleolar organizers, ribosomal RNA, proteins and ribosomal subunits that are formed here.
3. Composition and purpose of the cytoplasm
Cytoplasm is one of the two main parts of the cell, which provides its basic life processes.
Cytoplasm includes 4 components:
Cell membrane (plasmolemma).
Hyaloplasma.
Organelles (organelles).
Cellular inclusions.
Hyaloplasm is a colloidal matrix of the cytoplasm in which the main life processes of the cell occur, organelles and inclusions are located and function.
The cell membrane (plasmolemma) is built from a biological membrane, a supra-membrane complex and a sub-membrane apparatus. It retains cellular contents, maintains the shape of cells, carries out their motor reactions, performs barrier and receptor functions, ensures the processes of entry and exit of substances, as well as interaction with the pericellular environment, other cells and tissue elements.
The biological membrane as the basis of the plasmalemma is built from a bimolecular lipid layer into which protein molecules are mosaically included. The hydrophobic poles of lipid molecules face inward, forming a kind of hydraulic lock, and their hydrophilic heads ensure active interaction with the external and intracellular environment.
Proteins are located superficially (peripheral), enter the hydrophobic layer (semi-integral) or penetrate the membrane through (integral). Functionally, they form structural, enzymatic, receptor and transport proteins.
The supramembrane complex - the glycocalyx - membrane is formed by glycosaminoglycans. Performs protective and regulatory functions.
The submembrane apparatus is formed by microtubules and microfilaments. Acts as a musculoskeletal device.
Organelles are permanent structures of the cytoplasm that perform certain functions in it. There are general-purpose organelles (Golgi apparatus, mitochondria, cell center, ribosomes, lysosomes, peroxisomes, cytoplasmic reticulum, microtubules and microfilaments) and special ones (myofibrils - in muscle cells; neurofibrils, synaptic vesicles and tigroid substance - in neurocytes; tonofibrils, microvilli, cilia and flagella - in epithelial cells).
Inclusions are substances that enter the cell for nutritional purposes or are formed in it as a result of vital processes. There are trophic, secretory, pigment and excretory inclusions.
4. Cell organelles (definition, classification, characteristics of the structure and functions of mitochondria, lamellar complex, lysosomes, endoplasmic reticulum)
Organelles (organelles) are permanent structures of the cytoplasm that perform certain functions in it.
The classification of organelles takes into account the features of their structure and physiological functions.
Based on the nature of the functions performed, all organelles are divided into two large groups:
1. General purpose organelles, expressed in all cells of the body, provide the most general functions that support their structure and life processes (mitochondria, centrosome, ribosomes, lysosomes, peroxisomes, microtubules, cytoplasmic reticulum, Golgi complex)
2. Special - found only in cells that perform specific functions (myofibrils, tonofibrils, neurofibrils, synaptic vesicles, tigroid substance, microvilli, cilia, flagella).
Based on their structural characteristics, we distinguish organelles with membrane and non-membrane structure.
Organelles with a membrane structure basically have one or two biological membranes (mitochondria, lamellar complex, lysosomes, peroxisomes, endoplasmic reticulum).
Organelles of a non-membrane structure are formed by microtubules, globules from a complex of molecules and their bundles (centrosome, microtubules, microfilaments and ribosomes).
By size, we distinguish a group of organelles visible in a light microscope (Golgi apparatus, mitochondria, cell center), and ultramicroscopic organelles visible only in an electron microscope (lysosomes, peroxisomes, ribosomes, endoplasmic reticulum, microtubules and microfilaments).
The Golgi complex (lamellar complex) is visible under light microscopy in the form of short and long filaments (up to 15 µm in length). Under electron microscopy, each such filament (dictyosome) represents a complex of flat cisterns layered on top of each other, tubes and vesicles. The lamellar complex ensures the accumulation and removal of secretions, synthesizes some lipids and carbohydrates, and forms primary lysosomes.
Mitochondria under light microscopy are detected in the cytoplasm of cells in the form of small grains and short filaments (up to 10 microns in length), from the names of which the very name of the organelle is derived. Under electron microscopy, each of them appears in the form of round or oblong bodies, consisting of two membranes and a matrix. The inner membrane has comb-like protrusions - cristae. Mitochondrial DNA and ribosomes that synthesize some structural proteins are detected in the matrix. Enzymes localized on mitochondrial membranes provide the processes of oxidation of organic substances (cellular respiration) and ATP storage (energy function).
Lysosomes are represented by small vesicle-like formations, the wall of which is formed by a biological membrane, inside which is contained a wide range of hydrolytic enzymes (about 70).
They act as the digestive system of cells, neutralize harmful agents and foreign particles, and utilize their own obsolete and damaged structures.
There are primary lysosomes, secondary (phagolysosomes, autophagolysosomes) and tertiary telolysosomes (residual bodies).
The endoplasmic reticulum is a system of tiny cisterns and tubules that anastomose with each other and penetrate the cytoplasm. Their walls are formed by single membranes on which enzymes for the synthesis of lipids and carbohydrates are ordered - a smooth endoplasmic reticulum (agranular) or ribosomes are fixed - a rough (granular) reticulum. The latter is intended for accelerated synthesis of protein molecules for the general needs of the body (for export). Both types of EPS also provide circulation and transport of various substances.
veterinary histology cell organism
5. Structure and functions of the nucleus
The cell nucleus is its second most important component.
Most cells have one nucleus, but some liver cells and cardiomyocytes have two nuclei. In bone tissue macrophages there are from 3 to several dozen, and in striated muscle fiber there are from 100 to 3 thousand nuclei. On the contrary, mammalian red blood cells are anucleate.
The shape of the nucleus is often round, but in prismatic epithelial cells it is oval, in flat cells it is flattened, in mature granular leukocytes it is segmented, in smooth myocytes it elongates to rod-shaped. The nucleus is usually located in the center of the cell. In plasma cells it lies eccentrically, and in prismatic epithelial cells it is displaced towards the basal pole.
Chemical composition of the core:
Proteins - 70%, nucleic acids - 25%, carbohydrates, lipids and inorganic substances account for approximately 5%.
Structurally, the core is built from:
1. nuclear membrane (karyolemma),
2. nuclear juice (karyoplasm),
3. nucleolus,
4. chromatin. The nuclear envelope - karyolemma consists of 2 elementary biological membranes. Between them there is a pronounced perinuclear space. In some areas, two membranes are connected to each other and form the pores of the karyolemma, with a diameter of up to 90 nm. They contain structures that form the so-called pore complex of three plates. There are 8 granules along the edges of each plate, and one in their center. The finest fibrils (threads) come to it from the peripheral granules. As a result, peculiar diaphragms are formed to regulate the movement of organic molecules and their complexes through the shell.
Functions of the karyolemma:
1. delimiting,
2. regulatory.
Nuclear juice (karyoplasm) is a colloidal solution of carbohydrates, proteins, nucleotides and minerals. It is a microenvironment for ensuring metabolic reactions and the movement of information and transport RNAs to nuclear pores.
Chromatin is the form of existence of chromosomes. It is represented by a complex of DNA, RNA molecules, packaging proteins and enzymes (histones and non-histone proteins). Histones are directly associated with the chromosome. They ensure the helicalization of the DNA molecule in the chromosome. Non-histone proteins are enzymes: DNA - nucleases that destroy complementary bonds, causing its despiralization;
DNA and RNA polymerases, which ensure the construction of RNA molecules on unlinked DNA, as well as the self-duplication of chromosomes before division.
Chromatin is presented in the nucleus in two forms:
1. dispersed euchromatin, which is expressed in the form of fine granules and threads. In this case, sections of DNA molecules are in an untwisted state. RNA molecules that read information about the structure of the protein are easily synthesized on them, and transfer RNAs are built. The resulting i-RNA moves into the cytoplasm and is inserted into ribosomes, where protein synthesis processes take place. Euchromatin is a functionally active form of chromatin. Its predominance indicates a high level of cell vital processes.
2. Condensed heterochromatin. Under light microscopy it appears in the form of large granules and lumps. At the same time, histone proteins tightly spiral and pack DNA molecules, on which it is therefore impossible to build RNA, which is why heterochromatin represents a functionally inactive, unclaimed part of the chromosome set.
Nucleolus. It has a round shape, with a diameter of up to 5 microns. Cells can have from 1 to 3 nucleoli, depending on its functional state. It represents a collection of terminal sections of several chromosomes, which are called nucleolar organizers. Ribosomal RNAs are formed on the DNA of nucleolar organizers, which, when combined with the corresponding proteins, form ribosomal subunits.
Kernel functions:
1. Preservation of hereditary information received from the mother cell unchanged.
2. Coordination of vital processes and implementation of hereditary information through the synthesis of structural and regulatory proteins.
3. Transfer of hereditary information to daughter cells during division.
6. Types of cell division
Division represents a way for cells to reproduce themselves. It provides:
a) continuity of existence of cells of a certain type;
b) tissue homeostasis;
c) physiological and reparative regeneration of tissues and organs;
d) reproduction of individuals and preservation of animal species.
There are 3 ways of cell division:
1. amitosis - cell division without visible changes in the chromosomal apparatus. It occurs by simple constriction of the nucleus and cytoplasm. Chromosomes are not detected, the spindle is not formed. Characteristic of some embryonic and damaged tissues.
2. mitosis - a method of dividing somatic and germ cells at the reproduction stage. In this case, from one mother cell two daughter cells with a complete, or diploid, set of chromosomes are formed.
3. meiosis is a method of division of germ cells at the maturation stage, in which 4 daughter cells with a half, haploid set of chromosomes are formed from one mother cell.
7. Mitosis
Mitosis is preceded by interphase, during which the cell prepares for future division. This preparation includes
Cell growth;
Energy storage in the form of ATP and nutrients;
Self-duplication of DNA molecules and chromosome set. As a result of duplication, each chromosome consists of 2 sister chromatids;
Duplication of centrioles of the cell center;
Synthesis of special proteins such as tubulin for the construction of spindle filaments.
Mitosis itself consists of 4 phases:
Prophases,
Metaphases,
Anaphases,
Telophases.
In prophase, chromosomes spiral, become denser and shorten. They are now visible under light microscopy. The centrioles of the cell center begin to diverge towards the poles. A fission spindle is built between them. At the end of prophase, the nucleolus disappears and the nuclear membrane fragments.
In metaphase, the construction of the division spindle is completed. Short spindle filaments are attached to the centromeres of chromosomes. All chromosomes are located at the equator of the cell. Each of them is held in the equatorial plate with the help of 2 chromatin threads that go to the poles of the cell, and its central zone is filled with long achromatin fibrils.
In anaphase, due to the contraction of the chromatin filaments of the division spindles, the chromatids are separated from each other in the region of the centromeres, after which each of them slides along the central filaments to the upper or lower pole of the cell. From this point on, the chromatid is called a chromosome. Thus, an equal number of identical chromosomes appear at the poles of the cell, i.e. one complete, diploid set of them.
During telophase, a new nuclear envelope forms around each group of chromosomes. Condensed chromatin begins to loosen. Nucleoli appear. In the central part of the cell, the plasmalemma invaginates inward, the tubules of the endoplasmic reticulum connect to it, which leads to cytotomy and division of the mother cell into two daughter cells.
Meiosis (reduction division).
It is also preceded by interphase, in which the same processes occur as before mitosis. Meiosis itself includes two divisions: reduction, which produces haploid cells with double chromosomes, and equational, which leads mitotically to the formation of cells with single chromosomes.
The leading phenomenon that ensures a decrease in the chromosome set is the conjugation of paternal and maternal chromosomes in each pair, which takes place in the prophase of the first division. When homologous chromosomes, consisting of two chromatids, come together, tetrads are formed, which already include 4 chromatids.
In metaphase of meiosis, the tetrads are preserved and located at the equator of the cell. In anaphase, therefore, entire duplicated chromosomes move to the poles. As a result, two daughter cells with half the set of doubled chromosomes are formed. Such cells, after a very short interphase, divide again by normal mitosis, which leads to the appearance of haploid cells with single chromosomes.
The phenomenon of conjugation of homologous chromosomes simultaneously solves another important problem - the creation of prerequisites for individual genetic variability due to the processes of crossing over and gene exchange and multivariance in the polar orientation of tetrads in the metaphase of the first division.
8. The structure of sperm and their biological properties
Spermatozoa (male sex cells) are flagellated, whip-shaped cells. The sequential arrangement of organelles in the sperm makes it possible to distinguish the head, neck, body and tail in the cell.
The head of the sperm of representatives of agricultural mammals is asymmetrical - bucket-shaped, which ensures its rectilinear, translational-rotational movement. Most of the head is occupied by the nucleus, and the anterior one forms the head cap with the acrosome. The acrosome (modified Golgi complex) accumulates enzymes (hyaluronidase, proteases), which allow sperm to destroy the secondary membranes of the egg during fertilization.
Behind the nucleus, in the cell neck, two centrioles are located one after the other - proximal and distal. The proximal centriole lies free in the cytoplasm and is introduced into the egg during fertilization. An axial filament grows from the distal centriole - this is a special cell organelle that ensures that the tail beats in only one plane.
In the sperm body, around the axial filament, mitochondria are located sequentially one after another, forming a spiral filament - the energy center of the cell.
In the region of the tail, the cytoplasm gradually decreases, so that in its final part the axial filament is covered only by the plasmalemma.
Biological properties of sperm:
1. Carrying hereditary information about the paternal body.
2. Spermatozoa are not capable of division; their nucleus contains a half (haploid) set of chromosomes.
3. The size of the cells does not correlate with the weight of the animals and therefore in representatives of agricultural mammals varies within narrow limits (from 35 to 63 μm).
4. The moving speed is 2-5mm per minute.
5. Spermatozoa are characterized by the phenomenon of rheotaxis, i.e. movement against a weak current of mucus in the female genital tract, as well as the phenomenon of chemotaxis - the movement of sperm to chemical substances(gynogamones) produced by the egg.
6. In the epididymis, sperm acquire an additional lipoprotein coat, which allows them to hide their antigens, because For the female body, the male gametes act as foreign cells.
7. Spermatozoa have a negative charge, which allows them to repel each other and thereby prevent gluing and mechanical damage to cells (there are up to several billion cells in one ejaculate).
8. Spermatozoa of animals with internal fertilization cannot withstand the effects of environmental factors, in which they die almost immediately.
9. High temperature, ultraviolet irradiation, acidic environment, and heavy metal salts have a detrimental effect on sperm.
10. Adverse effects occur when exposed to radiation, alcohol, nicotine, drugs, antibiotics and a number of other medications.
11. At the animal’s body temperature, spermatogenesis processes are disrupted.
12. In low temperature conditions, male gametes are able to retain their vital properties for a long time, which made it possible to develop the technology of artificial insemination of animals.
13. In a favorable environment of the female reproductive tract, sperm retain fertilizing ability for 10-30 hours.
9. Spermatogenesis
It is carried out in the convoluted tubules of the testis in 4 stages:
1. reproduction stage;
2. growth stage;
3. stage of maturation;
4. formation stage.
During the first stage of reproduction, stem cells lying on the basement membrane (with a full set of chromosomes) divide repeatedly by mitosis, forming many spermatogonia. With each round of division, one of the daughter cells remains in this outer row as a stem cell, the other is forced into the next row and enters the growth stage.
During the growth stage, germ cells are called first-order spermatocytes. They are growing and preparing for the third stage of development. Thus, the second stage is simultaneously an interphase before future meiosis.
In the third stage of maturation, germ cells successively undergo two meiotic divisions. In this case, from 1st order spermatocytes, 2nd order spermatocytes with half the set of doubled chromosomes are formed. These cells, after a short interphase, enter the second meiotic division, as a result of which spermatids are formed. Spermatocytes of the 2nd order make up the third row in the spermatogenic epithelium. Due to the short duration of interphase, second-order spermatocytes are not found throughout the entire convoluted tubule. Spermatids are the smallest cells in the tubules. They form 2-3 cell rows at their inner edges.
During the fourth stage of formation, small round cells - spermatids - gradually transform into spermatozoa that have a flagellated shape. To ensure these processes, spermatids come into contact with trophic Sertoli cells, penetrating into niches between the processes of their cytoplasm. The arrangement of the nucleus, lamellar complex, and centrioles is ordered. An axial filament grows from the distal centriole, after which the cytoplasm with the plasmalemma shifts, forming the tail of the sperm. The lamellar complex is located in front of the nucleus and is transformed into an acrosome. Mitochondria descend into the cell body, forming a spiral thread around the axial filament. The heads of the formed sperm still remain in the niches of the supporting cells, and their tails hang into the lumen of the convoluted tubule.
10. Structure and classification of eggs
The egg is a stationary, round-shaped cell with a certain supply of yolk inclusions (nutrients of carbohydrate, protein and lipid nature). Mature eggs lack centrosomes (they are lost upon completion of the maturation stage).
Mammalian eggs, in addition to the plasmolemma (ovolemma), which is the primary shell, also have secondary shells with protective and trophic functions: a shiny, or transparent, shell consisting of glycosaminoglycans, proteins, and a corona radiata, formed by one layer of prismatic follicular cells glued between is hyaluronic acid.
In birds, the secondary membranes are weakly expressed, but the tertiary membranes are significantly developed: the albuginea, subshell, shell and suprashell. They act as protective and trophic formations during the development of embryos in dry conditions.
Eggs are classified according to their number and distribution in the yolk cytoplasm:
1. Oligolecithal - oocytes with few yolks. Characteristic of primitive chordates (lancelet) living in an aquatic environment, and female mammals in connection with the transition to the intrauterine path of embryo development.
2. Mesolecithal eggs with medium accumulation of yolk. Common to most fish and amphibians.
3. Polylecithal - multiyolk egg cells are characteristic of reptiles and birds due to the terrestrial conditions of embryo development.
Classification of eggs according to yolk distribution:
1. Isolecithal eggs, in which the yolk inclusions are distributed relatively evenly throughout the cytoplasm (oligolecithal eggs of lancelets and mammals);
2. Telolecithal eggs. The yolk in them moves to the lower vegetative pole of the cell, and the free organelles and nucleus move to the upper animal pole (in animals with meso- and telolecithal types of eggs).
11. Stages of embryo development
Embryonic development is a chain of interconnected transformations, as a result of which a multicellular organism is formed from a unicellular zygote, capable of existing in the external environment. In embryogenesis, as part of ontogenesis, the processes of phylogenesis are also reflected. Phylogeny is the historical development of a species from simple to complex forms. Ontogenesis is the individual development of a particular organism. According to the biogenetic law, ontogeny is a short form of phylogeny, and therefore representatives of different classes of animals have common stages of embryonic development:
1. Fertilization and formation of the zygote;
2. Fragmentation of the zygote and formation of the blastula;
3. Gastrulation and the appearance of two germ layers (ectoderm and endoderm);
4. Differentiation of ecto- and endoderm with the appearance of the third germ layer - mesoderm, axial organs (notochord, neural tube and primary gut) and further processes of organogenesis and histogenesis (development of organs and tissues).
