Embryonic development of mammals. The main stages of embryo development; characteristics of the stages of embryogenesis. What stages are distinguished in the embryonic development of animals?

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Embryonic development of mammals. The main stages of embryo development; characteristics of the stages of embryogenesis. What stages are distinguished in the embryonic development of animals?

The process of human embryonic development has 4 stages and lasts 8 weeks. It begins from the moment the male and female reproductive cells meet, their fusion and the formation of a zygote, and ends with the formation of an embryo.

What stages does embryogenesis consist of?

After the sperm merges with the egg, education It is she who moves through the fallopian tubes over the course of 3-4 days and reaches the uterine cavity. In this case, a period is observed. It is characterized by strong intensive cell division. At the end of this stage of embryo development blastula is formed- a cluster of individual blastomeres, in the form of a ball.

The third period, gastrulation, involves the formation of the second germ layer, resulting in gastrula is formed. After this, the third germ layer, the mesoderm, appears. Unlike vertebrates, embryogenesis in humans is complicated by the development of the axial complex of organs - the formation of the rudiments of the nervous system, as well as the axial skeleton and with it the muscles, occurs.

At the fourth stage of human embryonic development, isolation of the rudiments of future organs and systems formed at this moment. Thus, from the first germ layer the above-mentioned nervous system and partly the sense organs are formed. From the second endoderm, the epithelial tissue lining the digestive canal and the glands located in it. Connective, cartilage, bone tissue, as well as the vascular system are formed from mesenchyme.

What could cause the sequence of these stages to be disrupted?

The stages of human embryonic development presented in the table below do not always occur in the order in which it is necessary. Thus, under the influence of certain types of factors, mainly exogenous, the course of development of individual organs and systems may be disrupted. Among these reasons are:

What stages does embryogenesis consist of?

After the sperm merges with the egg, zygote formation. It is she who moves through the fallopian tubes over the course of 3-4 days and reaches the uterine cavity. In this case, a period of fragmentation is observed. It is characterized by strong intensive cell division. At the end of this stage of embryo development blastula is formed- a cluster of individual blastomeres, in the form of a ball.

The third period, gastrulation, involves the formation of the second germ layer, resulting in gastrula is formed. After this, the third germ layer, the mesoderm, appears. Unlike vertebrates, embryogenesis in humans is complicated by the development of the axial complex of organs—the formation of the rudiments of the nervous system, as well as the axial skeleton and, along with it, muscles.

What could cause the sequence of these stages to be disrupted?

violation of environmental conditions (temperature, chemicals, etc.);
disruption of interaction between individual cells;
heredity.

These are not all the reasons leading to impaired development of the embryo. There are so many of them that sometimes doctors are unable to determine exactly what in a particular case caused the failure of the embryonic development process. As a result of the fact that the stages of development of a human embryo violate their sequence, anomalies are formed, some of which can lead to the death of the embryo.

Embryonic period of development

In just 2 months, in the process of merging microscopic male and female germ cells, a new person is formed. For 8 weeks it is called an “embryo”, and only after this is it considered a fetus - a miniature copy of a person.

Embryonic development

Embryonic development is the most important stage in the formation of a new life, because during these 2 months the formation of all human organs and systems occurs. Let us take a more specific look at this period of organism development.

When does an embryo's heart start beating?

The period of embryonic development covers a two-month period from the beginning of gestation and by its end, an almost reduced copy of a little man lives in the mother’s womb - all organs and systems are laid down and develop at a rapid pace. When does the heart start beating? This question especially worries mothers.

The first month of pregnancy - what is possible, what is not?

Recommendations for expectant mothers will be very useful from the first days of pregnancy, because a woman will have to change her usual way of life, possibly her diet, and become calm and balanced. Read on for details.

EMBRYONAL DEVELOPMENT

(Greek embryon uterine fetus, embryo) - the stage of individual development of the body from the moment of fertilization to the completion of the basic processes of organogenesis.

In a number of animals, this period continues until the egg membranes are released. As for mammals, embryologists usually refer to their embryonic development as the entire period of intrauterine development and divide it into the embryonic and fetal periods.

In different mammals, the boundaries between these periods occur at different times of development and are defined differently. Sometimes, instead of the concept of “embryonic development,” the term “embryogenesis” is used. In obstetrics, embryonic development refers to the stage before the fetal period, the duration of which in humans corresponds to 8 weeks.

During this period, the main processes of organogenesis occur. Embryonic development in humans in the first 3 days after fertilization occurs in the fallopian tube, then in the uterus.

Based on morphological criteria, several periods are distinguished in embryonic development: the period of a one-cell embryo, or zygote (see), the period of egg fragmentation (see), the period of gastrulation (see), the period of separation of the main rudiments of organs and tissues, the period of organogenesis (see. ) and histogenesis (see).

The foundations for identifying periods of embryonic development were laid by K. M. Baer. The period of the zygote, which is a one-celled embryo formed as a result of the fusion of germ cells of parent organisms (see.

Fertilization in humans and many mammals lasts about 1 day. In this case, active physical and chemical processes occur in the cytoplasm of the zygote, the movement of organelles and inclusions occurs, and the plane of bilateral symmetry is determined.

The period of egg fragmentation proceeds from the division of the zygote into 2 cells (blastomere) until the formation of a single-layer multicellular embryo - blastula (see). In humans, it begins 1 day after fertilization and lasts 6 days. At this time, the embryo moves through the fallopian tube and moves into the uterus.

The fragmentation of the embryo in the fallopian tube occurs at a speed

one division per day. The rate of fragmentation in the uterine cavity increases sharply; in this case, the increase in the number of cells is accompanied by a progressive decrease in their size. In humans and viviparous mammals, the entire material of the zygote is fragmented (complete fragmentation), during which larger and darker blastomeres are separated - embryoblast and small, light blastomeres, overgrowing embryoblast cells - trophoblast (see).

In the crushing embryo, the amount of liquid appears and increases, it takes the form of a vesicle (blastocyst). Embryoblast cells are concentrated at one pole of the blastocyst, trophoblast cells make up its walls.

By the end of the 6th day, at the time of implantation, the embryo is an organism consisting of several hundred cells, the vast majority of which are trophoblasts. The trophoblast differentiates early into specialized epithelial tissue and is the source of the formation of the epithelial cover of the chorionic villi; formed from cells evicted from the embryoblast, the extraembryonic mesoderm (see) forms their connective tissue basis.

The embryoblast flattens, taking the shape of a disk and forming the germinal shield.

The period of gastrulation includes the transformation of a single-layer embryo into a three-layer one. In higher vertebrates and humans, the embryoblast, by delamination (cleavage), first turns into a two-layer formation, consisting of an outer germ layer - the epiblast, containing elements of the ectoderm (q.v.) and mesoderm, and an internal germ layer - the hypoblast, or endoderm (q.v.).

The formation of a two-layer embryo occurs in the 2nd week of embryo development (1st phase of gastrulation). In vertebrates, at the 3rd week of development, the third germ layer, the mesoderm, is formed from the epiblast (2nd phase of gastrulation). The result of gastrulation is the formation of an axial complex of primordia: the neural plate, which subsequently closes into the neural tube, notochord and mesoderm, which from the 4th week is actively divided into somites (see).

During the development process, contacts and interactions arise between embryonic anlages, which determines the determination of their cellular material. Ectoderm, mesoderm, endoderm - the sources of development of all tissues in the process of ontogenesis (see) - are unspecialized cells with basophilic cytoplasm, large nuclei, devoid of specialized organelles, with high mitotic activity, active growth, with the ability for targeted movements.

As part of the germ layers, heterogeneous rudiments of organs and tissues arise, the further development of which continues with different intensities and ends at different times, not even limited to the period of intrauterine development.

