Embryonic period. Embryonic period of development. Stages and stages of the embryonic period How the embryonic period of development begins and ends
Animal ontogeny
Comparison of vertebrate embryos at different stages of embryonic development. An infamous illustration from the work of Ernst Haeckel, in which the differences between embryos are artificially reduced in order to be more consistent with the theory of recapitulation (repetition of phylogeny in ontogeny). It should be noted that the falsification of this illustration does not negate the fact that embryos usually do appear to be more similar to each other than adult organisms, which was noted by embryologists even before the theory of evolution.
Ontogenesis is divided into two periods:
- embryonic - from the formation of the zygote to birth or exit from the egg membranes;
- postembryonic - from exit from the egg membranes or birth to the death of the organism.
Embryonic period
There are three main stages in the embryonic period: cleavage, gastrulation and primary organogenesis. Embryonic, or embryonic, the period of ontogenesis begins from the moment of fertilization and continues until the embryo emerges from the egg membranes. In most vertebrates it includes stages (phases) fragmentation, gastrulation, histo- and organogenesis.
Splitting up
Cleavage is a series of successive mitotic divisions of a fertilized or initiated egg. Cleavage represents the first period of embryonic development, which is present in the ontogenesis of all multicellular animals and leads to the formation of an embryo called a blastula (single-layer embryo). At the same time, the mass of the embryo and its volume do not change, that is, they remain the same as that of the zygote, and the egg is divided into smaller and smaller cells - blastomeres. After each cleavage division, the cells of the embryo become smaller and smaller, that is, the nuclear-plasma relationship changes: the nucleus remains the same, but the volume of the cytoplasm decreases. The process continues until these indicators reach values characteristic of somatic cells. The type of crushing depends on the amount of yolk and its location in the egg. If there is little yolk and it is evenly distributed in the cytoplasm (isolecithal eggs: echinoderms, flatworms, mammals), then crushing proceeds according to the type full uniform: blastomeres are identical in size, the entire egg is crushed. If the yolk is distributed unevenly (telolecithal eggs: amphibians), then crushing proceeds according to the type completely uneven: blastomeres are of different sizes, those containing the yolk are larger, the egg is crushed entirely. With incomplete crushing, there is so much yolk in the eggs that the crushing furrows cannot separate it entirely. The crushing of an egg, in which only the “cap” of cytoplasm concentrated at the animal pole, where the zygote nucleus is located, is crushed, is called incomplete discoidal(telolecithal eggs: reptiles, birds). At incomplete surface crushing in the depths of the yolk, the first synchronous nuclear divisions occur, not accompanied by the formation of intercellular boundaries. The nuclei, surrounded by a small amount of cytoplasm, are evenly distributed in the yolk. When there are enough of them, they migrate into the cytoplasm, where then, after the formation of intercellular boundaries, the blastoderm (centrolecithal eggs: insects) appears.
Gastrulation
One of the mechanisms of gastrulation is invagination (invagination of part of the blastula wall into the embryo) 1 - blastula, 2 - gastrula.
Primary organogenesis
Primary organogenesis is the process of formation of a complex of axial organs. In different groups of animals this process is characterized by its own characteristics. For example, in chordates, at this stage the formation of the neural tube, notochord and intestinal tube occurs.
During further development the formation of the embryo is carried out through the processes of growth, differentiation and morphogenesis. Growth ensures the accumulation of cell mass of the embryo. During the process of differentiation, variously specialized cells arise that form various tissues and organs. The process of morphogenesis ensures that the embryo acquires a specific shape.