Fertilization is the process of mutual assimilation of an egg and a sperm, during which a single-celled organism - a zygote - arises, combining two hereditary information.
Zygote cleavage is the repeated division of the zygote through mitosis without the growth of the resulting blastomeres. This is how the simplest multicellular organism is formed - the blastula. We distinguish:
Complete, or holoblastic, fragmentation, in which the entire zygote is fragmented into blastomeres (lancelet, amphibians, mammals);
Incomplete, or meroblastic, if only part of the zygote (animal pole) undergoes cleavage (birds).
Complete crushing, in turn, happens:
Uniform - blastomeres of relatively equal size are formed (lancelet) with their synchronous division;
Uneven - with asynchronous division with the formation of blastomeres of different sizes and shapes (amphibians, mammals, birds).
Gastrulation is the stage of formation of a two-layer embryo. Its superficial cellular layer is called the outer germ layer - ectoderm, and the deep cellular layer is called the inner germ layer - endoderm.
Types of gastrulation:
1. invagination - invagination of the blastomeres of the bottom of the blastula towards the roof (lancelet);
2. epiboly - fouling of the roof of the blastula of its marginal zones and bottom with rapidly dividing small blastomeres (amphibians);
3. delamination - separation of blastomeres and migration - movement of cells (birds, mammals).
Differentiation of the germ layers leads to the appearance of cells of different quality, giving rise to the rudiments of various tissues and organs. In all classes of animals, axial organs first appear - the neural tube, notochord, primary gut - and the third (medium in position) germ layer - the mesoderm.
12. Features of embryonic development of mammals (formation of trophoblast and fetal membranes)
Features of mammalian embryogenesis are determined by the intrauterine nature of development, as a result of which:
1. The egg does not accumulate large reserves of yolk (oligolecithal type).
2. Fertilization is internal.
3. At the stage of complete uneven fragmentation of the zygote, early differentiation of blastomeres occurs. Some of them divide faster and are characterized by light color and small size, others are dark color and large in size, since these blastomeres are delayed in dividing and fragment less frequently. The light blastomeres gradually envelop the slowly dividing dark ones, resulting in the formation of a spherical blastula without a cavity (morula). In the morula, dark blastomeres make up its internal contents in the form of a dense nodule of cells, which are later used to build the body of the embryo - this is the embryoblast.
Light blastomeres are located around the embryoblast in one layer. Their task is to absorb the secretion of the uterine glands (royal jelly) to ensure the nutritional processes of the embryo before the formation of the placental connection with the mother’s body. Therefore they form trophoblast.
4. The accumulation of royal jelly in the blastula pushes the embryoblast upward and makes it look like the discoblastula of birds. The embryo is now a germinal vesicle, or blastocyst. As a consequence, all further development processes in mammals repeat the already known paths characteristic of bird embryogenesis: gastrulation occurs through delamination and migration; the formation of axial organs and mesoderm occurs with the participation of the primitive streak and nodule, and the separation of the body and the formation of fetal membranes - the trunk and amniotic folds.
The trunk fold is formed as a result of the active proliferation of cells of all three germ layers in the zones bordering the germinal shield. The rapid growth of cells forces them to move inward and bend the leaves. As the trunk fold deepens, its diameter decreases, it increasingly isolates and rounds the embryo, simultaneously forming from the endoderm and the visceral layer of mesoderm the primary intestine and the yolk sac with the royal jelly enclosed in it.
The peripheral parts of the ectoderm and the parietal layer of mesoderm form an amniotic circular fold, the edges of which gradually move over the detached body and completely close over it. The fusion of the internal layers of the fold forms the internal water membrane - the amnion, the cavity of which is filled with amniotic fluid. Fusion of the outer layers of the amniotic fold ensures the formation of the outermost membrane of the fetus - the chorion (villous membrane).
Due to the blind protrusion through the umbilical canal of the ventral wall of the primary intestine, a middle membrane is formed - the allantois, in which a system of blood vessels (choroid) develops.
5. The outer shell - the chorion - has a particularly complex structure and forms multiple protrusions in the form of villi, with the help of which a close relationship is established with the mucous membrane of the uterus. The villi include areas of allantois with blood vessels that grow together with the chorion and trophoblast, the cells of which produce hormones to maintain the normal course of pregnancy.
6. The set of allantochorion villi and endometrial structures with which they interact form a special embryonic organ in mammals - the placenta. The placenta provides nutrition to the embryo, its gas exchange, removal of metabolic products, and reliable protection from unfavorable factors any etiology and hormonal regulation of development.
13. Placenta (structure, functions, classifications)
The placenta is a temporary organ that is formed during the embryonic development of mammals. There are baby and maternal placenta. The baby's placenta is formed by a collection of allanto-chorion villi. The maternal one is represented by areas of the uterine mucosa with which these villi interact.
The placenta ensures the supply of nutrients to the embryo (trophic function) and oxygen (respiratory), the release of the fetal blood from carbon dioxide and unnecessary metabolic products (excretory), the formation of hormones that support the normal course of pregnancy (endocrine), as well as the formation of the placental barrier (protective function) .
The anatomical classification of placentas takes into account the number and location of villi on the surface of the allantochorion.
1. Diffuse placenta is expressed in pigs and horses (short, unbranched villi are evenly distributed over the entire surface of the chorion).
2. Multiple, or cotyledonous, placenta is characteristic of ruminants. The villi of the allantochorion are arranged in islands called cotyledons.
3. The cingulate placenta in carnivores is a zone of accumulation of villi located in the form of a wide belt surrounding the fetal bladder.
4. In the discoidal placenta of primates and rodents, the chorionic villi zone has the shape of a disk.
Histological classification of placentas takes into account the degree of interaction of allantochorion villi with the structures of the uterine mucosa. Moreover, as the number of villi decreases, they become more branched in shape and penetrate deeper into the uterine mucosa, shortening the path of movement of nutrients.
1. Epitheliochorial placenta is characteristic of pigs and horses. Chorionic villi penetrate the uterine glands without destroying the epithelial layer. During childbirth, the villi easily move out of the uterine glands, usually without bleeding, which is why this type of placenta is also called a hemiplacenta.
2. Desmochorionic placenta is prominent in ruminants. The villi of the allanto-chorion penetrate into the lamina propria of the endometrium, in the area of its thickenings, the caruncles.
3. The endotheliochorial placenta is characteristic of carnivorous animals. The villi of the baby's placenta come into contact with the endothelium of the blood vessels.
4. Hemochorionic placenta is found in primates. The chorionic villi are immersed in blood-filled lacunae and washed with maternal blood. However, the mother's blood does not mix with the fetus' blood.
14. Morphological classification and brief characteristics of the main types of epithelium
The morphological classification of epithelial tissues is based on two characteristics:
1. number of layers of epithelial cells;
2. cell shape. In this case, in varieties of multilayered epithelium, only the shape of the epithelial cells of the surface (integumentary) layer is taken into account.
Single-layer epithelium, in addition, can be built from cells of the same shape and height, then their nuclei lie on the same level - single-row epithelium, and from significantly different epithelial cells.
In such cases, in low cells, the nuclei will form the bottom row, in medium-sized epithelial cells - the next one, located above the first, and in the tallest cells, one or two more rows of nuclei, which ultimately transforms the essentially single-layer tissue into a pseudo-multilayered form - multirow epithelium.
Considering the above, the morphological classification of the epithelium can be presented as follows:
Epithelium
Single layer Multilayer
Single Row Multi Row Flat: Transitional Cubic
Flat Prismatic keratinizing
Cubic ciliated non-keratinizing
Prismatic - (sciliating) edged Prismatic
In any type of single-layer epithelium, each of its cells has a connection with the basement membrane. Stem cells are located mosaically among the integumentary cells.
In multilayered epithelium, we distinguish three zones of epithelial cells with different shapes and degrees of differentiation. Only the lowest layer of prismatic or tall cubic cells is connected to the basement membrane. It is called basal and consists of stem, repeatedly dividing epithelial cells. The next, intermediate, zone is represented by differentiating (maturing) cells of various shapes, which can lie in one or several rows. Mature differentiated epithelial cells of a certain shape and properties are located on the surface. Multilayer epithelia provide protective functions.
Single-layer squamous epithelium is formed by flattened cells with irregular contours and a large surface area. Covers the serous membranes (mesothelium); forms the vascular lining (endothelium) and alveoli (respiratory epithelium) of the lungs.
Single-layer cubic epithelium is built from epithelial cells having approximately the same base width and height. The core is round in shape and characterized by a central position. Forms the secretory sections of the glands, the walls of the urine-forming renal tubules (nephrons).
Single-layer prismatic epithelium forms the walls of the excretory ducts of the exocrine glands, the uterine glands, and covers the mucous membrane of the intestinal type stomach, small and large intestine. Cells are characterized great height, narrow base and longitudinally oval shaped nucleus, displaced to the basal pole. The intestinal epithelium is bordered by microvilli at the apical poles of the enterocytes.
Single-layer multirow prismatic ciliated (ciliated) epithelium mainly covers the mucous membrane of the airways. The lowest wedge-shaped cells (basal) are constantly dividing, the middle ones are growing, not yet reaching the free surface, and the tall ones are the main type of mature epithelial cells bearing up to 300 cilia at the apical poles, which, contracting, move mucus with adsorbed foreign particles for coughing . Mucus is produced by nonciliated goblet cells.
Multilayered squamous non-keratinizing epithelium covers the conjunctiva and cornea of the eyes, the initial sections of the digestive tube, transition zones in the reproductive and urinary organs.
Multilayered squamous keratinizing epithelium consists of 5 layers of gradually keratinizing and exfoliating cells (keratinocytes) - basal, squamous spinous cells, granular, shiny, horny. Forms the epidermis of the skin, covers the external genitalia, the mucous membrane of the nipple canals in the mammary glands, and the mechanical papillae of the oral cavity.
Stratified transitional epithelium lines the mucous membranes of the urinary tract. The cells of the integumentary zone are large, longitudinally oval, secrete mucus, and have a well-developed glycocalyx in the plasma membrane to prevent the reabsorption of substances from urine.
Multilayered prismatic epithelium is expressed in the mouths of the main ducts of the wall salivary glands, in males - in the mucous membrane of the pelvic part of the genitourinary canal and in the canals of the testicular appendages, in females - in the lobar ducts of the mammary glands, in the secondary and tertiary follicles of the ovaries.
Multilayer cubic forms the secretory sections of the sebaceous glands of the skin, and in males, the spermatogenic epithelium of the convoluted tubules of the testes.
15. General characteristics of blood as a tissue of the internal environment of the body
Blood belongs to the tissues of the musculoskeletal group. Together with reticular and loose connective tissues, it plays a decisive role in the formation of the internal environment of the body. It has a liquid consistency and is a system consisting of two components - intercellular substance (plasma) and cells suspended in it - formed elements: erythrocytes, leukocytes and platelets ( blood platelets in mammals).
Plasma makes up about 60% of blood mass and contains 90-93% water and 7-10% dry matter. About 7% of it comes from proteins (4% - albumin, 2.8% - globulins and 0.4% - fibrinogen), 1% - from minerals, the same percentage remains from carbohydrates.
Functions of blood plasma proteins:
Albumin: - regulation of acid-base balance;
Transport;
Maintaining a certain level of osmotic pressure.
Globulins are immune proteins (antibodies) that perform a protective function and a variety of enzyme systems.
Fibrinogen - takes part in blood clotting processes.
Blood pH is 7.36 and is quite stably maintained at this level by a number of buffer systems.
Main functions of blood:
1. Continuously circulating through the blood vessels, it transfers oxygen from the lungs to the tissues, and carbon dioxide from the tissues to the lungs (gas exchange function); delivers nutrients absorbed in the digestive system to all organs of the body, and metabolic products to the excretory organs (trophic); transports hormones, enzymes and other biologically active substances to places of their active influence.
All of the above-mentioned aspects of the functional functions of blood can be combined into one general transport-trophic function.
2. Homeostatic - maintaining a constant internal environment of the body (creates optimal conditions for metabolic reactions);
3. Protective - ensuring cellular and humoral immunity, various forms of nonspecific protection, especially phagocytosis of foreign particles, blood clotting processes.
4. Regulatory function associated with maintaining a constant body temperature and a number of other processes provided by hormones and other biologically active substances.
Platelets - in mammals, non-nuclear cells, 3-5 microns in size, participate in blood clotting processes.
Leukocytes are divided into granulocytes (basophils, neutrophils and eosinophils) and agranulocytes (monocytes and lymphocytes). Perform various protective functions.
Erythrocytes in mammals are anucleate cells that have the shape of biconcave disks with an average diameter of 6-8 microns.
Part of the blood plasma constantly enters the tissues of organs through the vessels of the microvasculature and becomes tissue fluid. Giving up nutrients, receiving metabolic products, enriching itself in the hematopoietic organs with lymphocytes, the latter enters the vessels of the lymphatic system in the form of lymph and returns to the bloodstream.
Formed elements in the blood are in certain quantitative ratios and make up its hemogram.
The number of formed elements is calculated in 1 μl of blood or liter:
Red blood cells - 5-10 million per µl (x 1012 per l);
Leukocytes - 4.5-14 thousand per µl (x109 per l);
Blood platelets - 250-350 thousand per µl (x109 per l).
16. Structure and functional significance of granulocytes
Leukocytes in vertebrates are nucleated cells capable of active movement in the tissues of the body. The classification is based on taking into account the structural features of their cytoplasm.
Leukocytes, the cytoplasm of which contains specific granularity, are called granular, or granulocytes. Mature granular leukocytes have a nucleus divided into segments - segmented cells; in young ones it is unsegmented. Therefore, it is customary to divide them into young forms (bean-shaped nucleus), rod (nucleus in the form of a curved rod) and segmented - fully differentiated leukocytes, the nucleus of which contains from 2 to 5-7 segments. In accordance with the difference in the staining of cytoplasmic granules in the group of granulocytes, 3 types of cells are distinguished:
Basophils - granularity is colored violet with basic dyes;
Eosinophils - granularity is stained with acidic dyes in various shades of red;
Neutrophils - granularity is stained with both acidic and basic dyes in a pink-violet color.
Neutrophils are small cells (9-12 microns), the cytoplasm of which contains 2 types of granules: primary (basophilic), which are lysosomes, and secondary oxyphilic (containing cationic proteins and alkaline phosphatase). Neutrophils are characterized by the finest (pulverized) granularity and the most segmented nucleus. They are microphages and carry out the phagocytic function of small foreign particles of any nature and the utilization of antigen-antibody complexes. In addition, substances are released that stimulate the regeneration of damaged tissues.
Eosinophils often contain a two-segment nucleus and large oxyphilic granules in the cytoplasm. Their diameter is 12-18 microns. The granules contain hydrolytic enzymes (microphages in function). They exhibit antihistamine reactivity, stimulate the phagocytic activity of connective tissue macrophages and the formation of lysosomes in them, and utilize antigen-antibody complexes. But their main task is to neutralize toxic substances, so the number of eosinophils increases sharply during helminthic infestations.
Basophils, 12-16 microns in size, contain medium-sized basophil granules, which contain heparin (prevents blood clotting) and histamine (regulates vascular and tissue permeability). They also participate in the development of allergic reactions.
The percentage ratio between individual types of leukocytes is called the leukocyte formula, or leukogram. For granulocytes it looks like this:
Neutrophils - 25-40% - in pigs and ruminants; 50-70% - in horses and carnivores;
Eosinophils - 2-4%, in ruminants - 6-8%;
Basophils - 0.1-2%.
17. Structure and functional significance of agranulocytes
Non-granular leukocytes (agranulocytes) are characterized by the absence of specific granularity in the cytoplasm and large non-segmented nuclei. In the group of agranulocytes, there are 2 types of cells: lymphocytes and monocytes.
Lymphocytes are characterized by a predominantly round nuclear shape with compact chromatin. In small lymphocytes, the nucleus occupies almost the entire cell (its diameter is 4.5-6 microns), in medium-sized lymphocytes the rim of cytoplasm is wider, and their diameter increases to 7-10 microns. Large lymphocytes (10-13 μm) are extremely rare in peripheral blood. The cytoplasm of lymphocytes is stained basophilically, in various shades of blue.
Lymphocytes ensure the formation of cellular and humoral immunity. They are classified into T and B lymphocytes.
T-lymphocytes (thymus-dependent) undergo primary antigen-independent differentiation in the thymus. In the peripheral organs of the immune system, after contact with antigens, they turn into blast forms, multiply and now undergo secondary antigen-dependent differentiation, as a result of which effector types of T cells appear:
T-killers that destroy foreign cells and their own with defective phenocopies (cellular immunity);
T-helpers - stimulating the transformation of B-lymphocytes into plasma cells;
T-suppressors that suppress the activity of B-lymphocytes;
Memory T lymphocytes (long-lived cells) that retain information about antigens.
B lymphocytes (bursodependent). In birds, they are primarily differentiated in the bursa of Fabricius, in mammals - in the red bone marrow. During secondary differentiation, they turn into plasma cells, which produce large quantities of antibodies that enter the blood and other biological fluids of the body, which ensures the neutralization of antigens and the formation of humoral immunity.
Monocytes are the largest blood cells (18-25 microns). The kernel is sometimes bean-shaped, but more often irregular. The cytoplasm is significantly expressed, its share can reach up to half the volume of the cell, and is stained basophilic - smoky blue. Lysosomes are well developed in it. Monocytes circulating in the blood are the precursors of tissue and organ macrophages, forming a protective macrophage system in the body - the mononuclear phagocyte system (MPS). After a short stay in the vascular blood (12-36 hours), monocytes migrate through the endothelium of capillaries and venules into tissues and turn into fixed and free macrophages.
Macrophages, first of all, utilize dying and damaged cellular and tissue elements. But they play a more important role in immune reactions:
They convert antigens into molecular form and present them to lymphocytes (antigen-presenting function).
Produce cytokines to stimulate T and B cells.
They utilize complexes of antigens and antibodies.
Percentage of agranulocytes in the leukogram:
Monocytes - 1-8%;
Lymphocytes - 20-40% in carnivorous animals and horses, 45-56% in pigs, 45-65% in cattle.
18. Morphofunctional characteristics of loose connective tissue
Loose connective tissue is present in all organs and tissues, forming the basis for the placement of epithelium and glands, connecting the functional structures of organs into a single system. Accompanies blood vessels and nerves. Performs form-building, supporting, protective and trophic functions. Tissue consists of cells and intercellular substance. This is a polydifferent fabric, because... its cells came from different stem cells.