In the poorly differentiated cellular material of embryonic rudiments, cell multiplication, their specialization (differentiation), growth, spatial movements of individual cells and cell masses, their close interaction, and changes in biochemical composition occur. At the beginning of development, differences arise in the size and shape of cells of different rudiments, then gradually qualitative changes in structures and metabolic features appear.

In the cells of different primordia, unequal organelles and specific inclusions are formed, and extracellular derivatives are formed (for example, intercellular substance). As a result of differentiation, heterogeneous rudiments, specialized tissues and organs arise and, accordingly, their functional differences. In parallel with the process of differentiation, the process of integration develops and intensifies (the unification of parts of the embryo into one harmoniously developing whole), the degree of which increases as the embryo develops.

Integration is based on the interaction of the parts of the embryo, which becomes more and more perfect as development continues.

First, integration is expressed in the interaction of cells; subsequently, the integrating function is performed by the nervous and endocrine systems. Moreover, at each stage of development, certain components of the histogenesis process (reproduction, growth, migration of cells, intercellular and intertissue interactions - correlations, cell death) may be of primary importance.

The age of the embryo during embryonic development is calculated first in hours, then in days and weeks.

From the moment the mesoderm segmentation begins (20-21 days of development), the age of the embryo is determined by the number of somites, during the period of separation of the embryo from the provisional organs by measuring the length of its body from the crown to the coccyx, and with the development of the limbs - from the crown to the heels (see.

Embryo, Fruit).

During the period of embryonic development, due to the high intensity of metabolism and the high sensitivity of the embryo to various damaging factors (medicines, ionizing radiation, bacterial toxins, etc.), disturbances in the development process (dysembryogenesis) may occur, leading to the occurrence of diseases, malformations and even to the death of the embryo.

The degree of manifestation of various diseases (hereditary and non-hereditary) is also closely related to the conditions in which embryonic development occurs. Diseases of the mother, her use of a number of medications during pregnancy, and unfavorable environmental conditions during this period can have serious consequences for the embryo and manifest themselves both in the postnatal period and in the adult organism; under favorable conditions, pathology may not appear.

In the process of embryonic development, there are time intervals that coincide with the most critical morphogenetic processes, when the embryo is especially sensitive to damaging influences - the so-called critical periods (see.

Antenatal period). This is the period of implantation, corresponding to the end of the 1st and beginning of the 2nd week after conception, and the period of placenta formation, corresponding to the 3-7th weeks of development. Damage to the embryos, especially at this time, can lead to developmental delays, decreased body resistance, and termination of pregnancy.

When individual rudiments are damaged, local anomalies in organ development appear (cleft lip, missing limb, etc.). Due to the asynchrony of differentiation, different organs have their own time periods, unique to them, when they are most sensitive to damaging agents. The earlier the impact of unfavorable factors is noted, leading to deviations from normal development, the more organs and tissues they can manifest themselves in (see.

Antenatal pathology, Hereditary diseases, Developmental defects, Congenital heart defects, Embryopathies, Enzymopathies).

Bibliography: See bibliography. to Art. Germ, Embryology. O. V. Volkova.

0801-0810

801. To restore the ability to reproduce in hybrids bred by distant hybridization,
A) produce polyploid organisms
B) they are propagated vegetatively
C) get heterotic organisms
D) draw clean lines

Animal breeding is practically not used
A) mass selection
B) unrelated crossing
B) inbreeding
D) individual selection

Abstract

Which stage of embryonic development of vertebrates is represented by many unspecialized cells
A) blastula
B) two-layer gastrula
B) three-layered gastrula
D) neurula

Abstract

In flowering plants, the egg is formed from
A) microspores by mitosis
B) pollen grain
B) haploid nucleus of the embryo sac
D) diploid nucleus of the central cell

Animal sperm as opposed to egg
A) contains a lot of proteins and fats in the cytoplasm
B) has a haploid set of chromosomes
B) is formed as a result of mitosis
D) has a large number of mitochondria

Abstract

806. Spores in flowering plants, unlike bacterial spores, are formed in the process
A) adaptation to life in unfavorable conditions
B) mitosis of haploid cells
B) meiosis of diploid cells
D) sexual reproduction

Abstract

The sexual method of reproduction includes the process
A) parthenogenesis in bees
B) budding in yeast
B) spore formation in mosses
D) regeneration in freshwater hydra

Abstract

The rule of uniformity of the first generation will appear if the genotype of one of the parents is aabb, and the other
A) AABb
B) AaBB
B) AABB
D) AaBb

Abstract

809. Mutations differ from modifications in that they
A) are preserved in descendants in the absence of the factor that caused them
B) occur simultaneously in many individuals in a population
C) always have an adaptive character
D) cause a certain variability

Abstract

The loss of a section of a chromosome, in contrast to the crossing of chromatids in meiosis, is
A) conjugation
B) mutation
B) replication
D) crossing over

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EMBRYAL DEVELOPMENT (embryonic development), the development of an animal’s body that occurs inside the egg membranes or in the wall of the uterus of the mother’s body (in mammals, including humans, and some invertebrates - onychophorans). Embryonic development is preceded by the so-called pre-embryonic development, associated with the formation of male and female germ cells until their maturation (see Gametogenesis).

The initial moment of embryonic development is the fertilization of an egg by a sperm or its activation without the participation of a sperm. Following fertilization, fragmentation of the egg begins, as a result of which a multicellular embryo (blastula) is formed, equal in volume to the egg before the start of fragmentation. During the period of fragmentation or after its completion, the genes of the embryo are activated, which were in an inactive state for some time after fertilization.

Upon completion of fragmentation, a period of active movements of cells and entire cell layers (gastrulation) begins, as a result of which the embryos of all animals (except sponges) are dismembered into germ layers.

During gastrulation, at least two germ layers are formed: the outer (ectoderm) and the inner (endoderm), and the primary body cavity (blastocoel) is largely replaced by the cavity of the embryonic intestine (gastrocoel). From the outer leaf, in the course of further development, the integument of the body, the nervous system and sensory organs are formed, and from the inner leaf, the intestines with its derivatives (glands of the digestive tract, and in vertebrates also the lungs).

In addition, most animals (according to some ideas, all animals except sponges) also have a middle germ layer (mesoderm), the formation methods of which are very different. In lower deuterostomes (echinoderms, hemichordates, etc.), the mesoderm is formed during the process of gastrulation from part of the material of the so-called primary gut (enterocoelous anlage of mesoderm), and in many protostomes (annelids, mollusks) - from individual cells (teloblasts) immersed in the primary cavity of the embryo, regardless of gastrulation (teloblastic anlage of mesoderm).

Of these two main methods of laying down the mesoderm, more complex ones emerged in the process of evolution, characteristic, for example, of arthropods and vertebrates. Mesoderm lines the secondary body cavity (coelom).

From it muscles, internal skeleton, circulatory, excretory and reproductive systems develop. During the period of gastrulation, embryos represent integral systems, the individual parts of which interact with each other. Upon completion of gastrulation, the anlage of individual organs form subsystems that are relatively independent from each other.

In vertebrates, the first organ to develop is the central nervous system (CNS), which is then divided into the trunk and head parts - the future spinal cord and brain (see Neurulation).

The central nervous system arises as a result of the so-called primary embryonic induction - the effect of the embryonic notochord on the ectoderm. During the further development of organs, inductive interactions (so-called secondary inductions) continue between their parts.