Postembryonic development
Links
| Embryogenesis | |
|---|---|
| Biology Embryology Developmental biology | |
| Stages, processes/ result |
Egg cleavage/Zygote, Morula Blastulation/Blastula, Blastocyst Gastrulation (intussusception, epiboly)/Gastrula Neurulation/Neurula Organogenesis/Embryo |
| Germinal leaflets |
Ectoderm Endoderm Mesoderm |
| Differentiation cells |
Blastomer Embryoblast Trophoblast Epiblast Hypoblast |
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See what “Embryonic period” is in other dictionaries:
embryonic period- EMBRYOLOGY OF ANIMALS EMBRYONIC PERIOD, GERMINAL PERIOD – the period in which the embryo turns into a fetus. From a fertilized egg, an organism emerges that has the primitive structures of various organs and systems. Forming and taking their place... General embryology: Terminological dictionary
Embryonic period- (embryonic period). The second period of prenatal development, which lasts from the end of the 2nd week to the end of the 2nd month of pregnancy (counting from the moment of conception). All the major structures and organs of the individual are formed during this period... Developmental psychology. Dictionary by book
EMBRYONAL, embryonic, embryonic (book). adj. to an embryo, which is an embryo, germinal. Embryonic development. || Occurring during the development of the embryo, rudimentary. Embryonic period. Dictionary Ushakova. D.N. Ushakov... Ushakov's Explanatory Dictionary
EMBRYONAL STAGE- in plants that reproduce by seeds, the period of formation of the embryo and seed, from fertilization of the egg to the beginning of seed germination; in vegetatively propagated plants, the period of bud formation in the organs of vegetative propagation, from... ... Dictionary of botanical terms
Male puberty (increase in hair growth) Puberty is the period of puberty. It is individual and may vary to some extent depending on racial, geographic and other conditions. Conventionally, it is considered to be age from... ... Wikipedia
>> Individual development of organisms
Individual development of organisms.
1. Where does the mammalian embryo develop?
2. What is a zygote?
During sexual reproduction, the whole organism begins with one cell- zygote, in asexual reproduction - one cell or several cells of the parent individual. But in any case, in order for a small number of cells to turn into a full-fledged organism, a whole series of complex transformations that change each other is necessary. The process of individual development of an individual from the moment of its formation to the end of life is called ontogeny(from the Greek ontos - existing and genesis - origin).
Ontogenesis is divided into two periods: embryonic (from the Greek embrion - embryo) and postembryonic.
The embryonic period (embryogenesis) lasts from the formation of the zygote to birth(for example, in mammals) or exit from egg membranes (for example, in birds). The postembryonic period begins from the moment of birth and lasts until the end of the individual’s life.
Embryonic period.
In all multicellular organisms, the stages of embryonic development of the embryo are the same, but they can proceed differently. The eggs of some animals contain few nutrients, and the resulting zygote can develop freely. In other animals, the egg is supplied with a huge supply of nutrients compared to its size, and the development of the zygote occurs in a completely different way. An example of such animals are birds.
Let us analyze the embryonic development of the lancelet embryo (Fig. 48). Its first stage is called crushing. fertilized The egg - the zygote - begins to divide by mitosis. The first division occurs in the vertical plane, and the zygote is divided into two identical cells, which are called blastomeres (from the Greek blastos - embryo and meros - part).
The blastomeres do not separate, but divide again, and 4 cells are formed. The third division occurs in the horizontal plane, and from four 8 blastomeres are formed. Further, longitudinal and transverse divisions replace each other, and more and more blastomeres appear. Divisions occur very quickly, blastomeres do not grow, and even - as successive divisions - decrease in size. Gradually, the blastomeres are arranged in one layer and form a hollow ball - a blastula (Fig. 48). The cavity inside the blastula is called the primary body cavity or blastocoel.
At one of the poles of the blastula, the cells of its wall, rapidly dividing mitosis, begin to invaginate into the primary body cavity (Fig. 48). This forms the second, inner layer of embryonic cells. The resulting two-layer ball is called gastrula (from the Greek gaster - stomach) (Fig. 48).
The outer layer of cells is called ectoderm or outer germ layer, and the inner layer is called endoderm or inner germ layer. The cavity formed inside the gastrula is the primary intestine, and the opening leading to the primary intestine is called the primary mouth (Fig. 48). Then, between the endoderm and mesoderm, the third germ layer, the mesoderm, is formed. This stage of the embryo is called neurula. At the neurula stage, formation begins fabrics and organs of the future animal.
The neural plate is formed from the ectoderm (Fig. 48), which subsequently develops into the neural tube. In vertebrates, the spinal cord and brain are formed from the neural tube. The organs of the visual, olfactory and auditory systems, as well as the outer layer of the skin, are also formed from the ectoderm.
Endoderm cells form a tube - the future intestine, and outgrowths of the intestinal primordium subsequently turn into the liver, pancreas and lungs.
Most of the animal body is formed from the third germ layer - the mesoderm. From it the cartilaginous and bone skeleton, kidneys, muscular, reproductive and cardiovascular systems develop.
The entire organism develops from one cell - the zygote, and the cells of all organs and tissues, despite the diversity of structure, contain the same set of genes.
Postembryonic period.
At the moment of birth or the release of the organism from the egg shells, the period of postembryonic development begins.