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Lugansk National Agrarian University
Cytology, embryology, general histology
(lecture course)
Lugansk - 2005
Cytology, embryology, general histology
The course of lectures was compiled by the head of the Department of Animal Biology, Doctor of Biological Sciences, Professor G.D. Katsy.
2nd edition, revised and expanded.
Lectures prepared for students of the zoobiotechnological and veterinary medicine faculty of Lugansk National Agrarian University. I sincerely thank the graduate student of the Department of Animal Biology Krytsya Ya.P. and head of the laboratory Esaulenko V.P. for assistance in preparing the material for publication.
Introduction to Histology
1. The subject of histology and its place in the system of biological and veterinary sciences.
2. History and methods of microscopic research.
3. Cell theory, basic principles.
1. The specificity of agricultural production is due to the fact that despite the increasing role technical factors: the main tools and means of production remain biological objects. In terms of the scope of objects of study and in its depth, veterinary medicine represents, as Academician K.I. Scriabin said, the most interesting area of human knowledge: in which so many representatives of the animal kingdom are studied and protected.
Cytology, histology and embryology, along with physiology, biochemistry and other sciences, form the foundation of modern veterinary medicine.
Histology (Greek histos-tissue, logos-study) is the science of the development, structure and vital activity of tissues of animal organisms. Modern histology studies the structures of the body of animals and humans in connection with the processes occurring in them, reveals the relationship between function and structure, etc.
Histology is divided into 3 main sections: cytology, or the study of the cell; embryology, or the study of the embryo and general and particular histology, or the study of tissues, the microscopic structure of organs, their cellular and tissue composition.
Histology is closely related to a number of biological and veterinary sciences - general and comparative anatomy, physiology, pathological physiology and pathological anatomy, as well as some clinical disciplines (internal medicine, obstetrics and gynecology, etc.).
Future doctors need a good knowledge of the structure of cells and tissues of organs, which are the structural basis of all types of vital activity of the body. The importance of histology, cytology and embryology for doctors is also increasing because modern veterinary medicine is characterized by the widespread use of cytological and histological methods when conducting blood tests, bone marrow, organ biopsies, etc.
2. The concept of tissue was first introduced into biology by the brilliant young French scientist anatomist and physiologist Xavier Bichat (Bichat, 1771-1802), who was so impressed by the varied texture of the various layers and structures he discovered during anatomical studies that he wrote a book about tissues of the body, giving names to more than 20 of their species.
The term “histology” does not belong to Bichat, although he can be considered the first histologist. The term “histology” was proposed by the German researcher Meyer 17 years after Bisha’s death.
Tissue is a phylogenetically determined elementary system, united by a common structure, function and development (A.A. Zavarzin).
Advances in histology from its inception to the present day are primarily associated with the development of technology, optics and microscopy methods. The history of histology can be divided into three periods: 1st - domicroscopic (duration about 2000 years), 2nd - microscopic (about 300 years), 3rd - electron microscopic (about 40 years).
In modern histology, cytology and embryology, various research methods are used to comprehensively study the processes of development, structure and function of cells, tissues and organs.
The objects of research are living and dead (fixed) cells and tissues, their images obtained in light and electron microscopes or on a television screen. There are a number of methods that allow you to analyze these objects:
1) methods for studying living cells and tissues: a) intravital study of cells in the body (in vivo) - using methods of implanting transparent chambers into the body of animals, by transplantation;
b) study of living structures in cell and tissue culture (in vitro) - disadvantages: the relationship with other cells and tissues, the effect of a complex of neurohumoral regulatory factors, etc. are lost;
c) vital and supravital staining, that is, intravital staining and staining of living cells isolated from the body.
2) study of dead cells and tissues; The main object of study here is histological preparations prepared from fixed structures.
The process of making a histological specimen for light and electron microscopy includes the following main steps: 1) taking material and fixing it, 2) compacting the material, 3) preparing sections, 4) staining or color contrasting. For light microscopy, one more step is necessary - enclosing sections in balm or other transparent media (5).
3) study of the chemical composition and metabolism of cells and tissues:
Cyto- and histochemical methods,
Autoradiography method, which is based on the use of radioactive elements (for example, phosphorus-32P, carbon -14C, sulfur-35S, hydrogen-3H) or compounds labeled with it.
Differential centrifugation method - the method is based on the use of centrifuges producing from 20 to 150 thousand revolutions per minute. This separates and precipitates the various components of the cells and determines their chemical composition. - interferometry - the method allows you to estimate the dry mass and concentration of dense substances in living and fixed cells. - quantitative histochemical methods - cytospectrophotometry - a method for the quantitative study of intracellular substances by their absorption properties. Cytospectrofluorimetry is a method for studying intracellular substances using their fluorescence spectra.
4) methods of immunofluorescence analysis. They are used to study the processes of cell differentiation and identify specific chemical compounds and structures in them. They are based on antigen-antibody reactions.
Methods of microscopy of histological preparations:
Light microscopy: a) ultraviolet, b) fluorescent (luminescent).
Electron microscopy: a) transmission, b) scanning (reading). The first gives only a planar image, the second - a spatial one; The main advantage of the latter (raster) is the large depth of field (100-1000 times greater than that of light microscopes), a wide range of continuous changes in magnification (from tens to tens of thousands of times) and high resolution.
3. The body of higher animals consists of microscopic elements - cells and a number of their derivatives - fibers, amorphous matter.
The significance of a cell in a multicellular organism is determined by the fact that hereditary information is transmitted through it, and the development of multicellular animals begins with it; Thanks to the activity of cells, non-cellular structures and ground substance are formed, which, together with cells, form tissues and organs that perform specific functions in a complex organism. Dutrochet (1824, 1837) and Schwann (1839) should be considered the creator of the cell theory.
Dutrochet (1776-1847) - zoologist, botanist, morphologist, physiologist. In 1824, he published his book “Anatomical and physiological studies on the fine structure of animals and plants, as well as on their mobility.”
The creation of the cell theory was preceded by the following discoveries. In 1610, 46-year-old prof. Mathematics of the University of Padua G. Galileo designed a microscope. In 1665, Robert Hooke discovered the cell at 100x magnification. His contemporary, Felice Fontana, said: “...Everyone can look through a microscope, but only a few can judge what they see.” Hooke’s “micrography” included 54 observations, including “Observation 18. About the schematism or structure of a cork or about cells and pores in some other loose bodies.”
From the history. A company of young people (students) living in London in 1645 began to meet every day after classes to discuss the problems of experimental philosophy. Among them were Robert Boyle (18 years old), R. Hooke (17 years old), Ren (23 years old) and others. This is how the British Academy was born, then the Royal Society of London (Charles II was its honorary member).
The animal cell was discovered by Anton van Leeuwenhoek (1673-1695). He lived in Delft and traded in cloth. He brought his microscopes up to 275 x. Peter I was shown the blood circulation in the tail of an eel larva.
Currently, the cellular theory states: 1) a cell is the smallest unit of living things, 2) cells of different organisms are similar in structure, 3) cell reproduction occurs by dividing the original cell, 4) multicellular organisms are complex ensembles of cells and their derivatives, united in holistic integrated systems of tissues and organs, subordinate and interconnected by intercellular, humoral and nervous forms of regulation.
Cell is the elementary unit of living things
1. Composition and physicochemical properties of living matter.
2. Cell types. Theories of the origin of the eukaryotic cell.
3. Cell membranes, their molecular composition and functions.
1. A typical cell with a nucleus, cytoplasm and all the organelles contained in it cannot yet be considered the smallest unit of living matter, or protoplasm (Greek “protos” - first, “plasma” - formation). There are also more primitive or more simply organized units of life - the so-called prokaryotic organisms (Greek “karyon” - nucleus), which include most viruses, bacteria and some algae; In contrast to higher type cells with a true nucleus (eukaryotic cells), they lack a nuclear envelope and the nuclear substance is mixed or directly in contact with the rest of the protoplasm.
The composition of living matter includes proteins, nucleic acids (DNA and RNA), polysaccharides and lipids. The chemical components of a cell can be divided into inorganic (water and mineral salts) and organic (proteins, carbohydrates, nucleic acids, lipids, etc.).
The cytoplasm of a plant and animal cell contains 75-85% water, 10-20% protein, 2-3% lipids, 1% carbohydrates and 1% inorganic substances.
DNA is a molecule (0.4% of it) that contains genetic information that directs the synthesis of specific cellular proteins. For one DNA molecule there are about 44 RNA molecules, 700 protein molecules and 7000 lipid molecules.
The primary structure of RNA is similar to that of DNA, except that RNA contains ribose and uracil instead of thymine. It has now been established that there are three types of RNA, differing in molecular weight and other properties: ribosomal, messenger and transport. These three types of RNA are synthesized in the nucleus and are involved in protein synthesis.
2. Shatton (1925) divided all living organisms into two types (klister) - prokaryotes and eukaryotes. They diverged in the Precambrian (600-4500 million years ago). There are two concepts of the origin of the eukaryotic cell: exogenous (symbiotic) and endogenous. The first is based on the recognition of the principle of association of different prokaryotic organisms with each other. The endogenous concept is based on the principle of direct filiation, i.e. consistent evolutionary transformation of prokaryotic organisms into eukaryotic ones.
In the mammalian body, histologists count about 150 types of cells, and most of them are adapted to perform one specific task. The shape and structure of a cell depend on the function it performs.
Cell functions: irritability, contractility, secretion, respiration, conduction, absorption and assimilation, excretion, growth and reproduction.
3. Any cell is delimited by a plasma membrane. It is so thin that it cannot be seen under a light microscope. The plasma membrane, slightly damaged by a microneedle, is capable of recovery, but with more severe damage, especially in the absence of calcium ions, the cytoplasm flows out through the puncture and the cell dies.
According to modern theory, the plasma membrane consists of a bilayer of polar lipids and globular protein molecules embedded in it. Thanks to these layers, the membrane has elasticity and relative mechanical strength. The plasma membrane of most cell types consists of three layers, each approximately 2.5 nm wide. A similar structure, called the “elementary membrane,” is found in most intracellular membranes. Biochemical analysis showed that lipids and proteins are contained in them in a ratio of 1.0: 1.7. The protein component, called stromatin, is an acidic fibrillar protein with a high molecular weight. The bulk of the lipid components are formed by phospholipids, mainly lecithin and cephalin.
Plasmolemma is a cell membrane that performs delimiting, transport and receptor functions. It provides mechanical communication between cells and intercellular interactions, contains cellular receptors for hormones and other signals from the environment surrounding the cell, transports substances into the cell from the cell both along a concentration gradient - passive transfer, and with energy expenditure against the concentration gradient - active transfer.
The membrane consists of a plasma membrane, a non-membrane complex - the glycocalex, and a submembrane musculoskeletal apparatus.
The glycocalex contains about 1% carbohydrates, the molecules of which form long branching chains of polysaccharides associated with membrane proteins. Enzyme proteins located in the glycocalex are involved in the final extracellular breakdown of substances. The products of these reactions enter the cell in the form of monomers. During active transport, the transport of substances into the cell is carried out either by the entry of molecules in the form of a solution - pinocytosis, or by the capture of large particles - phagocytosis.
In accordance with the functional and morphological characteristics of tissues, the cell membrane forms their characteristic apparatus for intercellular contacts. Their main forms are: simple contact (or adhesion zone), tight (closing) and gap contact. Desmosomes are a type of tight junction.
Biological membranes act as diffusion barriers. Due to their selective permeability to K+, Na+, Cl-, etc. ions, as well as high-molecular compounds, they delimit intra- and intercellular reaction zones and create electrical gradients and concentration gradients of substances. This makes it possible for the existence of ordered biological structures with specific functions.
The penetration of substances into the cell is called endocytosis. But exocytosis also exists. For example, secretory vesicles are detached from the Golgi apparatus, migrating towards the cell membrane and throwing their contents out. In this case, the membrane of the vesicle merges with its homologous cell membrane.
Based on electron microscopic data, it can be assumed that the plasmalemma is a product of the Golgi apparatus. From this organelle, in the form of continuously separating vesicles, membrane material is constantly transported (“membrane flow”), restoring the used areas of the plasmalemma and ensuring its growth after cell division.
The membrane is the carrier of species-specific and cell-specific surface properties associated with the characteristic distribution of glycosaminoglycans and proteins on it. Their molecules can also cover the surface of cells in the form of thin films and form an intercellular matrix between neighboring cells. Cell contact properties and immune responses are determined by these membrane components.
Many cells, especially those specialized for absorption (intestinal epithelium), have hair-like outgrowths on the outside - microvilli. The formed or “brush border” carries enzymes and takes part in the breakdown of substances and transport processes. On the basal side of cells specialized for intense fluid transmission (during osmoregulation), for example, in the epithelium of the renal tubules and Malpighian vessels, the membrane forms multiple invaginations that make up the basal labyrinth. The product of cellular secretion, the basement membrane, often delimits the epithelium from the deeper cellular layers.
Special membrane structures arise at the points of contact between neighboring cells. There are areas where the membranes are so closely adjacent to each other that there is no room for intercellular substance (tight junction). In other areas, complex contact organelles - desmosomes - appear. They and other contact structures serve for mechanical connection and, most importantly, provide chemical and electrical integration of neighboring cells, facilitating intercellular ion transport due to their low electrical resistance.
The structure of an animal cell
1. Cytoplasm and organelles, their function.
2. The nucleus, its structure and functions.
3. Types of division, phases of the cell cycle.
1. Cytoplasm separated from environment plasmalemma, includes the hyaloplasm, the essential cellular components located in it - organelles, as well as various unstable structures - inclusions (Fig. 1).
Hyaloplasm (hyalinos - transparent) - the main plasma, or the matrix of the cytoplasm, is a very important part of the cell, its true internal environment.
In an electron microscope, the matrix appears as a homogeneous and fine-grained substance with low electron density. Hyaloplasm is a complex colloidal system that includes various biopolymers: proteins, nucleic acids, polysaccharides, etc. This system is capable of transitioning from a sol-like (liquid) state to a gel-like state and back. The hyaloplasm consists mainly of various globular proteins. They make up 20-25% of the total protein content in a eukaryotic cell. The most important enzymes of hyaloplasm include enzymes for the metabolism of sugars, nitrogenous bases, amino acids, lipids and other important compounds. The hyaloplasm contains enzymes for activating amino acids during protein synthesis and transfer RNAs (tRNAs). In the hyaloplasm, with the participation of ribosomes and polyribosomes, the synthesis of proteins necessary for the actual cellular needs, to maintain and ensure the life of a given cell occurs.
Organelles are microstructures that are constantly present and obligatory for all cells and perform vital functions.
There are membrane organelles - mitochondria, endoplasmic reticulum (granular and smooth), Golgi apparatus, lysosomes; the plasmalemma also belongs to the category of membrane organelles; non-membrane organelles: free ribosomes and polysomes, microtubules, centrioles and filaments (microfilaments). In many cells, organelles can take part in the formation of special structures characteristic of specialized cells. Thus, cilia and flagella are formed by centrioles and the plasma membrane, microvilli are outgrowths of the plasma membrane with hyaloplasm and microfilaments, the sperm acrosome is a derivative of elements of the Golgi apparatus, etc.
Figure 1. Ultramicroscopic structure of a cell in animal organisms (diagram)
1 – core; 2 – plasmalemma; 3 – microvilli; 4 – agranular endoplasmic reticulum; 5 - granular endoplasmic reticulum; 6 – Golgi apparatus; 7 – centriole and microtubules of the cell center; 8 – mitochondria; 9 – cytoplasmic vesicles; 10 – lysosomes; 11 – microfilaments; 12 – ribosomes; 13 – secretion of secretion granules.
Membrane organelles are single or interconnected compartments of the cytoplasm, delimited by a membrane from the surrounding hyaloplasm, having their own contents, different in composition, properties and functions:
Mitochondria are organelles for ATP synthesis. Their main function is related to oxidation organic compounds and using the energy released during the breakdown of these compounds for the synthesis of ATP molecules. Mitochondria are also called the energy stations of the cell, or the organelles of cellular respiration.
The term “mitochondrion” was coined by Benda in 1897. Mitochondria can be observed in living cells because... they have a fairly high density. In living cells, mitochondria can move, merge with each other, and divide. The shape and size of mitochondria in animal cells are varied, but on average their thickness is about 0.5 microns, and their length is from 1 to 10 microns. Their number in cells varies greatly - from single elements to hundreds. Thus, in a liver cell they make up more than 20% of the total cytoplasm. The surface area of all mitochondria in a liver cell is 4-5 times larger than the surface of its plasma membrane.
Mitochondria are bounded by two membranes about 7 nm thick. The outer mitochondrial membrane limits the actual internal contents of the mitochondrion, its matrix. Characteristic feature The inner membranes of mitochondria is their ability to form numerous invaginations into the mitochondria. Such invaginations often take the form of flat ridges, or cristae. The mitochondrial matrix strands are DNA molecules, and the small granules are mitochondrial ribosomes.
The endoplasmic reticulum was discovered by K.R. Porter in 1945. This organelle is a collection of vacuoles, flat membrane sacs or tubular formations that create a membrane network inside the cytoplasm. There are two types - granular and smooth endoplasmic reticulum.
The granular endoplasmic reticulum is represented by closed membranes, the distinctive feature of which is that they are covered with ribosomes on the hyaloplasmic side. Ribosomes are involved in the synthesis of proteins removed from a given cell. In addition, the granular endoplasmic reticulum takes part in the synthesis of enzyme proteins necessary for the organization of intracellular metabolism, and also used for intracellular digestion.
Proteins that accumulate in the cavities of the network can, bypassing the hyaloplasm, be transported to the vacuoles of the Golgi complex, where they are often modified and become part of either lysosomes or secretory granules.
The role of the granular endoplasmic reticulum is the synthesis of exported proteins on its polysomes, their isolation from the contents of the hyaloplasm inside the membrane cavities, the transport of these proteins to other parts of the cell, as well as the synthesis of the structural components of cell membranes.
The agranular (smooth) endoplasmic reticulum is also represented by membranes that form small vacuoles and tubes, tubules, which can branch with each other. Unlike the granular endoplasmic reticulum, there are no ribosomes on the membranes of the smooth endoplasmic reticulum. The diameter of vacuoles and tubules is usually about 50-100 nm.
The smooth endoplasmic reticulum arises and develops at the expense of the granular endoplasmic reticulum.
The activity of smooth ER is associated with the metabolism of lipids and some intracellular polysaccharides. Smooth ER is involved in the final stages of lipid synthesis. It is highly developed in cells secreting steroids in the adrenal cortex and sustentocytes (Sertoli cells) of the testes.
In striated muscle fibers, the smooth ER is capable of depositing calcium ions necessary for the function of muscle tissue.
The role of smooth ER is very important in the deactivation of various substances harmful to the body.
Golgi complex (CG). In 1898, C. Golgi, using the properties of binding heavy metals to cellular structures, identified mesh formations in nerve cells, which he called the internal mesh apparatus.