For example, during the development of the eye, the lens is formed under the inductive influence of the so-called eye cup, from which the retina develops. If the organ includes both epithelial cell layers and freely moving mesenchymal cells (limb buds, glands of the digestive system, hair, teeth, etc.), then the interactions of the epithelium with the mesenchyme are of utmost importance for their full development. In this case, directed, mutually coordinated movements occur, both individual cells and large cell groups.

Functional connections between organs are formed in the process of directed growth of nerve fibers (axons) towards strictly defined target cells. Cell movements and axon growth are completed by the establishment of so-called selective contacts between cells or between nerve endings and are determined by chemical and mechanical factors, and in some cases, electric fields.

Chemical factors act through the mechanism of chemotaxis - creating concentration gradients of certain substances that attract or repel growing axons.

Mechanical factors are associated with the formation of microstructures of the substrate (usually the extracellular matrix) along which cells move. The establishment of selective contacts is carried out by special molecular factors (the so-called cell adhesion molecules).

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Simultaneously with the development of organs, differentiation of the cells included in their composition occurs.

In the embryos of higher animals there are more than 200 different types of differentiated cells (not counting the cells of the immune system that produce antibodies, which are represented by millions of types).

Cell differentiation is associated with the inclusion or, on the contrary, suppression of the activity of large groups of genes. These processes are controlled by chemical and physical signals entering a given cell from other cells of the embryo, from the extracellular matrix, and in some cases from the external environment.

In many embryos, the eggs of which contain a large amount of yolk, as well as in all animals that reproduce in a terrestrial environment, from the very beginning of development, a division occurs into the embryonic part itself, which gives rise to the future organism, and extra-embryonic parts.

In higher vertebrates (amniotes), the extraembryonic parts are represented by embryonic membranes.

Despite the fact that the development of different groups of animals differs greatly from each other, important general patterns can be traced. The first of them was noted by K. M. Baer, who established the law of embryonic similarity, according to which, during embryonic development, the traits of the largest (type) and then smaller and smaller (class, family, etc.) systematic groups first appear.

This law does not apply, however, to the earliest stages of development, which may differ from each other more than the later ones.

Thus, in vertebrates, embryos are most similar to each other at the stage that occurs after the formation of the central nervous system. Another general pattern, which has practically no exceptions, is Driesch's law - the fate of a part of the embryo is a function of its position as a whole.

However, the factors underlying this law continue to be a matter of debate. Great progress has been made in studying the place and time of expression of various groups of genes, as well as intercellular signals during embryo development. It turned out that the set of active genes and intercellular signals in various classes of animals (for example, insects and amphibians), as well as in the same species at different stages of development, is very conservative.

In this case, homologous groups of genes in the embryos of different insect species are expressed on the ventral side of the body, and in vertebrate embryos - on the dorsal side. Similarly, the same groups of genes are involved in primary embryonic induction, in the development of limbs and other anlages. Clarification of the general laws of embryonic development is the most important problem in developmental biology. Progress in this area of knowledge is also relevant for solving applied problems of biotechnology, primarily for the creation of artificial tissues and organs used in medicine.

: Korochkin L.I. Biology of individual development (genetic aspect). M., 2002; Dondua A.K. Developmental biology. St. Petersburg, 2004-2005. T. 1-2; Gilbert S. F. Developmental biology. Sunderland, 2005; Belousov L.V. Fundamentals of general embryology. M., 2005.

Belousov.

The period of embryonic development is the most complex in higher animals and consists of several stages.

The period begins with the stage of zygote fragmentation (Fig. 1), i.e., a series of successive mitotic divisions of the fertilized egg. The two cells formed as a result of division (and all subsequent generations of them) at this stage are called blastomeres. One division follows another, and the resulting blastomeres do not grow and with each division the cells become smaller and smaller.

This feature of cell division determined the appearance of the figurative term “fragmentation of the zygote.”

1. Crushing and gastrulation of the lancelet egg (side view)

The figure shows: a - a mature egg with a polar body; b — 2-cell stage; c — 4-cell stage; d — 8-cell stage; e — 16-cell stage; e - 32-cell stage (in section to show the blastocoel); g - blastula; h — blastula section; and - early gastrula (at the vegetative pole - arrow - intussusception begins); j - late gastrula (intussusception has ended and a blastopore has formed; 1 - polar body; 2 - blastocoel; 3 - ectoderm; 4 - endoderm; 5 - cavity of the primary intestine; 6 - blastopore).

As a result of fragmentation (when the number of blastomeres reaches a significant number), a blastula is formed (see.

rice. 1, g, h). Often it is a hollow ball (for example, in a lancelet), the wall of which is formed by one layer of cells - the blastoderm. The cavity of the blastula, the blastocoel, or primary cavity, is filled with fluid.

At the next stage, the gastrulation process occurs - the formation of the gastrula. In many animals, it is formed by invagination of the blastoderm inward at one of the poles of the blastula during intensive proliferation of cells in this zone.

As a result, a gastrula appears (see Fig. 1, i, j).

The outer layer of cells is called ectoderm, and the inner layer is called endoderm. An internal cavity bounded by endoderm, the cavity of the primary gut communicates with the external environment by the primary mouth, or blastopore. There are other types of gastrulation, but in all animals (except sponges and coelenterates), this process ends with the formation of another cellular layer - mesoderm. It is located between the ento- and ectoderm.

At the end of the gastrulation stage, three cell layers (ecto-, endo- and mesoderm), or three germ layers, appear.

As a result of differentiation of germ layer cells, various tissues and organs of the developing organism are formed. The integument and nervous system are formed from the ectoderm.

Due to the endoderm, the intestinal tube, liver, pancreas, and lungs are formed. The mesoderm produces all other systems: musculoskeletal, circulatory, excretory, reproductive. The discovery of homology (similarity) of the three germ layers in almost all animals served as an important argument in favor of the point of view about the unity of their origin. The patterns outlined above were established at the end of the 19th century. I. I. Mechnikov and A. O. Kovalevsky and formed the basis of the “doctrine of germ layers” formulated by them.

During the embryonic period, there is an acceleration in the rate of growth and differentiation in the developing embryo.

It is only during the process of fragmentation of the zygote that growth does not occur and the blastula (in its mass) may even be significantly inferior to the zygote, but starting from the process of gastrulation, the mass of the embryo rapidly increases.

The formation of different types of cells begins at the stage of fragmentation and underlies primary tissue differentiation - the emergence of three germ layers.

Further development of the embryo is accompanied by an increasingly intensifying process of differentiation and morphogenesis. By the end of the embryonic period, the embryo already has all the main organs and systems that ensure viability in the external environment.

The embryonic period ends with the birth of a new individual capable of independent existence.

Krasnodembsky E.

G. "General biology: A manual for high school students and applicants to universities"

Lecture added 07/18/2012 at 08:37:45

I. Determine the stages of embryonic development of vertebrates.

-: Blastula → cleavage → zygote → gastrula → organ formation

+:zygote→cleavage→blastula→gastrula→organ formation

-: gastrula→cleavage→blastula→zygote→organ formation

-: zygote – cleavage – gastrula – blastula – organ formation

I.What features of a person’s structure are explained by his ability to work?

-: Well-developed facial muscles.

-: Wide chest.

-: Wide, durable basin.

+: Hand with the thumb opposed to the rest.

I. The muscular wall of the left ventricle is 2 times thicker than the muscular wall of the right ventricle, so it ensures the movement of blood:

-: in the pulmonary circulation;

+: in the systemic circulation;

-: from the ventricle to the atrium;

-: into the lungs.

I. Joint labor activity among human ancestors contributed to:

-: appearance of arched foot;

-: upright posture;

+: appearance of speech;

-: freeing your hand.

I.What experiment can be used to prove that low temperature affects the activity of gastric juice enzymes?