Postembryonic development can be direct when. from the egg or the mother’s body, a creature appears that is similar to the adult (reptiles, birds, mammals), and indirect, when the larva formed during the embryonic period is simpler in structure than the adult organism, and differs from it in the ways of feeding, movement, etc. (coelenterates, flat And annelids, crustaceans, insects, amphibians).
With direct development, the body grows, some organ systems develop, for example, the reproductive system, etc. Thus, the changes in the body are great, but overall plan its structure and mode of existence do not change.
With indirect development, a larva emerges from the egg: this often gives such organisms some advantages.
For example, in a sedentary toothless bivalve mollusk, a free-swimming larva can attach to the body of the fish with serrated shell valves and thus immediately move to new, distant habitats. In immobile ascidians, the larva itself is capable of moving long distances. As a rule, larvae and adults feed differently and do not compete with each other. For example, a frog tadpole lives in water and feeds on plant foods, while an adult frog lives on land and is a predator.
Butterfly caterpillars most often feed on leaves, while adult butterflies feed on flower nectar or do not feed at all. In some species, the larvae are even able to reproduce independently, for example, some flatworms and amphibians. During the larval period of life, intensive feeding, growth and settlement of animals occurs.
Indirect postembryonic development requires a complex restructuring during the transition to the adult form: some organs must disappear (the tail and gills of the tadpole), others must appear (the limbs and lungs of frogs).
1. How does the embryonic period of development begin and end?
2. How does the postembryonic period of development begin and end?
3. What organ systems are formed from the ectoderm? endoderm? mesoderm?
4. Give examples of animals with direct and indirect development.
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 9th grade
Submitted by readers from the website
The proembryonic period, preceding the formation of the zygote, is associated with the formation of gametes. Otherwise, this is gametogenesis (ovogenesis and spermatogenesis).
The processes characterizing oogenesis lead to the formation of a haploid set of chromosomes and the formation of complex structures in the cytoplasm. r-RNA and mRNA accumulate in the egg, and the yolk accumulates . The type of development of ontogenesis depends on the amount of yolk and its distribution.
Depending on the amount of yolk, there are 4 types of eggs:
1) polylecithales(a lot of yolk; lecitos - gr. yolk);
2) mesolecital(average amount of yolk);
3) oligolecithal(small amount of yolk);
4) alecithal(the yolk is almost absent).
Rice. 1. Types of eggs according to the distribution of the yolk: a – alecithal, b – isolecithal, c – telolecithal, d – centrolecithal.
According to the nature of the distribution of the yolk, eggs are divided into 3 types:
1) isolecithal or homolecithal(with an even distribution of yolk throughout the egg), in terms of the amount of yolk they are often oligo- or alecithal. Examples: eggs of echinoderms, lower chordates, mammals.
2) telolecithal(yolk is concentrated at the vegetative pole; Greek end); Based on the yolk content, these eggs are most often poly- or mesolecithal. Examples: eggs of mollusks, fish, amphibians, reptiles, birds.
3) centrolecithal(the yolk is concentrated in the center of the cell, and the cytoplasm is on the periphery and inside the nucleus). Examples: insect eggs, based on the yolk content these are most often oligo- or mesolecithal eggs.
1.5. Embryonic period
The embryonic period (Greek embryon - embryo) begins with fertilization and the formation of a zygote. The end of this period for different types of ontogenesis is associated with different moments of development.
Embryonic period is divided into the following stages:
1) fertilization - the formation of a zygote;
2) crushing – formation of a blastula;
3) gastrulation – formation of germ layers;
4) histo- and organogenesis - formation organs and tissues of the embryo.
In the larval form of ontogenesis, the embryonic period begins with the formation of the zygote and ends with the emergence from the egg membranes.
In the non-larval form of ontogenesis, the embryonic period begins with the formation of the zygote and ends with the emergence from the embryonic membranes.
In the intrauterine form of ontogenesis, the embryonic period begins with the formation of the zygote and lasts until birth.
1.5.1. Zygote.
Zygote is a single-celled stage of development of a new organism. In the zygote, the stage of two pronuclei and the stage of synkaryon are distinguished. The stage of two pronuclei precedes the synkaryon stage. The sperm has penetrated the egg, but the nuclei of the sperm and the egg have not yet fused. The synkaryon stage is characterized by the fusion of nuclei. As a result of synkaryogamy, the diploid set of chromosomes is restored. After the formation of the synkaryon, the zygote begins to fragment.