It is represented by membrane structures gathered together in a small area. A separate zone of accumulation of these membranes is called a dictyosome. There may be several such zones in a cell. In the dictyosome, 5-10 flat cisterns are located close to each other (at a distance of 20-25 nm), between which there are thin layers of hyaloplasm. In addition to the cisterns, many small bubbles (vesicles) are observed in the CG zone. KG is involved in the segregation and accumulation of products synthesized in the cytoplasmic reticulum, in their chemical rearrangements and maturation; in the CG tanks, the synthesis of polysaccharides occurs, their complexation with proteins and, most importantly, the removal of ready-made secretions outside the cell.
Lysosomes are a diverse class of 0.2-0.4 µm spherical structures bounded by a single membrane.
A characteristic feature of lysosomes is the presence in them of hydrolytic enzymes that break down various biopolymers. Lysosomes were discovered in 1949 by de Duve.
Peroxisomes are small oval-shaped bodies, 0.3-1.5 microns in size, bounded by a membrane. They are especially characteristic of liver and kidney cells. Enzymes that oxidize amino acids form hydrogen peroxide, which is destroyed by the enzyme catalase. Peroxisomal catalase plays an important protective role, since H2O2 is a toxic substance for the cell.
Non-membrane organelles
Ribosomes - the elementary apparatus for the synthesis of protein and polypeptide molecules - are found in all cells. Ribosomes are complex ribonucleoproteins that contain proteins and RNA molecules. The size of a functioning ribosome in eukaryotic cells is 25 x 20 x 20 nm.
There are single ribosomes and complex ribosomes (polysomes). Ribosomes can be located freely in the hyaloplasm and be associated with the membranes of the endoplasmic reticulum. Free ribosomes form proteins mainly for the cell’s own needs; bound ribosomes provide the synthesis of proteins “for export.”
Microtubules belong to the fibrillar components of a protein nature. In the cytoplasm they can form temporary formations (division spindle). Microtubules are part of centrioles and are also the main structural elements cilia and flagella. They are straight, unbranched long hollow cylinders. Their outer diameter is about 24 nm, the inner lumen is 15 nm, and the mesh thickness is 5 nm. Microtubules contain proteins called tubulins. By creating an intracellular skeleton, microtubules can be factors in the oriented movement of the cell as a whole and its intracellular components, creating factors for directed flows of various substances.
Centrioles. The term was proposed by T. Boveri in 1895 to refer to very small bodies. Centrioles are usually located in a pair - a diplosome, surrounded by a zone of lighter cytoplasm, from which radially thin fibrils extend (centrosphere). The collection of centrioles and centrosphere is called the cell center. These organelles in dividing cells take part in the formation of the division spindle and are located at its poles. In non-dividing cells they are located near the CG.
The structure of centrioles is based on 9 triplets of microtubules arranged around a circle, thus forming a hollow cylinder. Its width is about 0.2 microns and its length is 0.3-0.5 microns.
In addition to microtubules, the centriole includes additional structures - “handles” that connect triplets. The microtubule system of the centriole can be described by the formula: (9 x 3) + 0, emphasizing the absence of microtubules in its central part.
When cells prepare for mitotic division, centrioles double.
It is believed that centrioles are involved in the induction of polymerization by tubulin during the formation of microtubules. Before mitosis, the centriole is one of the centers of polymerization of microtubules of the cell division spindle.
Cilia and flagella. These are special movement organelles. At the base of the cilia and flagellum, small granules are visible in the cytoplasm - basal bodies. The length of cilia is 5-10 microns, flagella - up to 150 microns.
The cilium is a thin cylindrical outgrowth of the cytoplasm with a diameter of 200 nm. It is covered by a plasma membrane. Inside there is an axoneme (“axial filament”), consisting of microtubules.
The axoneme contains 9 doublets of microtubules. Here the microtubule system of the cilia is composed of (9 x 2) + 2.
Free cells with cilia and flagella have the ability to move. The method of their movement is “sliding threads”.
The fibrillar components of the cytoplasm include microfilaments with a thickness of 5-7 nm and so-called intermediate filaments, microfibrils, with a thickness of about 10 nm.
Microfilaments are found in all types of cells. They are different in structure and function, but it is difficult to distinguish them morphologically from each other. Their chemical composition is different. They can perform cytoskeletal functions and participate in movement within the cell.
Intermediate filaments are also protein structures. In epithelium they contain keratin. Bundles of filaments form tonofibrils, which approach the desmosomes. The role of intermediate microfilaments is most likely scaffolding.
Cytoplasmic inclusions. These are optional components of the cell that appear and disappear depending on the metabolic state of the cells. There are trophic, secretory, excretory and pigment inclusions. Trophic inclusions are neutral fats and glycogen. Pigment inclusions can be exogenous (carotene, dyes, dust particles, etc.) and endogenous (hemoglobin, melanin, etc.). Their presence in the cytoplasm can change the color of the tissue. Often tissue pigmentation serves as a diagnostic sign.
The core provides two groups of general functions: one associated with the storage and transmission of genetic information itself, the other with its implementation, ensuring protein synthesis.
In the nucleus, reproduction or reduplication of DNA molecules occurs, which makes it possible, during mitosis, for two daughter cells to receive exactly the same volumes of genetic information in qualitative and quantitative terms.
Another group of cellular processes provided by the activity of the nucleus is the creation of its own protein synthesis apparatus. This is not only the synthesis and transcription of various messenger RNAs on DNA molecules, but also the transcription of all types of transport and ribosomal RNAs.
Thus, the nucleus is not only the repository of genetic material, but also the place where this material functions and reproduces.
A nondividing, interphase cell usually has one nucleus per cell. The nucleus consists of chromatin, a nucleolus, karyoplasm (nucleoplasm) and a nuclear membrane that separates it from the cytoplasm (karyolemma).
Karyoplasm or nuclear juice is a microscopically structureless substance of the nucleus. It contains various proteins (nucleoproteins, glycoproteins), enzymes and compounds involved in synthesis processes nucleic acids, proteins and other substances that make up the karyoplasm. Electron microscopy reveals ribonucleoprotein granules 15 nm in diameter in the nuclear sap.
Glycolytic enzymes involved in the synthesis and breakdown of free nucleotides and their components, as well as enzymes of protein and amino acid metabolism, have also been identified in the nuclear sap. The complex life processes of the nucleus are provided by energy released during the process of glycolysis, the enzymes of which are contained in the nuclear juice.
Chromatin. Chromatin consists of DNA in complex with protein. Chromosomes, which are clearly visible during mitotic cell division, also have the same properties. The chromatin of interphase nuclei consists of chromosomes that at this time lose their compact shape, loosen, and decondense. Zones of complete decondensation are called euchromatin; incomplete loosening of chromosomes - heterochromatin. Chromatin is condensed to its maximum during mitotic cell division, when it is found in the form of dense chromosomes.
Nucleolus. This is one or more rounded bodies 1-5 microns in size that strongly refract light. It is also called nucleola. The nucleolus, the densest structure of the nucleus, is a derivative of the chromosome.
It is now known that the nucleolus is the site of formation of ribosomal RNA and polypeptide chains in the cytoplasm.
The nucleolus is heterogeneous in its structure: in a light microscope you can see its fine-fibrous organization. In an electron microscope, two main components are distinguished: granular and fibrillar. The fibrillar component is the ribonucleoprotein strands of ribosome precursors, the granules are the maturing subunits of ribosomes.
The nuclear envelope consists of an outer nuclear membrane and an inner envelope membrane, separated by a perinuclear space. The nuclear envelope contains nuclear pores. The nuclear membrane membranes are morphologically no different from other intracellular membranes.
The pores have a diameter of about 80-90 nm. There is a diaphragm across the pore. The pore sizes of a given cell are usually stable. The number of pores depends on the metabolic activity of cells: the more intense the synthetic processes in cells, the more pores per unit surface of the cell nucleus.
Chromosomes. Both interphase and mitotic chromosomes consist of elementary chromosomal fibrils - DNA molecules.
The morphology of mitotic chromosomes is best studied at the moment of their greatest condensation, in metaphase and at the beginning of anaphase. Chromosomes in this state are rod-shaped structures of varying lengths and fairly constant thickness. For most chromosomes, it is easy to find the zone of primary constriction (centromere), which divides the chromosome into two arms. Chromosomes with equal or almost equal arms are called metacentric, and those with arms of unequal length are called submetacentric. Rod-shaped chromosomes with a very short, almost imperceptible second arm are called acrocentric. The kinetochore is located in the region of the primary constriction. Microtubules of the cell spindle extend from this zone during mitosis. Some chromosomes also have secondary constrictions, located near one of the ends of the chromosome and separating a small area - a satellite of the chromosomes. DNA responsible for the synthesis of ribosomal RNA is localized in these places.
The totality of the number, size and structural features of chromosomes is called the karyotype of a given species. Karyotype of cattle - 60, horses - 66, pigs - 40, sheep - 54, humans - 46.
The time of existence of a cell as such, from division to division or from division to death is called the cell cycle (Fig. 2).
The entire cell cycle consists of 4 periods of time: mitosis itself, pre-synthetic, synthetic and postsynthetic periods of interphase. During the G1 period, cell growth begins due to the accumulation of cellular proteins, which is determined by an increase in the amount of RNA per cell. In the S period, the amount of DNA per nucleus doubles and the number of chromosomes accordingly doubles. Here, the level of RNA synthesis increases according to the increase in the amount of DNA, reaching its maximum in the G2 period. In the G2 period, the synthesis of messenger RNA necessary for the passage of mitosis occurs. Among the proteins synthesized at this time, tubulins, the proteins of the mitotic spindle, occupy a special place.
Rice. 2. Cell life cycle:
M – mitosis; G1 - pre-synthetic period; S - synthetic period; G2 - post-synthetic period; 1 - old cell (2n4c); 2- young cells (2n2c)
The continuity of the chromosome set is ensured by cell division, which is called mitosis. During this process, a complete restructuring of the core occurs. Mitosis consists of a sequential series of stages, changing in a certain order: prophase, metaphase, anaphase and telophase. During the process of mitosis, the nucleus of a somatic cell divides in such a way that each of the two daughter cells receives exactly the same set of chromosomes as the mother had.
The ability of cells to reproduce is the most important property of living matter. Thanks to this ability, continuous continuity of cellular generations is ensured, the preservation of cellular organization in the evolution of living things, growth and regeneration occur.
For various reasons (spindle disruption, chromatid nondisjunction, etc.), cells with large nuclei or multinucleated cells are found in many organs and tissues. This is the result of somatic polyploidy. This phenomenon is called endoreproduction. Polyploidy is more common in invertebrate animals. In some of them, the phenomenon of polyteny is also common - the construction of a chromosome from many DNA molecules.
Polyploid and polytene cells do not enter into mitosis and can only divide by amitosis. The meaning of this phenomenon is that both polyploidy - an increase in the number of chromosomes, and polyteny - an increase in the number of DNA molecules in a chromosome lead to a significant increase in the functional activity of the cell.
In addition to mitosis, science knows two more types of division - amitosis (a - without, mitosis - threads) or direct division and meiosis, which is the process of reducing the number of chromosomes by half through two cell divisions - the first and second division of meiosis (meiosis - reduction). Meiosis is characteristic of germ cells.
Gametogenesis, stages of early embryogenesis
1. The structure of vertebrate germ cells.
2. Spermatogenesis and oogenesis.
3. Stages of early embryogenesis.
1. Embryology is the science of embryo development. It studies the individual development of animals from the moment of conception (fertilization of the egg) until its hatching or birth. Embryology examines the development and structure of germ cells and the main stages of embryogenesis: fertilization, fragmentation, gastrulation, laying of axial organs and organogenesis, development of provisional (temporary) organs.
The achievements of modern embryology are widely used in animal husbandry, poultry farming, and fish breeding; in veterinary medicine and medicine when solving many practical problems relating to artificial insemination and fertilization, technology of accelerated reproduction and selection; increasing the fertility of farm animals, breeding animals through embryo transplantation, when studying the pathology of pregnancy, when recognizing the causes of infertility and other issues of obstetrics.
The structure of germ cells is similar to somatic cells. They also consist of a nucleus and cytoplasm, built from organelles and inclusions.
The distinctive properties of mature gametocytes are a low level of assimilation and dissimilation processes, inability to divide, and the content of a haploid (half) number of chromosomes in the nuclei.
Male germ cells (sperm) in all vertebrates have a flagellar shape (Fig. 3). They are formed in the testes in large quantities. One portion of semen (ejaculate) contains tens of millions and even billions of sperm.
Sperm of agricultural animals have mobility. Both the size and shape of sperm vary greatly between animals. They consist of a head, neck and tail. Sperm are heterogeneous because their nuclei contain different types of sex chromosomes. Half of the sperm have an X chromosome, the other half have a Y chromosome. Sex chromosomes carry genetic information that determines the sexual characteristics of a male. They differ from other chromosomes (autosomes) in their higher heterochromatin content, size and structure.
Sperm have a minimal supply of nutrients, which are consumed very quickly during cell movement. If the sperm does not merge with the egg, it usually dies in the female genital tract within 24-36 hours.
You can extend the life of sperm by freezing it. Quinine, alcohol, nicotine and other drugs have a detrimental effect on sperm.
The structure of eggs. The size of the egg is much larger than the sperm. The diameter of oocytes varies from 100 microns to several mm. Vertebrate eggs are oval-shaped, immobile, and consist of a nucleus and cytoplasm (Fig. 4). The nucleus contains a haploid set of chromosomes. Mammalian eggs are classified as homogametic, because their nucleus contains only the X chromosome. The cytoplasm contains free ribosomes, endoplasmic reticulum, Golgi complex, mitochondria, yolk and other components. Oocytes have polarity. In this connection, they distinguish two poles: apical and basal. The peripheral layer of the cytoplasm of the egg is called the cortical layer (cortex - cortex). It is completely devoid of yolk and contains many mitochondria.
The eggs are covered with membranes. There are primary, secondary and tertiary shells. The primary shell is the plasmalemma. The secondary membrane (transparent or shiny) is a derivative of the follicular cells of the ovary. Tertiary membranes are formed in the oviduct of birds: the albumen, subshell and shell membranes of the egg. Based on the amount of yolk, eggs with a small amount are distinguished - oligolecithal (oligos - few, lecytos - yolk), with an average amount - mesolecithal (mesos - medium) and with a large amount - polylecithal (poli - many).
Based on the location of the yolk in the cytoplasm, eggs with a uniform distribution of the yolk are distinguished - isolecithal, or homolecithal, and with the yolk localized at one pole - telolecithal (telos - edge, end). Oligolecithal and isolecithal ovules - in lancelets and mammals, mesolecithal and telolecithal - in amphibians, some fish, polylecithal and telolecithal - in many fish, reptiles, and birds.
2. The ancestors of germ cells are the primary germ cells - gametoblasts (gonoblasts). They are detected in the wall of the yolk sac near the blood vessels. Gonoblasts intensively divide by mitosis and migrate with the bloodstream or along the blood vessels to the rudiments of the gonads, where they are surrounded by supporting (follicular) cells. The latter perform a trophic function. Then, in connection with the development of the sex of the animal, the germ cells acquire properties characteristic of sperm and eggs.
The development of sperm (spermatogenesis) occurs in the testes of a sexually mature animal. There are 4 periods in spermatogenesis: reproduction, growth, maturation and formation.
Breeding period. The cells are called spermatogonia. They are small in size and have a diploid number of chromosomes. Cells rapidly divide by mitosis. Dividing cells are stem cells and replenish the supply of spermatogonia.
Growth period. The cells are called primary spermatocytes. They maintain a diploid number of chromosomes. The size of the cell increases and complex changes occur in the redistribution of hereditary material in the nucleus, and therefore four stages are distinguished: leptotene, zygotene, pachytene, diplotene
Maturation period. This is the process of developing spermatids with half the number of chromosomes.
During the maturation process, each primary spermatocyte produces 4 spermatids with a single number of chromosomes. Mitochondria, the Golgi complex, and the centrosome are well developed in them and are located near the nucleus. Other organelles and inclusions are almost absent. Spermatids are unable to divide.
Formation period. The spermatid acquires morphological properties characteristic of sperm. The Golgi complex is transformed into an acrosome, which encloses the spermatid nucleus in the form of a cap. Acrosome is rich in the enzyme hyaluronidase. The centrosome moves to the pole opposite to the nucleus, in which proximal and distal centrioles are distinguished. The proximal centriole remains in the neck of the sperm, and the distal centriole goes to build the tail.
The development of eggs, oogenesis, is a complex and very long process. It begins during the period of embryogenesis and ends in the organs of the reproductive system of a sexually mature female. Oogenesis consists of three periods: reproduction, growth, maturation.
The reproductive period occurs during fetal development and ends during the first months after birth. The cells are called oogonia and have a diploid number of chromosomes.
During the growth period, the cells are called primary oocytes. Changes in the nuclei are similar to primary spermatocytes. Then intensive synthesis and accumulation of yolk begins in the oocyte: the stage of previtellogenesis and the stage of vitellogenesis. The secondary membrane of the oocyte consists of a single layer of follicular cells. Previtellogenesis usually lasts until the female reaches sexual maturity. The maturation period consists of rapidly successive maturation divisions, during which a diploid cell becomes haploid. This process usually occurs in the oviduct after ovulation.
The first division of maturation ends with the formation of two unequal structures - the secondary oocyte and the first guiding or reduction body. During the second division, one mature egg and a second guide body are also formed. The first body also divides. Consequently, from one primary oocyte in the process of maturation, only one mature egg emerges and three guiding bodies, the latter soon die.
All eggs are genetically homogeneous, because they have only an X chromosome.
3. Fertilization - the fusion of sex gametes and the formation of a new single-celled organism (zygote). It differs from a mature egg in its doubled DNA mass and diploid number of chromosomes. Fertilization in mammals is internal, it occurs in the oviduct during its passive movement towards the uterus. The movement of sperm in the female genital tract is carried out due to the function of the movement apparatus of this cell (chemotaxis and rheotaxis), peristaltic contractions of the uterine wall, and the movement of cilia covering the inner surface of the oviduct. When the germ cells come together, the enzymes in the acrosome of the sperm head destroy the layer of follicular cells, the secondary shell of the egg. At the moment the sperm touches the plasmalemma of the egg, a protrusion of the cytoplasm forms on its surface - a fertilization tubercle. The head and neck penetrate the oocyte. In mammals, only one sperm is involved in fertilization - therefore the process is called monospermy: XY - male, XX - female.
Polyspermy is observed in birds and reptiles. In birds, all sperm have a Z chromosome, and eggs have a Z or W chromosome.