+: Pour the protein suspension and gastric juice into two test tubes; Place one of them in the snow, and the second in water with a temperature of 40 °C.

-: Pour the protein suspension and gastric juice into two test tubes and place both in the snow.

-: Pour a suspension of protein and gastric juice into two test tubes, place one in snow, and the other in boiling water.

-: Pour the protein suspension and gastric juice into two test tubes, place one in boiling water, and leave the other at room temperature.

Material taken from the site www.hystology.ru

The characteristics of the development of mammals will cover issues related to the structure of germ cells, fertilization, features of cleavage, gastrula formation, differentiation of germ layers and axial organs, development, structure and function of the fetal membranes (provisional, or temporary, organs).

The subtype of mammals is very diverse in the nature of embryogenesis. The increasing complexity of the structure of mammals, and therefore embryogenesis, necessitates the accumulation of more nutrients in the eggs. At a certain stage of development, this supply of nutrients cannot satisfy the needs of a qualitatively changed embryo, and therefore, in the process of evolution, mammals developed intrauterine development and in most animals of this subtype a secondary loss of yolk is observed by the eggs.

Sex cells. Fertilization. Splitting up. The most primitive mammals are oviparous (platypus, echidna). They have telolecithal eggs, meroblastic cleavage, so their embryogenesis is similar to the development of birds.

In marsupial mammals, the eggs contain a small amount of yolk, but the embryo is born underdeveloped and its further development takes place in the mother's pouch, where a connection is established between the mother's nipple and the baby's esophagus.

Higher mammals are characterized by intrauterine development and nutrition of the embryo at the expense of the mother's body, which is reflected in embryogenesis. The eggs have almost completely lost their yolk for the second time; they are considered secondary oligolecithal, isolecithal. They develop in the follicles (folliculus - sac, vesicle) of the ovary. After ovulation (rupture of the follicle wall and release of the egg from the ovary), they enter the oviduct.

Fertilization of the egg occurs in the upper part of the oviduct. In this case, the membranes of the egg are destroyed under the influence of the enzymes of the sperm acrosome.

Cleavage in higher mammals is complete, asynchronous: an embryo is formed, consisting of 3, 5, 7, etc. blastomeres. The latter usually lie in the form of a bunch of cells. This stage is called morula (Fig. 62). Two types of cells are distinguishable in it: small - light and large - dark. Light cells have the greatest mitotic activity. Dividing intensively, they are located on the surface of the morula in the form of an outer layer of trophoblast (trophe - nutrition, blastos - sprout). Dark blastomeres divide more slowly, so they are larger than light blastomeres and are located inside the embryo. The embryoblast is formed from dark cells.

The trophoblast performs a trophic function. It provides the embryo with nutritional material, since with its participation the connection between the embryo and the wall of the uterus is established. The embryoblast is the source of development of the body of the embryo and some of its extraembryonic organs.

If several babies are born to animals, then several eggs enter the oviduct at once.

Splitting, the embryo moves along the oviduct towards the uterus (Fig. 63, 64). The trophoblast absorbs the secretion of the glands. It accumulates between the embryoblast and trophoblast. The embryo greatly increases in size and turns into a blastoderm vesicle, or blastocyst (Fig. 65). The wall of the blastocyst is the trophoblast, and the embryoblast has the appearance of a bunch of cells and is called the germinal nodule.

Rice. 62. Scheme of crushing a mammal egg:

1 - shiny shell; 2 - polar bodies; 3 - blastomeres; 4 - light blastomeres forming trophoblast; 5 - dark blastomeres; 6 - trophoblast; 7 - germinal nodule.

Rice. 63. Scheme of movement of a splitting cow zygote along the oviduct.

The cavity of the blastocyst is filled with fluid. It was formed as a result of the absorption of uterine gland secretions by trophoblast cells. Initially, the blastocyst is free in 6h uterine cavity. Then, with the help of villi formed on the surface of the trophoblast, the blastocyst attaches to the wall of the uterus. This process is called implantation (im - penetration into, plantatio - planting) (Fig. 66). In cattle, implantation occurs on the 17th day, in the horse on the 63rd - 70th day, in the macaque - on the 9th day after fertilization. Then the cells of the germinal node line up in the form of a layer - a germinal disk is formed, similar to the germinal disk of birds. In its middle part, a compacted zone is differentiated - the embryonic shield. As in birds, the body of the embryo develops from the material of the embryonic shield, and the rest of the embryonic disk is used in the formation of provisional organs.

Thus, despite the fact that in higher mammals, due to the secondary loss of yolk, the eggs are oligolecithal with holoblastic cleavage, the structure of the blastula is similar to that which is formed after meroblastic cleavage. This can be explained by the fact that the predecessors of mammals had polylecithal, telolecithal eggs, and higher mammals inherited the structure of the blastula from their ancestors, the latter reminiscent of the blastula of birds.

Gastrulation. Formation of axial organs and their differentiation. Gastrulation occurs in the same way as in reptiles, birds, and lower mammals. By delamination of the germinal disc, ectoderm and endoderm are formed. If these leaves were formed from the material of the germinal scutellum, then they are called germinal, and if they arose from the non-embryonic zone of the germinal disc, then they are not germinal. Non-embryonic ectoderm and endoderm grow along the inner surface of the trophoblast. Soon the trophoblast located above the embryo is resorbed and the latter ends up lying for some time in the uterine cavity, uncovered.

Rice. 64. Scheme of ovulation, fertilization, crushing, implantation:

1 - primordial follicles; 2 - growing follicles; 3, 4 - vesicular follicles; 5 - ovulated egg; 6 - collapsed vesicular follicle; 7 - yellow body; 8 - fimbriae of the oviduct funnel; 9 - the egg at the moment of sperm penetration into it; 10 - sperm; 11 - zygote, pronuclei bringing together; 12 - zygote in metaphase; 13 - splitting up; 14 - morula; 15 - blastocyst; 16 - implantation.

The formation of mesoderm proceeds in the same way as in birds. The cells of the marginal zone of the discoblastula migrate in two streams to the posterior part of the embryo. Here these flows meet and change their direction of movement. Now they move forward in the center of the germinal disk, forming the primary streak with a longitudinal depression - the primary groove. At the anterior end of the primary stripe, a Hensen's node with a depression - the primary fossa - is formed. In this zone, the material of the future notochord is tucked in and grows forward between the ectoderm and endoderm in the form of a head (chordal) process (Fig. 67).

Mesoderm develops from the cells of the primitive streak. After migration, its material grows between the ectoderm and endoderm and turns into segmented mesoderm (somites), adjacent segmental legs and unsegmented mesoderm. Somites consist of a sclerotome (ventromedial part), a dermotome (lateral part), and a myotome (medial part). Somites can connect to unsegmented mesoderm through segmental stalks. The unsegmented part of the mesoderm has the appearance of a hollow sac. Its outer wall is called the parietal layer, and the inner wall is called the visceral layer. The cavity enclosed between them is called the secondary body cavity, or coelom (Fig. 68).

Rice. 65. Fragmentation of the zygote and formation of the pig blastocyst:

A - G- successive stages of crushing (black- - blastomeres, from which the body of the embryo will develop; white- blastomeres from which the trophoblast will develop); D- blastocyst; E - AND- development of the germinal disc and formation of endoderm; TO- formation of mesoderm and primary gut from endoderm; 1 - germinal nodule; 2 - trophoblast; 3 - blastocoel; 4 - shiny zone; 5 - endoderm cells; 6 - endoderm; 7 - germinal disc; 8 - ectoderm of the germinal disc; 9 - trophectoderm; 10 - mesoderm; 11 - primary gut (wall) (according to Patten).