Fig.2. Fertilization in mammals. A – sperm penetrates the egg; B – a nucleus was formed from the head of the sperm, and a centriole was formed from the neck. 1 - egg nucleus, 2 - sperm, 3 - receptive tubercle, 4 - centriole, 5 - sperm nucleus.
The individual development of each organism is a continuous process that begins from the formation of the zygote and continues until the death of the organism.
The concept of ontogenesis
Ontogenesis is a cycle of individual development of each organism; it is based on the implementation of hereditary information at all stages of existence. In this case, the influence of environmental factors plays an important role.
Ontogenesis is due to prolonged historical development each specific type. biogenetic law which was formulated by scientists Müller and Haeckel, reflects the relationship between individual and historical development.
Stages of ontogeny
When viewed from a biological perspective, the most significant event in all individual development is the ability to reproduce. It is this quality that ensures the existence of species in nature.
Based on the ability to reproduce, the entire ontogenesis can be divided into several periods.
- Pre-reproductive.
- Reproductive.
- Post-reproductive.
During the first period, the implementation of hereditary information occurs, which manifests itself in structural and functional transformations of the body. At this stage, the individual is quite sensitive to all influences.
The reproductive period realizes the most important purpose of each organism - procreation.
The last stage is inevitable in the individual development of each individual; it is manifested by aging and extinction of all functions. It always ends in the death of the organism.
The pre-reproductive period can still be divided into several stages:
- larval;
- metamorphosis;
- juvenile
All periods have their own characteristics, which manifest themselves depending on the organism’s belonging to a particular species.
Stages of the embryonic period
Taking into account the developmental features and responses of the embryo to damaging factors, all intrauterine development can be divided into the following stages:
The first stage begins with the fertilization of the egg and ends with the implantation of the blastocyst into the lining of the uterus. This occurs approximately 5-6 days after the formation of the zygote.
Crushing period
Immediately after the fusion of the egg with the sperm, the embryonic period of ontogenesis begins. A zygote is formed and begins to fragment. In this case, blastomeres are formed, the more they become in number, the smaller they are in size.
The crushing process does not proceed in the same way among representatives different types. This depends on the amount of nutrients and their distribution in the cytoplasm of the cell. The larger the yolk, the slower the division.
Crushing can be uniform or uneven, as well as complete or incomplete. Humans and all mammals are characterized by complete uneven fragmentation.
As a result of this process, a multicellular single-layer embryo with a small cavity inside is formed; it is called a blastula.
Blastula
This stage ends the first period of embryonic development of the organism. In blastula cells, one can already observe the ratio of nucleus and cytoplasm typical for a particular species.
From this moment on, the cells of the embryo are already called embryonic. This stage is characteristic of absolutely all organisms of any species. In mammals and humans, crushing is uneven due to the small amount of yolk.
In different blastomeres, division occurs at different rates, and one can observe the formation of light cells, which are located along the periphery, and dark cells, which line up in the center.
The trophoblast is formed from light cells; its cells are capable of:
- dissolve tissue, so the embryo has the opportunity to penetrate the wall of the uterus;
- peel off from the embryonic cells and form a vesicle filled with liquid.
The embryo itself is located on the inner wall of the trophoblast.
Gastrulation
After the blastula, the next embryonic period begins in all multicellular organisms - the formation of the gastrula. There are two stages in the gastrulation process:
- the formation of a two-layer embryo consisting of ectoderm and endoderm;
- the appearance of a three-layer embryo, the third germ layer is formed - the mesoderm.
Gastrulation occurs by intussusception, when blastula cells from one pole begin to invaginate. The outer layer of cells is called ectoderm, and the inner layer is called endoderm. The cavity that appears is called the gastrocoel.
The third germ layer, the mesoderm, is formed between the ectoderm and endoderm.
Formation of tissues and organs
The three germ layers formed at the end of the stage will give rise to all the organs and tissues of the future organism. The next embryonic period of development begins.
From the ectoderm develop:
- nervous system;
- leather;
- nails and hair;
- sebaceous and sweat glands;
- sense organs.
The endoderm gives rise to the following systems:
- digestive;
- respiratory;
- parts of the urinary system;
- liver and pancreas.
The third germ layer, the mesoderm, produces the most derivatives; from it the following is formed:
- skeletal muscles;
- gonads and most of the excretory system;
- cartilage tissue;
- circulatory system;
- adrenal glands and gonads.