After the sperm penetrates the egg, a fertilization membrane is formed around the latter, which prevents the penetration of other sperm into the oocyte, the nuclei of germ cells are called: male pronucleus, female pronucleus. The process of their connection is called synkaryon. The centriole brought by the sperm divides and diverges, forming an achromatin spindle. Crushing begins. Crushing is a further process of development of a single-celled zygote, during which a multicellular blastula is formed, which consists of a wall - the blastoderm and a cavity - the blastocoel. During the process of mitotic division of the zygote, new cells are formed - blastomeres.
The nature of cleavage in chordates is different and is largely determined by the type of egg. Cleavage can be complete (holoblastic) or partial (meroblastic). In the first type, the entire material of the zygote takes part, in the second - only that zone of it that is devoid of yolk.
Complete crushing is classified into uniform and uneven. The first is typical for oligo isolecithal eggs (lancelet, roundworm, etc.). In a fertilized egg, two poles are distinguished: the upper - animal and the lower - vegetative. After fertilization, the yolk moves to the vegetative pole.
The fragmentation ends with the formation of a blastula, the shape of which resembles a ball filled with liquid. The wall of the ball is formed by blastoderm cells. Thus, with complete uniform fragmentation, the material of the entire zygote participates in fragmentation and after each division the number of cells doubles.
Complete uneven fragmentation is characteristic of mesolecithal (average amount of yolk) and telolecithal eggs. These are amphibians. Their type of blastula is coeloblastula.
Partial or meroblastic (discoidal) cleavage is common in fish, birds and is characteristic of polylecithal and telolecithal eggs (the type of blastula is called discoblastula).
Gastrulation. With the further development of the blastula, in the process of division, growth, differentiation of cells and their movements, first a two- and then a three-layer embryo is formed. Its layers are ectoderm, endoderm and mesoderm.
Types of gastrulation: 1) invagination, 2) epiboly (fouling), 3) immigration (invasion), 4) delamination (stratification).
Laying of the axial organs. From these germ layers the axial organs are formed: the rudiment of the nervous system (neural tube), notochord and intestinal tube.
During the development of mesoderm in all vertebrates, a notochord, segmented mesoderm, or somites (dorsal segments), and unsegmented mesoderm, or splanchnotome, are formed. The latter consists of two layers: the outer - parietal and the inner - visceral. The space between these layers is called the secondary body cavity.
There are three primordia in somites: dermatome, myotome, and sclerotome. Nephrogonadotom.
When the germ layers differentiate, embryonic tissue is formed - mesenchyme. It develops from cells that have moved mainly from the mesoderm and ectoderm. Mesenchyme is the source of the development of connective tissue, smooth muscles, blood vessels and other tissues of the animal’s body. The processes of crushing in various representatives of chordates are very unique and depend on the promorphology of the eggs, especially on the quantity and distribution of the yolk. Gastrulation processes also vary widely within Chordata.
Thus, gastrulation in the lancelet is typically invaginative; it begins with invagination of the presumptive endoderm. Following the endoderm, the notochord material invaginates into the blastocoel, and the mesoderm penetrates through the lateral and ventral lips of the blastopore. The anterior (or dorsal) lip of the blastopore consists of material from the future nervous system, and from the inside from cells of the future notochord. As soon as the endodermal layer comes into contact with the inner side of the ectodermal layer, processes begin that lead to the formation of the rudiments of the axial organs.
The process of gastrulation in bony fishes begins when the multilayered blastodisc covers only a small part of the egg yolk, and ends when the entire “yolk ball” is completely covered. This means that gastrulation also includes the expansion of the blastodisc.
The cellular material of all three germinal layers along the anterior and lateral edges of the blastodisc begins to grow onto the yolk. In this way, the so-called yolk sac is formed.
The yolk sac, as part of the embryo, performs numerous functions:
1) this is an organ with a trophic function, since the differentiating endodermal layer produces enzymes that help break down yolk substances, and in the differentiating mesodermal layer blood vessels are formed that are in connection with the vascular system of the embryo itself.
2) the yolk sac is a respiratory organ. Gas exchange between the embryo and the external environment occurs through the walls of the sac vessels and the ectodermal epithelium.
3) “blood mesenchyme” is the cellular basis of hematopoiesis. The yolk sac is the first hematopoietic organ of the embryo.
Frogs, newts and sea urchins are the main objects of experimental embryological research in the twentieth century.
Intussusception in amphibians cannot occur in the same way as in the lancelet, because the vegetative hemisphere of the egg is very overloaded with yolk.
The first noticeable sign of beginning gastrulation in frogs is the appearance of a blastopore, i.e., a depression or slit in the middle of the gray falx.
The behavior of the cellular material of the nervous system and skin epidermis is worthy of special attention. Eventually, the future epidermis and nervous system material covers the entire surface of the embryo. The presumptive epidermis of the skin moves and thins in all directions. The totality of cells of the presumptive nervous system moves almost exclusively in meridional directions. The layer of cells of the future nervous system contracts in the transverse direction, the presumptive area of the nervous system appears elongated in the animal-vegetative direction.
Let us summarize what we know about the fate of each of the germ layers.
Derivatives of ectoderm. From the cells that make up the outer layer, multiplying and differentiating, the following are formed: the outer epithelium, skin glands, the surface layer of teeth, horny scales, etc. By the way, almost always each organ develops from the cellular elements of two, or even all three germ layers . For example, mammalian skin develops from ectoderm and mesoderm.
A large part of the primary ectoderm “sinks” inward, under the outer epithelium, and gives rise to the entire nervous system.
Endoderm derivatives. The inner germ layer develops into the epithelium of the midgut and its digestive glands. The epithelium of the respiratory system develops from the foregut. But its origin involves the cellular material of the so-called prechordal plate.
Mesoderm derivatives. From it develop all muscle tissue, all types of connective, cartilaginous, bone tissue, canals of excretory organs, peritoneum of the body cavity, circulatory system, part of the tissues of the ovaries and testes.
In most animals, the middle layer appears not only in the form of a collection of cells forming a compact epithelial layer, i.e., the mesoderm itself, but in the form of a loose complex of scattered, amoeba-like cells. This part of the mesoderm is called mesenchyme. Actually, mesoderm and mesenchyme differ from each other in their origin, there is no direct connection between them, they are not homologous. Mesenchyme is mostly of ectodermal origin, while mesoderm begins with endoderm. In vertebrates, however, the mesenchyme has a common origin with the rest of the mesoderm.
In all animals that tend to have a coelom (secondary body cavity), the mesoderm gives rise to hollow coelomic sacs. Coelomic pouches form symmetrically on the sides of the intestine. The wall of each coelomic sac facing the intestine is called the splanchnopleura. The wall facing the ectoderm of the embryo is called somatopleura.
Thus, during the development of the embryo, various cavities are formed that have important morphogenetic significance. First, Baer's cavity appears, turning into the primary body cavity - the blastocoel, then the gastrocoel (or gastric cavity) appears, and finally, in many animals, the coelom. With the formation of the gastrocoel and coelom, the blastocoel becomes increasingly smaller, so that all that remains of the former primary body cavity are gaps in the spaces between the walls of the intestine and the coelom. These gaps turn into cavities of the circulatory system. The gastrocoel eventually turns into the midgut cavity.
Features of embryogenesis of mammals and birds
1. Extraembryonic organs.
2. Placenta of mammals.
3. Stages of the prenatal period of ontogenesis of ruminants, pigs and birds.
1. The embryos of reptiles and birds also develop a yolk sac. All germ layers are involved in this. During the 2nd and 3rd days of chick embryo development, a network of blood vessels develops in the inner part of the area opaca. Their appearance is inextricably linked with the emergence of embryonic hematopoiesis. Thus, one of the functions of the yolk sac of bird embryos is embryonic hematopoiesis. In the embryo itself, only subsequently are hematopoietic organs formed - liver, spleen, bone marrow.
The fetal heart begins to function (contract) at the end of the second day, from which time blood flow begins.
In bird embryos, in addition to the yolk sac, three more provisional organs are formed, which are usually called embryonic membranes - amnion, serosa and allantois. These organs can be considered as developed during the evolutionary process of adaptation of embryos.
The amnion and serosa arise in a close relationship. The amnion, in the form of a transverse fold, grows, bends over the anterior end of the embryo's head and covers it like a hood. Subsequently, the lateral sections of the amniotic folds grow on both sides of the embryo itself and grow together. The amniotic folds consist of ectoderm and parietal mesoderm.
In conjunction with the wall of the amniotic cavity, another important provisional formation develops - the serosa, or serous membrane. It consists of an ectodermal layer, “looking” at the embryo, and a mesodermal layer, “looking” outwards. The outer shell grows over the entire surface under the shell. This is serosa.
The amnion and serosa are, of course, “membranes”, since they actually cover and unite the embryo itself from the external environment. However, these are organs, parts of the embryo with very important functions. Amniotic fluid creates an aquatic environment for embryos of animals that, in the course of evolution, became land animals. It protects the developing embryo from drying out, from shaking, and from sticking to the egg shell. It is interesting to note that the role of amniotic fluid in mammals was noted by Leonardo da Vinci.
The serous membrane takes part in respiration and resorption of remnants of the protein membrane (under the influence of enzymes secreted by the chorion).
Another provisional organ develops - the allantois, which first performs the function of the embryonic bladder. It appears as a ventral outgrowth of the hindgut endoderm. In the chick embryo, this protrusion appears already on the 3rd day of development. In the middle of the embryonic development of birds, the allantois grows under the chorion over the entire surface of the embryo with the yolk sac.
At the very end of the embryonic development of birds (and reptiles), the provisional organs of the embryo gradually cease their functions, they are reduced, the embryo begins to breathe the air present inside the egg (in the air chamber), breaks through the shell, is freed from the egg membranes and finds itself in the external environment.
The extraembryonic organs of mammals are the yolk sac, amnion, allantois, chorion and placenta (Fig. 5).
2. In mammals, the connection between the embryo and the maternal body is ensured by the formation of a special organ - the placenta (children's place). The source of its development is the allanto-chorion. Based on their structure, placentas are divided into several types. The classification is based on two principles: a) the nature of the distribution of chorionic villi and 2) the method of their connection with the uterine mucosa (Fig. 6).
There are several types of placenta based on their shape:
1) Diffuse placenta (epitheliochorionic) - its secondary papillae develop over the entire surface of the chorion. Chorionic villi penetrate the glands of the uterine wall without destroying the uterine tissue. The embryo is nourished through the uterine glands, which secrete royal jelly, which is absorbed into the blood vessels of the chorionic villi. During childbirth, chorionic villi move out of the uterine glands without tissue destruction. This placenta is typical for pigs, horses, camels, marsupials, cetaceans, and hippopotamuses.
Rice. 5. Scheme of development of the yolk sac and embryonic membranes in mammals (six successive stages):
A - the process of fouling of the amniotic sac cavity with endoderm (1) and mesoderm (2); B - formation of a closed endodermal vesicle (4); B - the beginning of the formation of the amniotic fold (5) and the intestinal groove (6); G - separation of the body of the embryo (7); yolk sac (8); D - closure of amniotic folds (9); the beginning of the formation of allantois development (10); E - closed amniotic cavity (11); developed allantois (12); chorionic villi (13); parietal layer of mesoderm (14); visceral layer of mesoderm (15); ectoderm (3).
2) Cotyledon placenta (desmochorial) - the chorionic villi are located in bushes - cotyledons. They connect to thickenings of the uterine wall, which are called caruncles. The cotyledon-caruncle complex is called the placentome. This type of placenta is characteristic of ruminants.
3) Belt placenta (endotheliochorial) - villi in the form of a wide belt surround the fetal bladder and are located in the connective tissue layer of the uterine wall, in contact with the endothelial layer of the wall of blood vessels.
4) Discoidal placenta (hemochorial) - the contact zone of the chorionic villi and the uterine wall has the shape of a disk. The chorionic villi are immersed in blood-filled lacunae lying in the connective tissue layer of the uterine wall. This type of placenta is found in primates.
3. Livestock workers, through their practical activities, breed and raise animals. These are complex biological processes, and in order to consciously manage or seek ways to improve them, the animal engineer and veterinarian must know the basic patterns of animal development throughout their individual lives. We already know that the chain of changes that an organism experiences from the moment of its origin to natural death is called ontogenesis. It consists of qualitatively different periods. However, the periodization of ontogenesis has not yet been sufficiently developed. Some scientists believe that the ontogenetic development of an organism begins with the development of germ cells, others - with the formation of a zygote.
Rice. 6. Types of histological structure of placentas:
A - epitheliochorial; B - desmochorial; B - endotheliochorial: G - hemochorial; I - germinal part; II - maternal part; 1 - epithelium: 2 - connective tissue and 3 - endothelium of the blood vessel of the chorionic villi; 4 - epithelium; 5 - connective tissue and 6 - blood vessels and lacunae of the uterine mucosa.
After the emergence of the zygote, the subsequent ontogenesis of agricultural animals is divided into intrauterine and postuterine development.
Duration of subperiods of intrauterine development of agricultural animals, days (according to G.A. Schmidt).
In the embryogenesis of animals, due to their relationship, there are some fundamentally similar features: 1) formation of the zygote, 2) fragmentation, 3) formation of germ layers, 4) differentiation of germ layers, leading to the formation of tissues and organs.
General histology. Epithelial tissue
1. Tissue development.
2. Classification of epithelial tissues.
3. Glands and criteria for their classification.
1. The animal body is made up of cells and non-cellular structures specialized to perform certain functions. Populations of cells, different in function, differ in structure and specificity of intracellular protein synthesis.
In the process of development, initially homogeneous cells acquired differences in metabolism, structure, and function. This process is called differentiation. In this case, genetic information emanating from the DNA of the cell nucleus is realized, which manifests itself in specific conditions. The adaptation of cells to these conditions is called adaptation.
Differentiation and adaptation determine the development of qualitatively new relationships and relationships between cells and their populations. At the same time, the importance of the integrity of the organism, i.e. integration, increases significantly. Thus, each stage of embryogenesis is not just an increase in the number of cells, but a new state of integrity.
Integration is the unification of cell populations into more complex functioning systems - tissues, organs. It can be disrupted by viruses, bacteria, X-rays, hormones and other factors. In these cases, the biological system gets out of control, which can cause the development of malignant tumors and other pathologies.
Morphofunctional and genetic differences, which arose during the process of phylogenesis, allowed cells and non-cellular structures to unite into so-called histological tissues.
A tissue is a historically developed system of cells and non-cellular structures, characterized by a common structure, function and origin.
There are four main types of tissues: epithelial, connective or musculoskeletal, muscle and nervous. There are other classifications.
2. Epithelial tissues communicate between the body and the external environment. They perform integumentary and glandular (secretory) functions. The epithelium is located in the skin, lining the mucous membranes of all internal organs; It has the functions of absorption and excretion. Most of the body's glands are made of epithelial tissue.
All germ layers take part in the development of epithelial tissue.
All epithelia are built from epithelial cells - epithelial cells. By connecting firmly to each other with the help of desmosomes, closure bands, gluing bands, and by interdigitation, epithelial cells form a cell layer that functions and regenerates. Typically, the layers are located on the basement membrane, which, in turn, lies on the loose connective tissue that nourishes the epithelium (Fig. 7).
Epithelial tissues are characterized by polar differentiation, which comes down to the different structure of either the layers of the epithelial layer or the poles of epithelial cells. For example, at the apical pole the plasmalemma forms a suction border or ciliated cilia, and at the basal pole there is a nucleus and most of the organelles.
Depending on the location and function performed, two types of epithelia are distinguished: integumentary and glandular.
The most common classification of integumentary epithelium is based on the shape of the cells and the number of layers in the epithelial layer, which is why it is called morphological.
3. The epithelium that produces secretions is called glandular, and its cells are called secretory cells, or secretory glandulocytes. Glands are built from secretory cells, which can be formed as an independent organ or be only a part of it.
There are endocrine and exocrine glands. Morphologically, the difference is in the presence of the excretory duct in the latter. Exocrine glands can be unicellular or multicellular. Example: goblet cell in simple columnar bordered epithelium. Based on the nature of the branching of the excretory duct, simple and complex ones are distinguished. Simple glands have a non-branching excretory duct, while complex glands have a branching one. The terminal sections of simple glands are branched and unbranched, while those of complex glands are branched.
Based on the shape of the end sections, exocrine glands are classified into alveolar, tubular and tubulo-alveolar. The cells in the terminal section are called glandulocytes.
Based on the method of secretion formation, glands are divided into holocrine, apocrine and merocrine. These are the sebaceous, then sweat and mammary glands of the stomach, respectively.
Regeneration. The integumentary epithelia occupy a borderline position. They are often damaged, therefore they are characterized by high regenerative ability. Regeneration is carried out mainly in a mitotic manner. The cells of the epithelial layer quickly wear out, age and die. Their restoration is called physiological regeneration. The restoration of epithelial cells lost due to injury is called reparative regeneration.
In single-layer epithelia, all cells have the ability to regenerate; in multilayer epithelia, stem cells have the ability to regenerate. In the glandular epithelium, during holocrine secretion, stem cells located on the basement membrane have this ability. In merocrine and apocrine glands, the restoration of epithelial cells occurs mainly through intracellular regeneration.
Rice. 7. Diagram of different types of epithelium
A. Single layer flat.
B. Single-layer cubic.
B. Single-layer cylindrical.
G. Multirow cylindrical ciliated.
D. Transitional.
E. Multilayer flat non-keratinizing.
G. Multilayer flat keratinizing.
Support-trophic tissues. blood and lymph
1. Blood. Blood cells.
3. Hemocytopoiesis.
4. Embryonic hemocytopoiesis.
With this topic we begin the study of a group of related tissues called connective tissues. This includes: the connective tissue itself, blood cells and hematopoietic tissues, skeletal tissues (cartilage and bone), connective tissues with special properties.
The manifestation of the unity of the above types of tissue is their origin from a common embryonic source - mesenchyme.
Mesenchyme is a set of embryonic network-like connected process cells that fill the gaps between the germ layers and organ rudiments. In the body of the embryo, mesenchyme arises mainly from cells of certain areas of the mesoderm - dermatomes, sclerotomes and splanchnotomes. Mesenchyme cells rapidly divide by mitosis. Numerous mesenchymal derivatives arise in its various parts - blood islands with their endothelium and blood cells, cells of connective tissue and smooth muscle tissue, etc.
1. Intravascular blood is a mobile tissue system with a liquid intercellular substance - plasma and formed elements - erythrocytes, leukocytes and blood platelets.
Constantly circulating in a closed circulatory system, blood unites the work of all body systems and maintains many physiological indicators of the body’s internal environment at a certain level that is optimal for metabolic processes. Blood performs a variety of vital functions in the body: respiratory, trophic, protective, regulatory, excretory and others.
Despite the mobility and variability of blood, its indicators at each moment correspond to the functional state of the body, therefore blood testing is one of the most important diagnostic methods.