Rice. 66. Macaque embryo at the age of 9 days at the time of implantation:

1 - embryoblast; 2 - part of the trophoblast that penetrates into the tissue of the uterus; 3 - 5 - uterine tissue (3 - epithelium, 4 - basis of the mucous membrane; 5 - gland in a state of dystrophy) (according to Vislotsky, Streeter).

The differentiation of the germ layers proceeds in the same way as in birds and other animals. On the dorsal part of the embryo, a neural plate is formed in the ectoderm; after its edges fuse, the neural tube is formed. The ectoderm grows on it, so very soon the neural tube becomes submerged under the ectoderm. The entire nervous system develops from the neural tube, and the superficial layer of skin (epidermis) develops from the ectoderm. The notochord does not function as an organ in adult animals. It is completely replaced by the vertebrae of the spinal column. Somite myotomes are the source of the formation of the trunk muscles, and sclerotomes are the mesenchyme, from which bone and cartilage tissue then develop. Derma-tom - the rudiment of the deep layers of the skin

Rice. 67. Rabbit embryo, top view:

1 - head process; 2 - Hensen's knot; 3 - primary fossa; 4 - primary stripe.

Rice. 68. Cross section of a mammalian embryo at the 11-segment stage. Visible connection with the uterus:

1 - uterine glands; 2 - visceral and 3 - parietal layers of mesoderm; 4 - myotome; 5 - aorta; 6 - intraembryonic coelom; 7 - extraembryonic coelom; S- endoderm of the yolk sac; 9 - chorionic villi; 10 - trophoblast; 11 - ectoderm.

cover. The urinary and reproductive systems are formed from the material of the segmental legs, which is why it is called nephrogonadotomy.

The superficial tissue (epithelium) of the parietal layer of the pleura and peritoneum is formed from the parietal layer of the splanchnotome, and the epithelium of the serous membranes of those organs that lie in the thoracic and abdominal cavities is formed from the visceral layer.

From the endoderm, epithelium develops, covering the inner surface of the digestive tube and organs - derivatives of the digestive tube: respiratory organs, liver, pancreas.

Thus, the development of germ layers and their further differentiation in mammals is similar to those in other animals. These signs are the most ancient; they reflect the path that mammals have traveled in their development. Such characteristics are classified as palingenetic (palin - again, genesis - birth) in contrast to coenogenetic, that is, acquired in connection with changes in living conditions, for example, the emergence of animals from water to land.

Not only the permanent organs of the embryo develop from the germ layers - ectoderm, endoderm and mesoderm. They participate in the laying of temporary, or provisional, organs - the membranes.

Formation of extraembryonic (temporary) organs(Fig. 69). One of the features of the development of mammals is considered to be that during the isolecithal egg cell and holoblastic fragmentation, the formation of temporary organs occurs. As is known, in the evolution of chordates, provisional organs are the acquisition of vertebrates with telolecithal, polylecithal eggs and meroblastic cleavage.

Rice. 69. 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); IN- formation of a closed endodermal vesicle (4); IN - the beginning of the formation of the amniotic fold (5) and intestinal philtrum (6); G- separation of the body of the embryo (7); yolk sac (8); D- closure of amniotic folds (9); beginning of 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).

Another feature of the development of mammals is the very early separation of the embryonic from the non-embryonic part. Thus, already at the beginning of crushing, blastomeres are formed, forming an extra-embryonic auxiliary membrane - the trophoblast, with the help of which the embryo begins to receive nutrients

Rice. 70. Diagram of the relationship between the uterus and the yolk sac in a rabbit:

1 - allantoic placenta; 2 - yolk sac; 3 - wall of the uterus; 4 - amnion.

substances from the uterine cavity. After the formation of the germ layers, the trophoblast located above the embryo is reduced. The unreduced part of the trophoblast, merging with the ectoderm, forms a single layer. Adjacent to this layer on the inner side, sheets of unsegmented mesoderm and extraembryonic ectoderm grow.

Simultaneously with the formation of the embryo's body, the development of the fetal membranes occurs: the yolk sac, amnion, chorion, allantois.

The yolk sac, as in birds, is formed from the extraembryonic endoderm and the visceral layer of mesoderm. Unlike birds, it does not contain yolk, but a protein liquid. Blood vessels form in the wall of the yolk sac. This membrane performs hematopoietic and trophic functions. The latter comes down to the processing and delivery of nutrients from the mother’s body to the embryo (Fig. 70,71). The duration of yolk sac function varies from animal to animal.

As in birds, in mammals the development of membranes begins with the formation of two folds - the trunk and the amniotic. The trunk fold lifts the embryo above the yolk sac and separates its embryonic part from the non-embryonic part, and the embryonic endoderm closes into the intestinal tube. However, the intestinal tube remains connected to the yolk sac by a narrow vitelline stalk (duct). The tip of the trunk fold is directed under the body of the embryo, while all the germ layers bend: ectoderm, unsegmented mesoderm, endoderm.

The formation of the amniotic fold involves the trophoblast, fused with the extraembryonic ectoderm and the parietal layer of mesedermis. The amniotic fold has two parts: internal and external. Each of them is built from leaves of the same name, but differs in the order of their arrangement. So, the inner layer of the inner part of the amniotic fold is the ectoderm, which in the outer part of the amniotic fold will be on the outside. This also applies to the sequence of occurrence of the parietal layer of mesoderm. The amniotic fold is directed above the body of the embryo. After its edges have fused, the embryo becomes surrounded by two membranes at once - the amnion and the chorion.

Rice. 71. Scheme of migration of primary germ cells from the yolk sac to the gonad primordium (different stages of migration are conventionally plotted on the same cross section of the embryo):

1 - epithelium of the yolk sac; 2 - mesenchyme; 3 - vessels; 4 - primary kidney; 5 - gonad primordium; 6 - primary germ cells; 7 - rudimentary epithelium.

The amnion develops from the inner part of the amniotic fold, the chorion - from the outer part. The cavity that forms around the embryo is called the amniotic cavity. It is filled with a transparent watery liquid, in the formation of which the amnion and the embryo take part. Amniotic fluid protects the embryo from excessive loss of water, serves as a protective environment, softens shocks, creates the possibility of embryo mobility, and ensures the exchange of amniotic fluid. The amnion wall consists of extraembryonic ectoderm directed into the amnion cavity and the parietal layer of mesoderm located outside the ectoderm.

The chorion is homologous to the serosa of birds and other animals. It develops from the outer part of the amniotic fold, and is therefore built from a trophoblast connected to the ectoderm and a parietal layer of mesoderm. On the surface of the chorion, processes are formed - secondary villi, growing into the wall of the uterus. This zone is greatly thickened, abundantly supplied with blood vessels and is called the baby's place, or placenta. The main function of the placenta is to supply the embryo with nutrients, oxygen and free its blood from carbon dioxide and unnecessary metabolic products. The flow of substances into and out of the blood of the embryo is carried out diffusely or through active transfer, that is, with the cost of this process

Rice. 72. Scheme of relationships between organs in the fetus of animals with epitheliochorial type of placentation:

1 - allanto-amnion; 2 - allanto-chorion; 3 - chorionic villi; 4 - cavity of the urinary sac; 5 - amnion cavity; 6 - yolk sac.

energy. However, it should be noted that the mother’s blood does not mix with the blood of the fetus either in the placenta or in other parts of the chorion.

The placenta, being an organ of nutrition, excretion, and respiration of the fetus, also performs the function of an organ of the endocrine system. Hormones synthesized by the trophoblast and then by the placenta ensure the normal course of pregnancy.

There are several types of placenta based on their shape.