After the formation of tissues, the next embryonic period of ontogenesis begins - the formation of organs.
Two phases can be distinguished here.
- Neurulation. A complex of axial organs is formed, which includes the neural tube, notochord and intestine.
- Construction of other organs. Individual areas of the body acquire their characteristic shapes and outlines.
Organogenesis ends completely when the embryonic period comes to an end. It is worth noting that development and differentiation continue after birth.
Control of embryonic development
All stages of the embryonic period are based on the implementation of hereditary information received from parents. The success and quality of implementation depends on the influence of external and internal factors.
The scheme of ontogenetic processes consists of several stages.
- Genes receive all the information from neighboring cells, hormones and other factors in order to come into an active state.
- Information from genes for protein synthesis at the stages of transcription and translation.
- Information from protein molecules to stimulate the formation of organs and tissues.
Immediately after the fusion of the egg with the sperm, the first period of embryonic development of the organism begins - fragmentation, which is completely regulated by the information that is in the egg.
At the blastula stage, activation occurs by sperm genes, and gastrulation is controlled by the genetic information of the germ cells.
The formation of tissues and organs occurs due to the information contained in the cells of the embryo. The separation of stem cells begins, which give rise to various tissues and organs.
The formation of external characteristics of the body during the human embryonic period depends not only on hereditary information, but also on the influence of external factors.
Factors influencing embryonic development
All influences that can negatively affect the development of a child can be divided into two groups:
- environmental factors;
- mother's illnesses and lifestyle.
The first group of factors includes the following.
- Radioactive radiation. If such an effect occurred at the first stage of the embryonic period, when implantation has not yet occurred, then most often a spontaneous miscarriage occurs.
- Electromagnetic radiation. Such exposure is possible when you are near operating electrical appliances.
- Impact chemical substances, This includes benzene, fertilizers, dyes, chemotherapy.
The expectant mother can also cause disruption of embryonic development; the following dangerous factors can be mentioned:
- chromosomal and genetic diseases;
- use of drugs, alcoholic beverages, any stages of the embryonic period are considered vulnerable;
- infectious diseases of the mother during pregnancy, for example rubella, syphilis, influenza, herpes;
- heart failure, bronchial asthma, obesity - with these diseases, there may be a disruption in the supply of oxygen to the tissues of the embryo;
- taking medications; the features of the embryonic period are such that the most dangerous in this regard are the first 12 weeks of development;
- excessive addiction to synthetic vitamin preparations.
If you look at the following table, you can see that not only a lack of vitamins is harmful, but also their excess.
| Vitamin name | Dangerous dose of the drug | Developmental disorders |
| A | 1 million IU | Developmental disorders brain, hydrocephalus, miscarriage. |
| E | 1 g | Anomalies in the development of the brain, visual organs, and skeleton. |
| D | 50,000 IU | Skull deformation. |
| K | 1.5 g | Reduced blood clotting. |
| C | 3 g | Miscarriage, stillbirth. |
| B2 | 1 g | Fusion of fingers, shortening of limbs. |
| PP | 2.5 g | Chromosomal mutation. |
| B5 | 50 g | Disturbance in the development of the nervous system. |
| B6 | 10 g | Stillbirth. |
Fetal diseases in the last stages of embryonic development
In the last weeks of development, the child’s vital organs mature and prepare to endure all sorts of disorders that may arise during childbirth.
Before birth, the fetal body creates high level passive immunization. At this stage, various diseases that the fetus can get are also possible.
Thus, despite the child’s practically formed body, some negative factors are quite capable of causing serious disorders and congenital diseases.
Dangerous periods of embryonic development
Throughout embryonic development, periods can be identified that are considered the most dangerous and vulnerable, since at this time the formation of vital organs occurs.
- 2-11 weeks, as the formation of the brain occurs.
- 3-7 weeks - the formation of the organs of vision and heart begins.
- 3-8 weeks - the formation of limbs occurs.
- Week 9 - the belly is filled.
- 4-12 weeks - the formation of the genital organs begins.
- 10-12 weeks - laying of the sky.
The considered characteristics of the embryonic period once again confirm that for fetal development the most dangerous periods are considered to be from 10 days to 12 weeks. It is at this time that all the main organs of the future organism are formed.
Lead a healthy lifestyle, try to protect yourself from the harmful effects of external factors, avoid communicating with sick people, and then you can be almost sure that your baby will be born healthy.
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