Plasma is a liquid component of blood, containing 90-92% water and 8-10% dry substances, including 9% organic and 1% mineral substances. The main organic substances of blood plasma are proteins (albumin, various fractions of globulins and fibrinogen). Immune proteins (antibodies), and most of them are contained in the gamma globulin fraction, are called immunoglobulins. Albumins ensure the transport of various substances - free fatty acids, bilirubin, etc. Fibrinogen takes part in blood clotting processes.
Red blood cells are the main type of blood cells, as they are 500-1000 times more numerous than white blood cells. 1mm3 of blood contains 5.0-7.5 million in cattle, 6-9 million in horses, 7-12 million in sheep, 12-18 million in goats, 6-7.5 million in pigs. chickens - 3-4 million red blood cells.
Having lost their nucleus during development, mature erythrocytes in mammals are anucleate cells and have the shape of a biconcave disk with an average circle diameter of 5-7 µm. The red blood cells of camel and llama are oval. The discoid shape increases the total surface of the red blood cell by 1.64 times.
There is an inverse relationship between the number of red blood cells and their size.
Red blood cells are covered with a membrane - plasmalemma (6 nm thick), containing 44% lipids, 47% proteins and 7% carbohydrates. The erythrocyte membrane is easily permeable to gases, anions, and Na ions.
The internal colloidal content of erythrocytes consists of 34% hemoglobin - a unique complex colored compound - a chromoprotein, in the non-protein part of which (heme) there is divalent iron, capable of forming special weak bonds with an oxygen molecule. It is thanks to hemoglobin that the respiratory function of red blood cells is carried out. Oxyhemoglobin = hemoglobin + O2.
The presence of hemoglobin in erythrocytes causes their pronounced oxyphilia when staining a blood smear according to Romanovsky-Giemsa (eosin + azure II). The red blood cells are stained red with eosin. In some forms of anemia, the central pale colored part of the red blood cells is enlarged - hypochromic red blood cells. When supravital blood is stained with brilliant cresyl blue, young forms of erythrocytes containing granular-mesh structures can be detected. Such cells are called reticulocytes, they are the immediate precursors of mature red blood cells. Reticulocyte counting is used to obtain information about the rate of red blood cell production.
The lifespan of an erythrocyte is 100-130 days (in rabbits 45-60 days). Red blood cells have the property of resisting various destructive influences - osmotic, mechanical, etc. When the concentration of salts in the environment changes, the erythrocyte membrane ceases to retain hemoglobin, and it releases into the surrounding fluid - the phenomenon of hemolysis. The release of hemoglobin can occur in the body under the influence of snake venom and toxins. Hemolysis also develops with transfusion of incompatible blood group. It is practically important when introducing liquids into the blood of animals to ensure that the injected solution is isotonic.
Red blood cells have a relatively high density compared to plasma and blood leukocytes. If blood is treated with anticoagulants and placed in a vessel, erythrocyte sedimentation is noted. The erythrocyte sedimentation rate (ESR) is not the same in animals of different ages, sexes and species. ESR is high in horses and, conversely, low in cattle. ESR has diagnostic and prognostic significance.
Leukocytes are vascular blood cells of various morphological characteristics and functions. In the animal body, they perform diverse functions, aimed primarily at protecting the body from foreign influences through phagocytic activity, participation in the formation of humoral and cellular immunity, as well as in restoration processes in case of tissue damage. There are 4.5-12 thousand of them in 1 mm3 of blood in cattle, 7-12 thousand in horses, 6-14 thousand in sheep, 8-16 thousand in pigs, 20-40 thousand in chickens. Increased number of leukocytes - leukocytosis - characteristic feature for many pathological processes.
Having formed in the hematopoietic organs and entering the blood, leukocytes remain in the vascular bed for only a short time, then migrate to the surrounding vascular connective tissue and organs, where they perform their main function.
The peculiarity of leukocytes is that they have mobility due to the formation of pseudopodia. Leukocytes are divided into a nucleus and a cytoplasm containing various organelles and inclusions. The classification of leukocytes is based on the ability to stain with dyes and granularity.
Granular leukocytes (granulocytes): neutrophils (25-70%), eosinophils (2-12%), basophils (0.5-2%).
Non-granular leukocytes (agranulocytes): lymphocytes (40-65) and monocytes (1-8%).
A certain percentage ratio between individual types of leukocytes is called the leukocyte formula - leukogram.
An increase in the percentage of neutrophils in the leukogram is typical for purulent-inflammatory processes. In mature neutrophils, the nucleus consists of several segments connected by thin bridges.
On the surface of basophils there are special receptors through which immunoglobulins E bind. They participate in immunological reactions of the allergic type.
Monocytes circulating in the blood are the precursors of tissue and organ macrophages. After remaining in the vascular blood (12-36 hours), monocytes migrate through the endothelium of capillaries and venules into tissues and turn into motile macrophages.
Lymphocytes are the most important cells involved in various immunological reactions of the body. A large number of lymphocytes are found in lymph.
There are two main classes of lymphocytes: T- and B-lymphocytes. The first develop from bone marrow cells in the cortical part of the thymus lobules. The plasmalemma contains antigenic markers and numerous receptors, with the help of which foreign antigens and immune complexes are recognized.
B lymphocytes are formed from stem progenitors in the bursa of Fabricius (Bursa). The place of their development is considered to be myeloid tissue of the bone marrow.
Effector cells in the T-lymphocyte system are three main subpopulations: T-killers (cytotoxic lymphocytes), T-helpers (helpers) and T-suppressors (inhibitors). The effector cells of B lymphocytes are plasmablasts and mature plasmacytes, capable of producing immunoglobulins in increased quantities.
Blood plates are nuclear-free elements of the vascular blood of mammals. These are small cytoplasmic fragments of red bone marrow megakaryocytes. There are 250-350 thousand blood platelets in 1 mm3 of blood. In birds, cells with similar functions are called platelets.
Blood plates have essential knowledge in ensuring the main stages of stopping bleeding - hemostasis.
2. Lymph is an almost transparent yellowish liquid located in the cavity of the lymphatic capillaries and vessels. Its formation is due to the transition components blood plasma from blood capillaries into tissue fluid. In the formation of lymph, the relationship between the hydrostatic and osmotic pressure of blood and tissue fluid, the permeability of the wall of blood capillaries, etc. are essential.
Lymph consists of a liquid part - lymphoplasm and formed elements. Lymphoplasm differs from blood plasma in having lower protein content. Lymph contains fibrinogen, so it is also capable of coagulation. The main formed elements of lymph are lymphocytes. The composition of lymph in different vessels of the lymphatic system is not the same. There are peripheral lymph (before the lymph nodes), intermediate (after the lymph nodes) and central (lymph of the thoracic and right lymphatic ducts), which is the richest in cellular elements.
3. Hematopoiesis (hemocytopoiesis) is a multi-stage process of successive cellular transformations leading to the formation of mature peripheral vascular blood cells.
In the postembryonic period in animals, the development of blood cells occurs in two specialized, intensively renewed tissues - myeloid and lymphoid.
Currently, the most recognized scheme of hematopoiesis proposed by I.L. Chertkov and A.I. Vorobyov (1981), according to which all hemocytopoiesis is divided into 6 stages (Fig. 8).
The ancestor of all blood cells (according to A.A. Maksimov) is a pluripotent stem cell (colony-forming unit in the spleen and CFU). In an adult body, the largest number of stem cells is located in the red bone marrow (there are about 50 stem cells per 100,000 bone marrow cells), from which they migrate to the thymus and spleen.
The development of erythrocytes (erythrocytopoiesis) in the red bone marrow proceeds according to the following scheme: stem cell (SC) - semi-stem cells (CFU - GEMM, CFU - GE, CFU - MGCE) - unipotent precursors of erythropoiesis (PFU - E, CFU - E) - erythroblast - pronormocyte - basophilic normocyte - polychromatophilic normocyte - oxyphilic normocyte - reticulocyte - erythrocyte.
Development of granulocytes: red bone marrow stem cell, semi-stem (CFU - GEMM, CFU - GM, CFU - GE), unipotent precursors (CFU - B, CFU - Eo, CFU - Gn), which through the stages of recognizable cellular forms turn into mature segmented ones There are three types of granulocytes - neutrophils, eosinophils and basophils.
Lymphocyte development is one of the most complex processes differentiation of hematopoietic stem cells.
With the participation of various organs, the formation of two cell lines closely related in the functioning - T- and B-lymphocytes - is gradually carried out.
The development of blood platelets occurs in the red bone marrow and is associated with the development of special giant cells in it - megakaryocytes. Megakaryocytopoiesis consists of the following stages: SC - semi-stem cells (CFU - GEMM and CFU - MGCE) - unipotent precursors, (CFU - MGC) - megakaryoblast - promegakaryocyte - megakaryocyte.
4. At the earliest stages of ontogenesis, blood cells are formed outside the embryo, in the mesenchyme of the yolk sac, where clusters - blood islands - are formed. The central cells of the islets round and transform into hematopoietic stem cells. The peripheral cells of the islets stretch into strips of interconnected cells and form the endothelial lining of the primary blood vessels (yolk sac vasculature). Some stem cells turn into large basophilic blast cells - primary blood cells. Most of these cells, rapidly multiplying, become increasingly stained with acidic dyes. This occurs due to the synthesis and accumulation of hemoglobin in the cytoplasm, and condensed chromatin in the nucleus. Such cells are called primary erythroblasts. In some primary erythroblasts, the nucleus disintegrates and disappears. The resulting generation of nuclear and non-nuclear primary erythrocytes is varied in size, but the most common are large cells - megaloblasts and megalocytes. The megaloblastic type of hematopoiesis is characteristic of the embryonic period.
Some of the primary blood cells are converted into a population of secondary erythrocytes, and a small number of granulocytes - neutrophils and eosinophils - develop outside the vessels, i.e. myelopoiesis occurs.
The stem cells generated in the yolk sac are transported through the blood to the organs of the body. After the liver is formed, it becomes a universal hematopoietic organ (secondary erythrocytes, granular leukocytes and megakaryocytes develop). By the end of the prenatal period, hematopoiesis in the liver stops.
At 7-8 weeks of embryonic development (in cattle), thymic lymphocytes and T-lymphocytes migrating from it differentiate from stem cells in the developing thymus. The latter populate the T-zones of the spleen and lymph nodes. At the beginning of its development, the spleen is also the organ in which all types of blood cells are formed.
At the last stages of embryonic development in animals, the main hematopoietic functions begin to be performed by red bone marrow; it produces erythrocytes, granulocytes, blood platelets, and some lymphocytes (B-l). In the postembryonic period, red bone marrow becomes an organ of universal hematopoiesis.
During embryonic erythrocytopoiesis, there is a characteristic process of changing generations of erythrocytes, differing in morphology and the type of hemoglobin formed. The population of primary erythrocytes forms the embryonic type of hemoglobin (Hb - F). at subsequent stages, red blood cells in the liver and spleen contain the fetal type of hemoglobin (Hb-H). The definitive type of red blood cells with the third type of hemoglobin (Hb-A and Hb-A 2) are formed in the red bone marrow. Different types of hemoglobins differ in the composition of amino acids in the protein part.
cell embryogenesis tissue histology cytology
Connective tissue itself
1. Loose and dense connective tissue.
2. Connective tissue with special properties: reticular, adipose, pigmented.
1. Widespread tissues in the animal body with a highly developed system of fibers in the intercellular substance, thanks to which these tissues perform versatile mechanical and shape-forming functions - they form a complex of partitions, trabeculae or layers inside organs, are part of numerous membranes, form capsules, ligaments, fascia , tendons.
Depending on the quantitative relationship between the components of the intercellular substance - fibers and ground substance and in accordance with the type of fibers, three types of connective tissue are distinguished: loose connective tissue, dense connective tissue and reticular tissue.
The main cells that create the substances necessary for building fibers in loose and dense connective tissue are fibroblasts, and in reticular tissue - reticular cells. Loose connective tissue is characterized by a particularly wide variety of cellular composition.
Loose connective tissue is the most common. It accompanies all blood and lymphatic vessels, forms numerous layers inside organs, etc. It consists of a variety of cells, ground substance and a system of collagen and elastic fibers. In the composition of this tissue, more sedentary cells (fibroblasts - fibrocytes, lipocytes) and mobile cells (histiocytes - macrophages, tissue basophils, plasmacytes) are distinguished - Fig. 9.
The main functions of this connective tissue are: trophic, protective and plastic.
Types of cells: Adventitial cells - poorly differentiated, capable of mitotic division and transformation into fibroblasts, myofibroblasts and lipocytes. Fibroblasts are the main cells directly involved in the formation of intercellular structures. During embryonic development, fibroblasts arise directly from mesenchymal cells. There are three types of fibroblasts: poorly differentiated (function: synthesis and secretion of glycosaminoglycans); mature (function: synthesis of procollagen, proelastin, enzyme proteins and glycosaminoglycans, especially protein synthesis of collagen fibers); myofibroblasts that promote wound closure. Fibrocytes lose their ability to divide and reduce their synthetic activity. Histiocytes (macrophages) belong to the mononuclear phagocyte system (MPS). This system will be discussed in the next lecture. Tissue basophils (mast cells, mast cells), located near small blood vessels, are one of the first cells to respond to the penetration of antigens from the blood.
Plasmocides - functionally - are effector cells of immunological reactions of the humoral type. These are highly specialized cells of the body that synthesize and secrete the bulk of various antibodies (immunoglobulins).
The intercellular substance of loose connective tissue makes up a significant part of it. It is represented by collagen and elastic fibers and the main (amorphous) substance.
An amorphous substance is a product of the synthesis of connective tissue cells (mainly fibroblasts) and the intake of substances from the blood, transparent, slightly yellowish, capable of changing its consistency, which significantly affects its properties.
It consists of glycosaminoglycans (polysaccharides), proteoglycans, glycoproteins, water and inorganic salts. The most important chemical high-polymer substance in this complex is a non-sulfated type of glycosaminoglycans - hyaluronic acid.
Collagen fibers consist of fibrils formed by tropocollagen protein molecules. The latter are peculiar monomers. The formation of fibrils is the result of a characteristic grouping of monomers in the longitudinal and transverse directions.
Depending on the amino acid composition and the form of association of chains into a triple helix, there are four main types of collagen, which have different localizations in the body. Type I collagen is found in the connective tissue of the skin, tendons and bones. Type II collagen is found in hyaline and fibrous cartilage. Collagen II? type - in the skin of embryos, the wall of blood vessels, ligaments. Type IV collagen is found in basement membranes.
There are two ways to form collagen fibers: intracellular and extracellular synthesis.
Elastic fibers are homogeneous threads that form a network. They do not combine into bundles and have low strength. There is a more transparent amorphous central part, consisting of the protein elastin, and a peripheral part, consisting of microfibrils of a glycoprotein nature, shaped like tubes. Elastic fibers are formed due to the synthetic and secretory function of fibroblasts. It is believed that first, a framework of microfibrils is formed in the immediate vicinity of fibroblasts, and then the formation of an amorphous part from the elastin precursor, proelastin, is enhanced. Proelastin molecules, under the influence of enzymes, are shortened and converted into tropoelastin molecules. The latter, during the formation of elastin, are connected to each other using desmosine, which is absent in other proteins. Elastic fibers predominate in the occipito-cervical ligament and abdominal yellow fascia.
Dense connective tissue. This tissue is characterized by a quantitative predominance of fibers over the ground substance and cells. Depending on the relative position of the fibers and the networks formed from the lower bundles, two main types of dense connective tissue are distinguished: unformed (dermis) and formed (ligaments, tendons).
2. Reticular tissue consists of branched reticular cells and reticular fibers (Fig. 10). Reticular tissue forms the stroma of the hematopoietic organs, where, in combination with macrophages, it creates a microenvironment that ensures the reproduction, differentiation and migration of various blood cells.
Reticular cells develop from mesenchymocytes and are similar to fibroblasts, chondroblasts, etc. Reticular fibers are derivatives of reticular cells and are thin branching fibers that form a network. They contain fibrils of different diameters, enclosed in an interfibrillar substance. Fibrils are composed of type III collagen.
Adipose tissue is formed by fat cells (lipocytes). The latter are specialized in the synthesis and accumulation of storage lipids, mainly triglycerides, in the cytoplasm. Lipocytes are widely distributed in loose connective tissue. During embryogenesis, fat cells arise from mesenchymal cells.
The precursors for the formation of new fat cells in the postembryonic period are adventitial cells accompanying blood capillaries.
There are two types of lipocytes and actually two types of adipose tissue: white and brown. White adipose tissue is found in the body of animals differently depending on the species and breed. There is a lot of it in fat depots. The total amount of it in the body of animals of various species, breeds, sex, age, and fatness ranges from 1 to 30% of fat mass. Fat as a source of energy (1 g of fat = 39 kJ), water depot, shock absorber.
Rice. 11. Structure of white adipose tissue (scheme according to Yu.I. Afanasyev)
A - adipocytes with removed fat in a light optical microscope; B - ultramicroscopic structure of adipocytes. 1 - fat cell nucleus; 2 - large drops of lipids; 3 - nerve fibers; 4 - hemocapillaries; 5 - mitochondria.
Rice. 12. Structure of brown adipose tissue (scheme according to Yu.I. Afanasyev)
A - adipocytes with removed fat in a light optical microscope; B - ultramicroscopic structure of adipocytes. 1 - adipocyte nucleus; 2 - finely crushed lipids; 3 - numerous mitochondria; 4 - hemocapillaries; 5 - nerve fiber.
Brown adipose tissue is found in significant quantities in rodents and hibernating animals; as well as in newborns of other species. Cells, when oxidized, generate heat, which is used for thermoregulation.
Pigment cells (pigmentocytes) have many dark brown or black grains of pigment from the melanin group in their cytoplasm.
The immune system and cellular interactions in immune reactions
1. The concept of antigens and antibodies, their varieties.
2 The concept of cellular and humoral immunity.
3 Genesis and interaction of T- and B-lymphocytes.
4 Mononuclear system of macrophages.
1. In industrial livestock farming, in conditions of concentration and intensive exploitation of livestock, stressful effects of technogenic and other environmental factors, the role of preventing diseases of animals, especially young animals, caused by the influence of various agents of infectious and non-infectious nature against the background of a decrease in the natural protective abilities of the body, increases significantly.
In this regard, the problem of monitoring the physiological and immunological state of animals in order to timely increase their general and specific resistance becomes of great importance (Tsymbal A.M., Konarzhevsky K.E. et al., 1984).
Immunity (immunitatis - liberation from something) is the body’s protection from everything genetically foreign - microbes, viruses, foreign cells. or genetically modified own cells.
The immune system unites organs and tissues in which the formation and interaction of cells occurs - immunocytes, which perform the function of recognizing genetically foreign substances (antigens) and carrying out a specific reaction.