1. Diffuse placenta (Fig. 72) - its secondary papillae develop over the entire surface of the chorion. It is found in pigs, horses, camels, marsupials, cetaceans, and hippopotamus. Chorionic villi penetrate the glands of the uterine wall without destroying the uterine tissue. Since the latter is covered with epithelium, according to its structure this type of placenta is called epitheliochorial, or hemiplacenta (Fig. 73). The embryo is nourished in the following way - the uterine glands secrete royal jelly, which is absorbed into the blood vessels of the chorionic villi. During childbirth, the chorionic villi move out of the uterine glands without tissue destruction, so there is usually no bleeding.

2. Cotyledon placenta (Fig. 74) - 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. In this zone, the epithelium of the uterine wall dissolves and the cotyledons are immersed in a deeper (connective tissue) layer of the uterine wall. Such a placenta is called desmochorial and is characteristic of artiodactyls. According to some scientists, ruminants also have an epitheliochorionic placenta.

3. Belt placenta (Fig. 75). The zone of chorionic villi in the form of a wide belt surrounds the amniotic sac. The connection between the embryo and the uterine wall is closer: the chorionic villi are located in the connective tissue layer of the uterine wall, in contact with the endothelial layer of the blood vessel wall. This. The placenta is called endotheliochorionic.

4. Discoidal placenta. The contact area between the chorionic villi and the uterine wall has the shape of a disc. The chorionic villi are immersed in blood-filled lacunae lying in the connective tissue layer of the uterine wall. This type of placenta is called hemochorionic and is found in primates.

Allantois is an outgrowth of the ventral wall of the hindgut. Like the intestine, it consists of endoderm and a visceral layer of mesoderm. In some mammals, nitrogenous metabolic products accumulate in it, so it functions like a bladder. In most animals, due to the very early development of the embryo with the maternal organism, the allantois is developed much less well than in birds. Blood vessels from the embryo and placenta pass through the wall of the allantois. After blood vessels grow into the allantois, the latter begins to take part in the metabolism of the embryo.

The junction of the allantois with the chorion is called the chorioallantois or allantoic placenta. The embryo is connected to the placenta through the umbilical cord. It consists of a narrow duct of the yolk sac, allantois and

Rice. 73. Scheme of placentas:

A- epitheliochorial; b- desmochorial; V- endotheliochorial; G- hemochorial; 1 - chorion epithelium; 2 - epithelium of the uterine wall; 3 - connective tissue of the chorionic villi; 4 - connective tissue of the uterine wall; 5 - blood vessels of the chorionic villi; 6 - blood vessels of the uterine wall; 7 ~ maternal blood.

Rice. 74 Amniotic sac with the fetus of a cow at the age of 120 days:

1 - cotyledons; 2 - umbilical cord.

blood vessels. In some animals, the Et yolk sac is associated with the placenta. This type of placenta is called yolk placenta.

Thus, the duration of embryogenesis varies in different placental animals. It is determined by the maturity of the birth of the babies and the nature of the connection between the embryo and the mother’s body, that is, the structure of the placenta.

Embryogenesis of farm animals proceeds similarly and differs from primates. These developmental features will be briefly discussed below.

In obstetric practice, intrauterine development is divided into three periods: embryonic (fetal), prefetal and fetal. The embryonic period is characterized by the development of characteristics typical of all vertebrates and mammals. During the prefetal period, the characteristics characteristic of this family are laid down. During the fertile period, species, breed and individual structural features develop.

In cattle, the duration of intrauterine development is 270 days (9 months). According to G. A. Schmidt, the germinal (embryonic) period lasts the first 34 days, the pre-fertal period - from the 35th to the 60th day, the fetal period - from the 61st to the 270th day.

During the first week, the zygote is fragmented and the trophoblast is formed. The embryo is nourished by the yolk of the egg. In this case, oxygen-free breakdown of nutrients occurs.

From the 8th to the 20th day is the stage of development of the germ layers, axial organs, amnion and yolk sac (Fig. 76). Nutrition and respiration are carried out, as a rule, with the help of trophoblast.

On the 20th - 23rd day, the trunk fold develops, the digestive tube and allantois are formed. Nutrition and respiration occur with the participation of blood vessels.

24 - 34 days - the stage of formation of the placenta, chorion cotyledons, and many organ systems. Nutrition and respiration of the embryo

Rice. 75. Zonar (belt) placenta of carnivorous animals.

Rice. 76. Cow embryo at the stage of closure of the neural tube ridges (age 21 days):

1 - neural plate; 2 - general structures of skeletal muscles and skeleton; 3 - laying of the allantois.

Rice. 77. Cross section of a 15-day-old primate embryo at the level of the primitive streak:

1 - plasmodiotrophoblast; 2 - cytotrophoblast; 3 - connective tissue of the chorion; 4 - amniotic leg; 5 - amnion ectoderm; 6 - outer layer of the embryonic shield; 7 - mitotically dividing cell; 8 - endoderm; 9 - mesoderm of the primitive streak; 10 - amniotic cavity; 11 - cavity of the yolk sac.

carried out through the vessels of the allantois connected to the trophoblast.

35 - 50 days - early pre-fetal period. During this period, the number of cotyledons increases, the cartilaginous skeleton and mammary gland are formed.

50 - 60 days - the late pre-fetal period, characterized by the formation of the bone skeleton, the development of signs of the animal's sex.

Rice. 78. Scheme of a sagittal section of a 3-week human embryo:

1 - cutaneous ectoderm; 2 - amnion ectoderm; 3 - amnion mesoderm; 4 - intestinal endoderm; 5 - vitelline endoderm; 6 - chord; 7 - allantois; 8 - rudiments of the heart; 9 - blood islands; 10 - amniotic leg; 11 - chorion; 12 - chorionic villi.

61 - 120 days - early fetal period: development of breed characteristics.

121 - 270 days - late fetal period: formation and growth of all organ systems, development of individual structural features.

In other species of farm animals, the periods of intrauterine development have been studied in less detail. In sheep, the embryonic period occurs during the first 29 days after fertilization. The prefetal period lasts from the 29th to the 45th day. Then comes the fertile period.

The duration of the periods of intrauterine development of pigs differs from cattle and sheep. The embryonic period lasts 21 days, the prefertal period lasts from the 21st day to the beginning of the second month, and then the fertile period begins.

Embryogenesis of primates is characterized by the following features: there is no correlation in the development of the trophoblast, extraembryonic mesoderm and embryo; early formation of the amnion and yolk sac; thickening of the trophoblast lying above the embryoblast, which helps to strengthen the connection between the embryo and the maternal body.

Trophoblast cells synthesize enzymes that destroy uterine tissue and the germinal vesicle, plunging into them, comes into contact with the mother’s body.

From the expanding endoderm, which is formed by delamination of the embryoblast, the yolk vesicle is formed. The ectoderm of the embryoblast splits. In the cleavage zone, a first insignificant and then rapidly enlarging cavity is formed - the amniotic sac (Fig. 77).

The area of the embryoblast bordering the vitelline and amniotic sacs thickens and becomes a two-layer embryonic shield. The layer facing the amniotic sac is the ectoderm, and the layer facing the yolk sac is the endoderm. In the embryonic shield, the primary streak with Hensen's node is formed - the sources of development of the notochord and mesoderm. The outside of the embryo is covered with trophoblast. Its inner layer is the extraembryonic mesoderm, or the so-called amniotic leg. The allantois is located here. The latter also develops from the intestinal endoderm. The vessels of the allantois wall connect the embryo with the placenta (Fig. 78).

Further stages of embryogenesis in primates proceed in the same way as in other mammals.