Antibodies are complex proteins found in the immunoglobulin fraction of animal blood plasma, synthesized by plasma cells under the influence of various antigens. Several classes of immunoglobulins have been studied (Y, M, A, E, D).
At the first encounter with an antigen (primary response), lymphocytes are stimulated and undergo transformation into blast forms, which are capable of proliferation and differentiation into immunocytes. Differentiation leads to the appearance of two types of cells - effector and memory cells. The former are directly involved in the elimination of foreign material. Effector cells include activated lymphocytes and plasma cells. Memory cells are lymphocytes that return to an inactive state, but carry information (memory) about an encounter with a specific antigen. When this antigen is reintroduced, they are able to provide a rapid immune response (secondary response) due to increased proliferation of lymphocytes and the formation of immunocytes.
2. Depending on the mechanism of antigen destruction, cellular immunity and humoral immunity are distinguished.
In cellular immunity, effector (motor) cells are cytotoxic T-lymphocytes, or killer lymphocytes, which are directly involved in the destruction of foreign cells of other organs or pathological own cells (for example, tumor cells) and secrete lytic substances.
In humoral immunity, effector cells are plasma cells that synthesize and release antibodies into the blood.
In the formation of cellular and humoral immunity in humans and animals, the cellular elements of lymphoid tissue, in particular T- and B-lymphocytes, play an important role. Information on the populations of these cells in the blood of cattle is sparse. According to Korchan N.I. (1984), calves are born with a relatively mature B-lymphocyte system and an insufficiently developed B-lymphocyte system and the regulatory relationships between these cells. Only by 10-15 days of life do the indicators of these cell systems approach those of adult animals.
The immune system in the body of an adult animal is represented by: red bone marrow - a source of stem cells for immunocytes, central organs of lymphocytopoiesis (thymus), peripheral organs of lymphocytopoiesis (spleen, lymph nodes, accumulation of lymphoid tissue in organs), blood and lymph lymphocytes, as well as populations of lymphocytes and plasma cells, penetrating all connective and epithelial tissues. All organs of the immune system function as a single whole thanks to neurohumoral regulatory mechanisms, as well as the constantly occurring processes of migration and recycling of cells through the circulatory and lymphatic systems. The main cells that carry out control and immunological defense in the body are lymphocytes, as well as plasma cells and macrophages.
3. There are two main types of lymphocytes: B-lymphocytes and T-lymphocytes. Stem cells and B cell progenitor cells are produced in the bone marrow. In mammals, differentiation of B lymphocytes also occurs here, characterized by the appearance of immunoglobulin receptors in the cells. Next, such differentiated B lymphocytes enter the peripheral lymphoid organs: the spleen, lymph nodes, and lymph nodes of the digestive tract. In these organs, under the influence of antigens, proliferation and further specialization of B lymphocytes occurs with the formation of effector cells and memory B cells.
T lymphocytes also develop from stem cells of bone marrow origin. The latter are transported with the bloodstream to the thymus and turn into blasts, which divide and differentiate in two directions. Some blasts form a population of lymphocytes that have special receptors that perceive foreign antigens. The differentiation of these cells occurs under the influence of a differentiation inducer produced and secreted by the epithelial elements of the thymus. The resulting T-lymphocytes (antigen-reactive lymphocytes) populate special T-zones (thymus-dependent) in the peripheral lymphoid organs. There, under the influence of antigens, they can undergo transformation into T-blasts, proliferate and differentiate into effector cells involved in transplantation (killer T-cells) and humoral immunity (T-helper and T-suppressor cells), as well as memory T cells. Another part of the descendants of T-blasts differentiate to form cells that carry receptors for the antigens of their own body. These cells are destroyed.
Thus, it is necessary to distinguish between antigen-independent and antigen-dependent proliferation, differentiation and specialization of B and T lymphocytes.
In the case of the formation of cellular immunity under the influence of tissue antigens, the differentiation of T-lymphoblasts leads to the appearance of cytotoxic lymphocytes (T-killers) and memory T-cells. Cytotoxic lymphocytes are capable of destroying foreign cells (target cells) or through the special mediator substances they secrete (lymphokines).
During the formation of humoral immunity, most soluble and other antigens also have a stimulating effect on T-lymphocytes; in this case, T-helpers are formed, which secrete mediators (lymphokines) that interact with B-lymphocytes and cause their transformation into B-blasts, which specialize in secreting plasma cell antibodies. The proliferation of antigen-stimulated T lymphocytes also leads to an increase in the number of cells that turn into inactive small lymphocytes that retain information about a given antigen for several years and are therefore called memory T cells.
T-helper determines the specialization of B-lymphocytes in the direction of the formation of antibody-forming plasmacytes, which provide “humoral immunity” by producing and releasing immunoglobulins into the blood. At the same time, the B lymphocyte receives antigenic information from the macrophage, which captures the antigen, processes it and transfers it to the B lymphocyte. On the surface of the B lymphocyte there are a larger number of immunoglobulin receptors (50-150 thousand).
Thus, to ensure immunological reactions, cooperation between the activities of three main types of cells is necessary: B-lymphocytes, macrophages and T-lymphocytes (Fig. 13).
4. Macrophages play an important role in both natural and acquired immunity of the body. The participation of macrophages in natural immunity is manifested in their ability to phagocytose. Their role in acquired immunity is the passive transfer of antigen to immunocompetent cells (T and B lymphocytes) and the induction of a specific response to antigens.
Most of the processed antigen material released by macrophages has a stimulating effect on the proliferation and differentiation of T- and B-lymphocyte clones.
In the B-zones of the lymph nodes and spleen there are specialized macrophages (dendritic cells), on the surface of their numerous processes many antigens are stored that enter the body and are transmitted to the corresponding clones of B-lymphocytes. In the T-zones of lymphatic follicles there are interdigitating cells that influence the differentiation of T-lymphocyte clones.
Thus, macrophages are directly involved in the cooperative interaction of cells (T- and B-lymphocytes) in the body’s immune reactions.
There are two types of migration of immune system cells: slow and fast. The first is more typical for B lymphocytes, the second - for T lymphocytes. The processes of migration and recycling of cells of the immune system ensure the maintenance of immune homeostasis.
see also tutorial“Methods for assessing the protective systems of the mammalian body” (Katsy G.D., Koyuda L.I. - Lugansk - 2003. - p. 42-68).
Skeletal tissues: cartilage and bone
1. Development, structure and types of cartilage tissue.
2. Development, structure and types of bone tissue.
1. Cartilage tissue is a specialized type of connective tissue that performs a supporting function. In embryogenesis, it develops from mesenchyme and forms the skeleton of the embryo, which is subsequently largely replaced by bone. Cartilage tissue, with the exception of the articular surfaces, is covered with dense connective tissue - perichondrium, containing vessels that feed the cartilage and its cambial (chondrogenic) cells.
Cartilage consists of chondrocyte cells and intercellular substance. In accordance with the characteristics of the intercellular substance, three types of cartilage are distinguished: hyaline, elastic and fibrous.
During the embryonic development of the embryo, the mesenchyme, intensively developing, forms islands of protochondral tissue cells tightly adjacent to each other. Its cells are characterized by high values of nuclear-cytoplasmic ratios, small dense mitochondria, an abundance of free ribosomes, weak development of granular EPS, etc. During development, primary cartilaginous (prechondral) tissue is formed from these cells.
As the intercellular substance accumulates, the cells of the developing cartilage are isolated in separate cavities (lacunae) and differentiate into mature cartilage cells - chondrocytes.
Further growth of cartilage tissue is ensured by the continued division of chondrocytes and the formation of intercellular substance between daughter cells. The formation of the latter slows down over time. Daughter cells, remaining in the same lacuna, form isogenic groups of cells (Isos - equal, genesis - origin).
As cartilage tissue differentiates, the intensity of cell reproduction decreases, the nuclei become pictonized, and the nucleolar apparatus is reduced.
Hyaline cartilage. In the adult body, hyaline cartilage is part of the ribs, sternum, covers articular surfaces, etc. (Fig. 14).
Cartilage cells - chondrocytes - of its various zones have their own characteristics. Thus, immature cartilage cells - chondroblasts - are localized directly under the perichondrium. They are oval in shape, the cytoplasm is rich in RNA. In the deeper zones of cartilage, chondrocytes become rounded and form characteristic “isogenic groups”.
The intercellular substance of hyaline cartilage contains up to 70% of the dry weight of fibrillar collagen protein and up to 30% of amorphous substance, which includes glycosaminoglycans, proteoglycans, lipids and non-collagen proteins.
The orientation of the fibers of the intercellular substance is determined by the patterns of mechanical tension characteristic of each cartilage.
Collagen fibrils of cartilage, unlike collagen fibers of other types of connective tissue, are thin and do not exceed 10 nm in diameter.
The metabolism of cartilage is ensured by the circulation of tissue fluid of the intercellular substance, which accounts for up to 75% of the total mass of the tissue.
Elastic cartilage forms the skeleton of the outer ear and the cartilage of the larynx. In addition to the amorphous substance and collagen fibrils, its composition includes a dense network of elastic fibers. Its cells are identical to the cells of hyaline cartilage. They also form groups and lie singly only under the perichondrium (Fig. 15).
Fibrous cartilage is localized in the intervertebral discs, in the area where the tendon attaches to the bones. The intercellular substance contains coarse bundles of collagen fibers. Cartilage cells form isogenic groups, elongated into chains between bundles of collagen fibers (Fig. 16).
Cartilage regeneration is ensured by the perichondrium, the cells of which retain cambiality - chondrogenic cells.
2. Bone tissue, like other types of connective tissue, develops from mesenchyme and consists of cells and intercellular substance. Performs the function of support, protection and is actively involved in metabolism. Red bone marrow is localized in the spongy substance of skeletal bones, where the processes of hematopoiesis and differentiation of cells of the body's immune defense are carried out. Bone deposits salts of calcium, phosphorus, etc. In total, minerals make up 65-70% of the dry mass of the tissue.
Bone tissue contains four different types of cells: osteogenic cells, osteoblasts, osteocytes and osteoclasts.
Osteogenic cells are cells at an early stage of specific differentiation of mesenchyme in the process of osteogenesis. They retain the potency for mitotic division. These cells are localized on the surface of bone tissue: in the periosteum, endosteum, Haversian canals and other areas of bone tissue formation. As they multiply, they replenish the supply of osteoblasts.
Osteoblasts are cells that produce organic elements of the intercellular substance of bone tissue: collagen, glycosaminoglycans, proteins, etc.
Osteocytes lie in special cavities of the intercellular substance - lacunae, interconnected by numerous bone tubules.
Osteoclasts are large, multinucleated cells. They are located on the surface of bone tissue in places of its resorption. Cells are polarized. The surface facing the resorbable tissue has a corrugated border due to thin branching processes.
The intercellular substance consists of collagen fibers and amorphous substances: glycoproteins, glycosaminoglycans, proteins and inorganic compounds. 97% of the body's total calcium is concentrated in bone tissue.
In accordance with the structural organization of the intercellular substance, coarse-fiber bone and lamellar bone are distinguished (Fig. 17). Rough fibrous bone is characterized by a significant diameter of bundles of collagen fibrils and a variety of their orientation. It is typical for bones of the early stage of animal ontogenesis. In lamellar bone, collagen fibrils do not form bundles. Arranged in parallel, they form layers - bone plates with a thickness of 3-7 microns. The plates contain cellular cavities - lacunae and bone tubules connecting them, in which osteocytes and their processes lie. Tissue fluid circulates through the system of lacunae and tubules, ensuring metabolism in the tissue.
Depending on the position of the bone plates, spongy and compact bone tissue is distinguished. In the spongy substance, in particular in the epiphyses of long bones, groups of bone plates are located at different angles to each other. The cells of spongy bone contain red bone marrow.
In the compact substance, groups of bone plates 4-15 microns thick fit tightly to each other. Three layers are formed in the diaphysis: the outer common system of plates, the osteogenic layer and the internal common system.
Through the external common system, perforating tubules pass from the periosteum, carrying blood vessels and coarse bundles of collagen fibers into the bone.
In the osteogenic layer of the tubular bone, the osteon channels containing blood vessels and nerves are mainly oriented longitudinally. The system of tube-shaped bone plates surrounding these canals - osteons - contain from 4 to 20 plates. Osteons are delimited from each other by a cement line of the main substance; they are a structural unit of bone tissue (Fig. 18).
The internal common system of bone plates borders the endosteum of the bone band and is represented by plates oriented parallel to the canal surface.
There are two types of osteogenesis: directly from mesenchyme (“direct”) and by replacing embryonic cartilage with bone (“indirect”) osteogenesis - Fig. 19.20.
The first is characteristic of the development of coarse-fiber bone of the skull and lower jaw. The process begins with the intensive development of connective tissue and blood vessels. Mesenchymal cells, anastomosing processes with each other, form a network. Cells pushed to the surface by the intercellular substance differentiate into osteoblasts, which are actively involved in osteogenesis. Subsequently, the primary coarse-fiber bone tissue is replaced by lamellar bone. The bones of the torso, limbs, etc. are formed in place of cartilaginous tissue. In tubular bones, this process begins in the area of the diaphysis with the formation under the perichondrium of a network of crossbars of coarse-fiber bone - the bone cuff. The process of replacing cartilage with bone tissue is called enchondral ossification.
Simultaneously with the development of enchondral bone, an active process of perichondral osteogenesis occurs from the side of the periosteum, forming a dense layer of periosteal bone, extending along its entire length to the epiphyseal growth plate. Periosteal bone is the compact bone substance of the skeleton.
Later, ossification centers appear in the epiphyses of the bone. Bone tissue here replaces cartilage. The latter is preserved only on the articular surface and in the epiphyseal growth plate, which separates the epiphysis from the diaphysis throughout the entire period of growth of the organism until the animal reaches sexual maturity.
The periosteum (periosteum) consists of two layers: the inner layer contains collagen and elastic fibers, osteoblasts, osteoclasts and blood vessels. External - formed by dense connective tissue. It is directly connected to the muscle tendons.
Endosteum is a layer of connective tissue lining the medullary canal. It contains osteoblasts and thin bundles of collagen fibers that pass into the bone marrow tissue.
Muscle tissue
1. Smooth.
2. Cardiac striated.
3. Skeletal striated.
4. Development, growth and regeneration of muscle fibers.
1. The leading function of muscle tissue is to ensure movement in space of the body as a whole and its parts. All muscle tissues make up a morphofunctional group, and depending on the structure of organelles, contractions are divided into three groups: smooth, skeletal striated and cardiac striated muscle tissues. These tissues do not have a single source of embryonic development. They are mesenchyme, myotomes of segmented mesoderm, visceral layer of splanchnotome, etc.
Smooth muscle tissue of mesenchymal origin. The tissue consists of myocytes and a connective tissue component. A smooth myocyte is a spindle-shaped cell 20-500 µm long and 5-8 µm thick. The rod-shaped core is located in its central part. There are many mitochondria in the cell.
Each myocyte is surrounded by a basement membrane. There are holes in it, in the area of which gap-like connections (nexuses) are formed between neighboring myocytes, ensuring functional interactions of myocytes in the tissue. Numerous reticular fibrils are woven into the basement membrane. Around muscle cells, reticular, elastic and thin collagen fibers form a three-dimensional network - endomysium, which connects neighboring myocytes.
Physiological regeneration of smooth muscle tissue usually manifests itself under conditions of increased functional load, mainly in the form of compensatory hypertrophy. This is most clearly observed in the muscular lining of the uterus during pregnancy.
Elements of muscle tissue of epidermal origin are myoepithelial cells developing from the ectoderm. They are located in the sweat, mammary, salivary and lacrimal glands, differentiating simultaneously with their secretory epithelial cells from common precursors. By contracting, the cells promote the excretion of gland secretions.
Smooth muscles form muscle layers in all hollow and tubular organs.
2. Sources of development of cardiac striated muscle tissue are symmetrical sections of the visceral layer of the splanchnotome. Most of its cells differentiate into cardiomyocytes (cardiac myocytes), the rest into epicardial mesothelial cells. Both have common progenitor cells. During histogenesis, several types of cardiomyocytes are differentiated: contractile, conductive, transitional and secretory.
The structure of contractile cardiomyocytes. The cells have an elongated shape (100-150 microns), close to cylindrical. Their ends are connected to each other by insertion disks. The latter perform not only a mechanical function, but also conductive and provide electrical communication between cells. The nucleus is oval in shape and located in the central part of the cell. It has a lot of mitochondria. They form chains around special organelles - myofibrils. The latter are built from constantly existing, orderly filaments of actin and myosin - contractile proteins. To secure them, special structures are used - telophragm and mesophragm, built from other proteins.
The section of myofibril between two Z lines is called a sarcomere. A-bands - anisotropic, microfilaments are thick, contain myosin: I-bands - isotropic, microfilaments are thin, contain actin; The H-band is located in the middle of the A-band (Fig. 21).
There are several theories about the mechanism of myocyte contraction:
1) Under the influence of the action potential, which propagates through the cytolemma, calcium ions are released, enter the myofibrils and initiate a contractile act, which is the result of the interaction of actin and myosin microfilaments; 2) The most common theory at present is the sliding thread model (G. Huxley, 1954). We are supporters of the latter.
Features of the structure of conducting cardiomyocytes. The cells are larger than working cardiomyocytes (length is about 100 µm and thickness is about 50 µm). Cytoplasm contains all organelles general meaning. Myofibrils are few in number and lie along the periphery of the cell. These cardiomyocytes are connected into fibers with each other not only by their ends, but also by their lateral surfaces. The main function of conducting cardiomyocytes is that they perceive control signals from pacemaker elements and transmit information to contractile cardiomyocytes (Fig. 22).
In the definitive state, cardiac muscle tissue does not retain either stem cells or progenitor cells, therefore, if cardiomyocytes die (infarction), they are not restored.
3. The source of development of elements of skeletal striated muscle tissue are myocyte cells. Some of them differentiate in place, while others migrate from myotomes to the mesenchyme. The former participate in the formation of myosymplast, the latter differentiate into myosatellite cells.
The main element of skeletal muscle tissue is muscle fiber, formed by myosymplast and myosatellite cells. The fiber is surrounded by sarcolemma. Since symplast is not a cell, the term “cytoplasm” is not used, but “sarcoplasm” (Greek sarcos - meat). Organelles of general importance are located in the sarcoplasm at the poles of the nuclei. Special organelles are represented by myofibrils.
The mechanism of fiber contraction is the same as in cardiomyocytes.
Inclusions, primarily myoglobin and glycogen, play a major role in the activity of muscle fibers. Glycogen serves as the main source of energy necessary both to perform muscle work and to maintain the thermal balance of the entire body.