The most primitive mammals are oviparous (platypus, echidna). They have telolecithal eggs, meroblastic cleavage, so their embryogenesis is similar to the development of birds. In marsupial mammals, the eggs contain a small amount of yolk, but the embryo is born underdeveloped and its further development takes place in the mother's pouch, where a connection is established between the mother's nipple and the baby's esophagus. Higher mammals are characterized by intrauterine development and nutrition of the embryo at the expense of the mother's body, which is reflected in embryogenesis. The eggs have almost completely lost their yolk for the second time; they are considered secondary oligolecithal, isolecithal. They develop in the follicles (folliculus - sac, vesicle) of the ovary. After ovulation (rupture of the follicle wall and release of the egg from the ovary), they enter the oviduct.

Mammalian eggs are microscopic in size. Their diameter is 100 - 200 microns. They are covered with two shells - primary and secondary. The first is the plasmalemma of the cell. The second layer is the follicular cells. The wall of the follicle is built from them, where the eggs are located in the ovary. Fertilization of the egg occurs in the upper part of the oviduct. In this case, the membranes of the egg are destroyed under the influence of the enzymes of the sperm acrosome. After internal fertilization, it usually takes longer for the first two blastomeres to form because a more complex process of differentiation in the zygote (in humans, up to 28 hours). As a result of differentiation, material moves inside the zygote, fields are formed, from which certain rudiments will be formed in the future.

After the formation of the first cleavage furrow, two blastomeres are formed, which differ in size and contrast (one dark, the other light). One of the blastomeres contains the material of the trophoblast, the future provisional organ, and it is more homogeneous, while the other blastomere contains the material of the future embryoblast, so it is more complex in composition. Light blastomeres fragment faster than dark ones and begin to overgrow them. Therefore, with subsequent fragmentation, not 4 blastomeres are formed, but 3, then 5, 1. i.e. blastomeres are fragmented unevenly, and this type of fragmentation is called asynchronous. As a result of crushing, an embryo is formed in the form of a dense nodule - a steroblastula (at this moment it does not yet have a cavity).

Cleavage in higher mammals is complete, asynchronous: an embryo is formed, consisting of 3, 5, 7, etc. blastomeres. The latter usually lie in the form of a bunch of cells. This stage is called morula. Two types of cells are distinguishable in it: small - light and large - dark. Light cells have the greatest mitotic activity. Dividing intensively, they are located on the surface of the morula in the form of an outer layer of trophoblast (trophe - nutrition, blastos - sprout). Dark blastomeres divide more slowly, so they are larger than light blastomeres and are located inside the embryo. The outer cells are lighter in color and form the trophoblast. The inner cells are darker and form the embryoblast.

Because the embryo does not have nutritional material, then the trophoblast cells, moving along the genital tract, begin to secrete enzymes and break down the mucus of the genital tract and absorb it. As a result, products of this cleavage appear inside the embryo, which gradually push away the embryoblast material - a small cavity appears and from this time the embryo takes the form of a vesicle - a blastocyst. It is suspended, and the cavity increases, and the embryoblast cells seem to float above the cavity at its upper pole.

Only after this stage in higher mammals do changes begin to occur in the internal cells of the embryo, i.e. in the embryoblast. Its cells are split into 2 plates (gastrulation by delamination), the inner plate is endoderm, and the outer plate is ectoderm and mesoderm. The trophoblast above the embryo is resorbed and this area occupies the outer germinal layer.

Late gastrulation

Gastrulation occurs in the same way as in reptiles, birds, and lower mammals. By delamination of the germinal disc, ectoderm and endoderm are formed. If these leaves were formed from the material of the germinal scutellum, then they are called germinal, and if they arose from the non-embryonic zone of the germinal disc, then they are not germinal. Non-embryonic ectoderm and endoderm grow along the inner surface of the trophoblast. Soon the trophoblast located above the embryo is resorbed and the latter ends up lying for some time in the uterine cavity, uncovered. In the anterior section of the embryonic shield, blastomeres are intensively formed, which begin to move to the posterior section, forming the primary streak, primary node, and the putative material of the notochord and neural plate. Next comes the formation of mesoderm, notochord and neural tube.

Then the torso fold is formed; The amniotic fold forms with the formation of the amnion and the creation of an aqueous environment for the development of the embryo. A yolk sac is formed that does not contain yolk, so instead of a trophic function it performs a hematopoietic and reproductive one. The allantois, which has lost its excretory function, also forms from the caudal part of the intestinal tube.

The trophoblast forms the villi. The parietal mesoderm grows towards it, which penetrates into the villi of the trophoblast and blood vessels are formed in it. From this moment, the trophoblast turns into chorion, the villi of which penetrate into the mucous membrane of the uterus and together with it form the placenta - a new provisional organ.

Features of the development of mammals are the early development of the trophoblast, and its subsequent transformation into the chorion. Also new is the formation of the placenta (the avian analogue is the serosa). Thus, in all mammals, gastrulation is divided into two stages. The first is almost hidden, but as a result of it, extra-embryonic material is evicted, which is used to build extra-embryonic organs. The second stage is gastrulation itself.

Formation of extraembryonic (temporary) organs

One of the features of the development of mammals is considered to be that during the isolecithal egg cell and holoblastic fragmentation, the formation of temporary organs occurs. As is known, in the evolution of chordates, provisional organs are the acquisition of vertebrates with telolecithal, polylecithal eggs and meroblastic cleavage.
Another feature of the development of mammals is the very early separation of the embryonic from the non-embryonic part. Thus, already at the beginning of crushing, blastomeres are formed, forming an extra-embryonic auxiliary membrane - the trophoblast, with the help of which the embryo begins to receive nutrients from the uterine cavity. After the formation of the germ layers, the trophoblast located above the embryo is reduced. The unreduced part of the trophoblast, merging with the ectoderm, forms a single layer. Adjacent to this layer on the inner side, sheets of unsegmented mesoderm and extraembryonic ectoderm grow.

Simultaneously with the formation of the embryo's body, the development of the fetal membranes occurs: the yolk sac, amnion, chorion, allantois. The yolk sac, as in birds, is formed from the extraembryonic endoderm and the visceral layer of mesoderm. Unlike birds, it does not contain yolk, but a protein liquid. Blood vessels form in the wall of the yolk sac. This membrane performs hematopoietic and trophic functions. The latter comes down to the processing and delivery of nutrients from the mother’s body to the embryo. The duration of yolk sac function varies from animal to animal.

Types of placentas

During the development of the embryo in a mammal, certain contacts between the fetus and maternal tissues occur, i.e. The mother-fetus system is formed and this connection is carried out through the provisional organ - the placenta.

The placenta has undergone changes in the process of evolution. In birds this was a serous membrane. In lower mammals, this is a trophoblast, which, as it matures, turns into the chorion and then into the placenta. Contact with the maternal tissues of the chorion is different, therefore there are four main types of placenta.

1. In lower animals (in pigs), the chorionic villi are in contact with the entire surface of the uterine mucosa and directly with its epithelium, and this type of placenta is called epitheliochoriapic. In this case, the epithelium of the uterine mucosa is not destroyed. Anatomically, such a placenta is called diffuse, because. The entire mucous membrane is involved and the villi are arranged one at a time.

2. Ruminants have a desmochorionic type of placenta. Here the chorionic villi come into contact with the connective tissue, growing into the epithelium, which is destroyed. The connection is more complex, stronger and closer. The villi spread in the form of cotyledons (clusters), and not diffusely, therefore such a placenta is anatomically called cotyledonous (multiple).

3. The third type of placenta is endotheliochorionic. Found in predators. The chorionic villi grow to the endothelium of the blood capillaries, partially destroying the walls of the blood vessels. The contact with the maternal body is even closer; the villi are concentrated in the form of a belt, occupying part of the endometrium. This type of placenta is anatomically called cingulum.