Rice. 22. Ultramicroscopic structure of three types of cardiomyocytes: conducting (A), intermediate (B) and working (C) (scheme according to G.S. Katinas)
1 - basement membrane; 2 - cell nuclei; 3 - myofibrils; 4 - plasmalemma; 5 - connection of working cardiomyocytes (intercalated disc); connections between the intermediate cardiomyocyte and the working and conducting cardiomyocytes; 6 - connection of conducting cardiomyocytes; 7 - transverse tubule systems (general purpose organelles are not shown).
Myosatellite cells are adjacent to the surface of the symplast so that their plasmalemmas are in contact. A significant number of satellite cells are associated with one symplast. Each myosatellite cell is a mononuclear cell. The nucleus is smaller than the myosymplast nucleus and more rounded. Mitochondria and the endoplasmic reticulum are evenly distributed in the cytoplasm, the Golgi complex and the cell center are located next to the nucleus. Myosatellite cells are cambial elements of skeletal muscle tissue.
Muscle as an organ. Between the muscle fibers there are thin layers of loose connective tissue - endomysium. Its reticular and collagen fibers intertwine with the fibers of the sarcolemma, which helps to combine forces during contraction. Muscle fibers are grouped into bundles, between which there are thicker layers of loose connective tissue - perimysium. It also contains elastic fibers. The connective tissue surrounding the muscle as a whole is called the epimysium.
Vascularization. The arteries entering the muscle branch in the perimysium. Next to them there are many tissue basophils that regulate the permeability of the vascular wall. Capillaries are located in the endomysium. Venules and veins lie in the perimysium next to the arterioles and arteries. Lymphatic vessels also pass through here.
Innervation. Nerves entering the muscle contain both efferent and afferent fibers. The process of a nerve cell, bringing an efferent nerve impulse, penetrates the basement membrane and branches between it and the plasmolemma of the symplast, participating in the formation of a motor or motor plaque. The nerve impulse releases mediators here, which cause excitation that spreads along the plasmalemma of the symplast.
So, each muscle fiber is innervated independently and is surrounded by a network of hemocapillaries. This complex forms the morphofunctional unit of skeletal muscle - the myon; sometimes the muscle fiber itself is called a myon, which does not correspond to the International Histological Nomenclature.
4. The cells from which striated muscle fibers are formed during embryogenesis are called myoblasts. After a series of divisions, these mononuclear cells, which do not contain myofibrils, begin to merge with each other, forming elongated multinuclear cylindrical formations - microtubules, in which myofibrils and other organelles characteristic of striated muscle fibers appear in due course. In mammals, most of these fibers are formed before birth. During postnatal growth, muscles must become longer and thicker in order to maintain proportionality with the growing skeleton. Their final value depends on the work that falls to their share. After the first year of life, further muscle growth is entirely due to the thickening of individual fibers, i.e., it represents hypertrophy (hyper - over, over, and trophy - nutrition), and not an increase in their number, which would be called hyperplasia (from plasis - formation).
Thus, striated muscle fibers grow in thickness by increasing the number of myofibrils (and other organelles) they contain.
Muscle fibers lengthen as a result of fusion with satellite cells. In addition, in the postnatal period, elongation of myofibrils is possible by attaching new sarcomeres to their ends.
Regeneration. Satellite cells not only provide one of the mechanisms for the growth of striated muscle fibers, but also remain throughout life as a potential source of new myoblasts, the fusion of which can lead to the formation of completely new muscle fibers. Satellite cells are capable of dividing and giving rise to myoblasts after muscle injury and in some dystrophic conditions, when attempts to regenerate new fibers are observed. However, even minor defects in muscle tissue after severe injuries are filled with fibrous tissue formed by fibroblasts.
Growth and regeneration of smooth muscles. Like other types of muscle, smooth muscle responds to increased functional demands by compensatory hypertrophy, but this is not the only response possible. For example, during pregnancy not only the size of smooth muscle cells in the wall of the uterus increases (hypertrophy), but also their number (hyperplasia).
In animals during pregnancy or after the administration of hormones, mitotic figures can often be seen in the muscle cells of the uterus; Therefore, it is generally accepted that smooth muscle cells retain the ability to undergo mitotic division.
Nervous tissue
1. Tissue development.
2. Classification of nerve cells.
3. Neuroglia, its variety.
4. Synapses, fibers, nerve endings.
1. Nervous tissue is a specialized tissue that forms the main integrating system of the body - the nervous system. The main function is conductivity.
Nervous tissue consists of nerve cells - neurons, which perform the function of nervous excitation and conduction of nerve impulses, and neuroglia, which provide support, trophic and protective functions.
Nervous tissue develops from the dorsal thickening of the ectoderm - the neural plate, which during development differentiates into the neural tube, neural ridges (ridges) and neural placodes.
In subsequent periods of embryogenesis, the brain and spinal cord are formed from the neural tube. The neural crest forms sensory ganglia, ganglia of the sympathetic nervous system, melanocytes of the skin, etc. Neural placodes are involved in the formation of the organs of smell, hearing, and sensory ganglia.
The neural tube consists of a single layer of prismatic cells. The latter, multiplying, form three layers: the inner - ependymal, the middle - mantle and the outer - marginal veil.
Subsequently, the cells of the inner layer produce ependymal cells that line the central canal of the spinal cord. The cells of the mantle layer differentiate into neuroblasts, which further turn into neurons and spongioblasts, which give rise to various types neuroglia (astrocytes, oligodendrocytes).
2. Nerve cells (neurocytes, neurons) of various parts of the nervous system are characterized by a variety of shapes, sizes and functional significance. According to their function, nerve cells are divided into receptor (afferent), associative and effector (efferent).
With a wide variety of shapes of nerve cells, a common morphological feature is the presence of processes that ensure their connection as part of reflex arcs. The length of the processes is different and ranges from several microns to 1-1.5 m.
Nerve cell processes are divided into two types based on their functional significance. Some receive nervous excitation and conduct it to the perikaryon of the neuron. They are called dendrites. Another type of processes conducts an impulse from the cell body and transmits it to another neurocyte or to an axon (axos - axis), or neurite. All nerve cells have only one neurite.
Based on the number of processes, nerve cells are divided into unipolar - with one process, bipolar and multipolar (Fig. 23).
The nuclei of nerve cells are large, round or slightly oval, located in the center of the perikaryon.
The cytoplasm of cells is characterized by an abundance of various organelles, neurofibrils, and chromatophilic substances. The surface of the cell is covered with plasmalemma, which is characterized by excitability and the ability to conduct excitation.
Rice. 23. Types of nerve cells (scheme according to T.N. Radostina, L.S. Rumyantseva)
A – unipolar neuron; B - pseudounipolar neuron; B – bipolar neuron; G – multipolar neuron.
Neurofibrils are a collection of fibers and cytoplasmic structures that form a dense plexus in the perikaryon.
Chromatophilic (basophilic) substance is detected in the perikarya of nephrocytes and in their dendrites, but is absent in axons.
Ependymocytes line the cavities of the central nervous system: the ventricles of the brain and the spinal canal. The cells facing the cavity of the neural tube contain cilia. Their opposite poles turn into long processes that support the skeleton of the neural tube tissues. Ependymocytes participate in secretory function, releasing various active substances into the blood.
Astrocytes are either protoplasmic (short-rayed) or fibrous (long-rayed). The former are localized in the gray matter of the CNS (central nervous system). They participate in the metabolism of nervous tissue and perform a delimiting function.
Fibrous astrocytes are characteristic of the white matter of the central nervous system. They form the supporting apparatus of the central nervous system.
Oligodendrocytes are a large group of cells in the central nervous system and PNS (peripheral nervous system). They surround the bodies of neurons, are part of the sheaths of nerve fibers and nerve endings, and participate in their metabolism.
Microglia (glial macrophages) are a specialized system of macrophages that perform a protective function. They develop from mesenchyme and are capable of amoeboid movement. They are characteristic of the white and gray matter of the central nervous system.
4. The processes of nerve cells, together with the neuroglial cells covering them, form nerve fibers. The processes of nerve cells located in them are called axial cylinders, and the oligodendroglial cells covering them are called neurolemmocytes (Schwann cells).
There are myelinated and unmyelinated nerve fibers.
Unmyelinated (non-myelinated) nerve fibers are characteristic of the autonomic nervous system. Lemmocytes adhere tightly to each other, forming continuous strands. The fiber contains several axial cylinders, i.e., processes of various nerve cells. The plasmalemma forms deep folds that form a double membrane - mesaxon, on which the axial cylinder is suspended. With light microscopy, these structures are not detected, which gives the impression of immersion of the axial cylinders directly into the cytoplasm of glial cells.
Myelinated (meaty) nerve fibers. Their diameter ranges from 1 to 20 microns. They contain one axial cylinder - a dendrite or neurite of a nerve cell, covered with a membrane formed by lemmocytes. In the fiber sheath, two layers are distinguished: the inner - myelin, thicker and the outer - thin, containing the cytoplasm and nuclei of lemmocytes.
At the border of two lemmocytes, the sheath of the myelin fiber becomes thinner, and a narrowing of the fiber is formed - a nodal interception (interception of Ranvier). The section of nerve fiber between two nodes is called the internodal segment. Its shell corresponds to one lemmocyte.
Nerve endings differ in their functional significance. There are three types of nerve endings: effector, receptor and terminal apparatus.
Effector nerve endings - these include motor nerve endings of striated and smooth muscles and secretory endings of glandular organs.
The motor nerve endings of striated skeletal muscles - motor plaques - are a complex of interconnected structures of nervous and muscle tissue.
Sensitive nerve endings (receptors) are specialized terminal formations of the dendrites of sensory neurons. There are two large groups of receptors: exteroceptors and interoreceptors. Sensitive endings are divided into mechanoreceptors, chemoreceptors, thermoreceptors, etc. They are divided into free and non-free nerve endings. The latter are covered with a connective tissue capsule and are called encapsulated. This group includes lamellar corpuscles (Vater-Pacini corpuscles), tactile corpuscles (Meissner corpuscles), etc.
Lamellar bodies are characteristic of the deep layers of the skin and internal organs. Tactile corpuscles are also formed by glial cells.
Synapses are specialized contacts between two neurons that provide one-sided conduction of nerve excitation. Morphologically, the synapse is divided into presynaptic and postsynaptic poles, and between them there is a gap. There are synapses with chemical and electrical transmission.
According to the place of contact, synapses are distinguished: axosomatic, axodendritic and axoaxonal.
The presynaptic pole of the synapse is characterized by the presence of synaptic vesicles containing a mediator (acetylcholine or norepinephrine).
The nervous system is represented by sensory and motor cells, united by interneuronal synapses into functionally active formations - reflex arcs. A simple reflex arc consists of two neurons - sensory and motor.
The reflex arcs of higher vertebrates also contain a significant number of associative neurons located between sensory and motor neurons.
A nerve is a bundle of fibers surrounded by a dense perineurium sheath. Small nerves consist of only one fascicle surrounded by an endoneurium. The number and diameter of nerve fibers in a bundle are highly variable. The distal portions of some nerves have more fibers than the more proximal portions. This is explained by the branching of the fibers.
Blood supply to nerves. The nerves are abundantly supplied with vessels that form many anastomoses. There are epineural, interfascicular, perineural and intrafascicular arteries and arterioles. The endoneurium contains a network of capillaries.
Literature
1. Aleksandrovskaya O.V., Radostina T.N., Kozlov N.A. Cytology, histology and embryology.-M: Agropromizdat, 1987.- 448 p.
2. Afanasyev Yu.I., Yurina N.A. Histology.- M: Medicine, 1991.- 744 p.
3. Vrakin V.F., Sidorova M.V. Morphology of farm animals. - M: Agropromizdat, 1991. - 528 p.
4. Glagolev P.A., Ippolitova V.I. Anatomy of farm animals with the basics of histology and embryology. - M: Kolos, 1977. - 480 p.
5. Ham A., Cormack D. Histology. -M: Mir, 1982.-T 1-5.
6. Seravin L.N. Origin of the eukaryotic cell // Cytology. - 1986 / - T. 28.-No. 6-8.
7. Seravin L.N. The main stages of the development of cell theory and the place of the cell among living systems // Tsitology.-1991.-T.33.-No. 12/-C. 3-27.
What do we know about the science of histology? Indirectly, one could become familiar with its main provisions at school. But this science is studied in more detail in higher school(universities) in medicine.
At the level school curriculum we know that there are four types of tissues, and they are one of the basic components of our body. But people who are planning to choose or have already chosen medicine as their profession need to become more familiar with such a branch of biology as histology.
What is histology
Histology is a science that studies the tissues of living organisms (humans, animals and others), their formation, structure, functions and interactions. This section of science includes several others.
How academic discipline this science includes:
- cytology (the science that studies cells);
- embryology (study of the development process of the embryo, features of the formation of organs and tissues);
- general histology (the science of the development, functions and structure of tissues, studies the characteristics of tissues);
- private histology (studies the microstructure of organs and their systems).
Levels of organization of the human body as an integral system
This hierarchy of the object of histology study consists of several levels, each of which includes the next. Thus, it can be visually represented as a multi-level matryoshka doll.
- Organism. This is a biologically integral system that is formed in the process of ontogenesis.
- Organs. This is a complex of tissues that interact with each other, performing their basic functions and ensuring that organs perform basic functions.
- Fabrics. At this level, cells are combined with their derivatives. Types of fabrics are studied. Although they may be composed of a variety of genetic data, their basic properties are determined by the underlying cells.
- Cells. This level represents the main structural and functional unit of tissue - the cell, as well as its derivatives.
- Subcellular level. At this level, the components of the cell are studied - the nucleus, organelles, plasmalemma, cytosol, etc.
- Molecular level. This level is characterized by the study of the molecular composition of cell components, as well as their functioning.
Tissue Science: Challenges
As with any science, histology also has a number of tasks that are performed in the course of the study and development of this field of activity. Among these tasks, the most important are:
- histogenesis study;
- interpretation of the general histological theory;
- studying the mechanisms of tissue regulation and homeostasis;
- study of such cell features as adaptability, variability and reactivity;
- development of the theory of tissue regeneration after damage, as well as methods of tissue replacement therapy;
- interpretation of the device of molecular genetic regulation, creation of new methods, as well as the movement of embryonic stem cells;
- the study of the process of human development in the embryonic phase, other periods of human development, as well as problems with reproduction and infertility.
Stages of development of histology as a science
As you know, the field of studying the structure of tissues is called “histology”. What it is, scientists began to find out even before our era.
Thus, in the history of the development of this area, three main stages can be distinguished - domestic microscopic (until the 17th century), microscopic (until the 20th century) and modern (until today). Let's look at each stage in more detail.
Pre-microscopic period
At this stage, histology in its initial form was studied by such scientists as Aristotle, Vesalius, Galen and many others. At that time, the object of study were tissues that were separated from the human or animal body by dissection. This stage began in the 5th century BC and lasted until 1665.
Microscopic period
The next, microscopic, period began in 1665. Its dating is explained by the great invention of the microscope in England. The scientist used a microscope to study various objects, including biological ones. The results of the study were published in the publication “Monograph”, where the concept of “cell” was first used.
Prominent scientists of this period who studied tissues and organs were Marcello Malpighi, Antonie van Leeuwenhoek and Nehemiah Grew.
The structure of the cell continued to be studied by such scientists as Jan Evangelista Purkinje, Robert Brown, Matthias Schleiden and Theodor Schwann (his photo is posted below). The latter eventually formed which is still relevant today.
The science of histology continues to develop. What it is is currently being studied by Camillo Golgi, Theodore Boveri, Keith Roberts Porter, and Christian Rene de Duve. Also related to this are the works of other scientists, such as Ivan Dorofeevich Chistyakov and Pyotr Ivanovich Peremezhko.
The current stage of development of histology
The last stage of science, studying the tissues of organisms, begins in 1950. The time frame is determined this way because it was then that an electron microscope was first used to study biological objects, and new research methods were introduced, including the use of computer technology, histochemistry and historadiography.
What are fabrics
Let us move directly to the main object of study of such a science as histology. Tissues are evolutionarily evolved systems of cells and non-cellular structures that are united due to the similarity of structure and have common functions. In other words, tissue is one of the components of the body, which is a combination of cells and their derivatives, and is the basis for the construction of internal and external human organs.
Tissue is not made exclusively of cells. The tissue may include the following components: muscle fibers, syncytium (one of the stages of development of male germ cells), platelets, erythrocytes, horny scales of the epidermis (postcellular structures), as well as collagen, elastic and reticular intercellular substances.
The emergence of the concept of “fabric”
The concept of “fabric” was first used by the English scientist Nehemiah Grew. While studying plant tissue at that time, the scientist noticed the similarity of cellular structures with textile fibers. Then (1671) fabrics were described by this concept.
Marie François Xavier Bichat, a French anatomist, in his works further firmly established the concept of tissues. Varieties and processes in tissues were also studied by Alexey Alekseevich Zavarzin (theory of parallel series), Nikolai Grigorievich Khlopin (theory of divergent development) and many others.
But the first classification of tissues in the form in which we know it now was first proposed by German microscopists Franz Leydig and Köliker. According to this classification, tissue types include 4 main groups: epithelial (borderline), connective (support-trophic), muscle (contractile) and nervous (excitable).
Histological examination in medicine
Today, histology, as a science that studies tissue, is very helpful in diagnosing the condition of human internal organs and prescribing further treatment.
When a person is diagnosed with a suspicion of the presence of a malignant tumor in the body, one of the first things to be done is a histological examination. This is, in essence, the study of a tissue sample from the patient’s body obtained by biopsy, puncture, curettage, surgical intervention (excisional biopsy) and other methods.
Thanks to the science that studies the structure of tissues, it helps to prescribe the most correct treatment. In the photo above you can see a sample of tracheal tissue stained with hematoxylin and eosin.
Such an analysis is carried out if necessary:
- confirm or refute a previously made diagnosis;
- establish an accurate diagnosis in cases where controversial issues arise;
- determine the presence of a malignant tumor in the early stages;
- monitor the dynamics of changes in malignant diseases in order to prevent them;
- carry out differential diagnostics of processes occurring in organs;
- determine the presence of a cancerous tumor, as well as the stage of its growth;
- analyze the changes occurring in tissues during the already prescribed treatment.
Tissue samples are examined in detail under a microscope in a traditional or accelerated manner. The traditional method takes longer and is used much more often. In this case, paraffin is used.
But the accelerated method makes it possible to obtain analysis results within an hour. This method is used when there is an urgent need to make a decision regarding the removal or preservation of a patient’s organ.
The results of histological analysis, as a rule, are the most accurate, since they make it possible to study tissue cells in detail for the presence of a disease, the degree of damage to the organ and methods of its treatment.
Thus, the science that studies tissue makes it possible not only to study the suborganism, organs, tissues and cells of a living organism, but also helps to diagnose and treat dangerous diseases and pathological processes in the body.
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