4. In primates and rodents, hemochorial type of placenta is found. The chorionic villi come into contact with maternal blood. During the formation of the placenta, the epithelium is destroyed, then grows into the connective tissue and destroys it, and the blood vessels are also destroyed. Blood leaves the blood vessels, and lacunae (lakes) are formed with which the villi come into contact. This is a more advanced form of placenta. The villi are already located in a small area, forming the placenta in the form of a disk or cake (in humans, 2-3 cm thick). Anatomically, this type of placenta is called discoid.

The placenta performs the following functions:

Trophic;

Respiratory; fertilization mammals gastrulation placenta

excretory;

Immunobiological - protection of the fetus from antigens that may be in the mother's blood. But this protection is poor, so suppressor cells act intensely in the mother’s body. suppress maternal immunity, so pregnancy occurs against the background of immunodeficiency (from the day of fertilization);

Barrier - the placental barrier is unstable for many compounds and a number of medicinal substances, as well as for alcohol;

Endocrine - the placenta begins to produce hormones early that support the process of embryonic development;

Protein-synthesizing function; according to this function, all placentas can be divided into two types:

Type 1 - epitheliochorial and desmochorial. With these types of placentas, complex compounds are absorbed from the mother’s body and from the blood. Then in the placenta they are broken down into simple ones and in this form they are delivered to the fetus, where embryo-specific or “organ-specific compounds” are synthesized, i.e. organs build themselves. Therefore, by the time of birth, the fetal organs are more mature.

Type 2 - endotheliochorial and hemochorial. Simple compounds are taken from the mother’s blood, so during pregnancy there is no particular danger to the mother’s body. For example, there are no deaths with histosis. In the placenta, complex compounds are synthesized from these simple compounds and delivered ready-made to the fetus (after the 7th month of embryogenesis, the fetus itself synthesizes some of the compounds it needs). Therefore, at the time of birth, such an organism is functionally less mature.

The development of the mammalian embryo goes through stages characteristic of vertebrate amniotes. Lancelet, amphibians, and fish are anamniotes. They do not have an amnion. They do not need it, since their development occurs in a natural aquatic environment. Early embryogenesis occurs in the oviducts, and final development occurs in the uterus. The uterine period of development is divided into two periods: embryonic and fetal. The duration of the uterine period varies among different classes of mammals, from 2-3 months to a year. In mammals, in parallel with the development of the embryo, the formation of extraembryonic organs occurs, which ensure the development of the embryo.

In the proembryonic period, germ cells are formed - gametogenesis (progenesis). The formation and growth of female germ cells occurs in the ovary, from where, at the 1st order oocyte stage, they are released into the abdominal space and captured by the villi (fimbriae) of the fallopian tubes. The first division of maturation begins during ovulation, and meiosis is completed in the lumen of the fallopian tube (oviduct).

As a result of the first division of maturation (reduction), the 1st order oocyte turns into a 2nd order oocyte, which has a haploid set of chromosomes. As a result of the second division of maturation, the 2nd order oocyte turns into a mature female reproductive cell - an oocyte, which remains viable from several hours to 1 day.

In most cases, one germ cell matures in each of the ovaries. With the simultaneous maturation of two or more germ cells in some classes, the formation of several embryos is possible - multiple pregnancy. The mammalian egg is secondary isolecithal, has a rounded shape, surrounded by the zona pellucida and a layer of follicular cells forming the corona radiata. The cytoplasm of the egg is fine-grained and contains a small number of yolk grains. The diameter of the egg is on average 120-150 microns.

Male germ cells (flagellar sperm) develop in the convoluted tubules of the testes (testes or testes), enter the vas deferens, and have a haploid set of chromosomes. At the same time, millions of them develop, then they enter the spermatic tract, where they are deposited. The spermatozoon consists of a head, neck, body, tail in the form of a flagellum and in its organization differs little in different types of placental animals: head shape, size.

The development of the early stages of embryogenesis (fertilization, cleavage and the first stage of blastulation) occurs in the oviducts (fallopian tubes).

Fertilization: monospermia, non-free - in the ampullary part of the oviducts.

Splitting up: full, uneven, irregular. As a result, after the first division, two types of blastomeres are formed. Small light ones are embryoblasts, and large dark ones are trophoblasts.

Blastulation occurs in two stages. 1) formation of a dense blastula or blastocyst in the form of a berry (morula). The appearance of the blastula is round. Embryoblast cells are located in the center. An embryo will develop from them. Along the periphery, trophoblast cells with microvilli are located in a single layer. They actively absorb nutrients from the tissue fluid of the oviducts, providing nutrition to the embryo. At this stage, the embryo enters the uterine cavity from the oviducts. The glands of its mucous membrane produce a mucous secretion - royal jelly, containing nutrients. Trophoblast cells actively absorb its components and transfer them to embryoblast cells. The embryo floats in the uterine cavity. Excess trophic material accumulates and compresses the embryoblast in the form of a disk. This 2nd stage of blastulation is called blastocyst.

Subsequently, the development processes of the embryo proceed in parallel, i.e. simultaneously with the development of the embryonic membranes.

Gastrulation in mammals it occurs in two stages, as in birds.

Stage 1 – delamination: splitting of the germinal disc into two layers or anlages: ectoderm and endoderm. In this case, the ectoderm moves towards the trophoblast and displaces it above itself, i.e. is integrated into the trophoblast. The trophoblast cells above it are exfoliated - Rauber's leaf. In the middle part of the two-leaf embryo, the germinal shield is distinguished. Actively dividing cells, especially at the leading edge of the germinal shield. The cells move along the sides of the embryo to the rear edge, two streams collide, forming the primitive streak. Its cells divide by mitosis and invaginate toward the endoderm. In this area, two leaves grow together. The cells between the sheets, continuing to divide, grow with wings between the ectoderm and endoderm, forming the mesodermal anlage. On the surface, the cells of the primitive streak divide by mitosis and rush to the anterior edge of the embryo. But since the density of cellular material at the leading edge is high, the cells of the primitive streak accumulate, forming Hensen's node. Its cells, continuing to divide by mitosis, migrate to the endoderm and grow forward between the wings of the mesoderm. Thus, the first axial organ, the notochord, is immediately formed at the gastrula stage. The remains of Hensen's node cells grow on the surface of the ectoderm towards the anterior edge, forming a nerve rudiment. Thus, at the stage of gastrulation, embryonic anlages were formed - sources of tissue development.

Formation of axial organs occurs according to the general principle, as in the lancelet. At this stage, the process of histogenesis begins - tissue development. In the area of the axial organs from the material of the anlages from which they are formed.

Formation of the embryo body and embryonic membranes(provisional organs occur, as in birds, with the help of trunk and amniotic folds. Due to two lateral and two anteroposterior trunk folds, the body(torso) and yolk sac. It does not contain yolk. Trunk folds are formed in the area of fusion of trophoblast and ectoderm. At the same time, cells in the area of contact between the trophoblast and ectoderm begin to move in the opposite direction from the trunk folds to the dorsal surface of the embryo, forming amniotic folds, there are also four of them. Thus, the ectoderm remains internal, but is divided into the germinal ectoderm and the ectoderm that forms the amnion wall. The ridges of the amniotic folds grow together. As a result of their fusion around the embryo, a bowl-shaped cavity is formed - amnion. Gradually it fills with liquid, in which further development of the embryo will occur. The amnion grows in the extraembryonic cavity of the coelom, reaching its greatest development compared to other membranes. From the outer surface after the fusion of the amniotic folds, a chorion(analogous to the serous membrane). The surface of the chorion is divided into two parts: smooth and villous. The smooth chorion performs a protective function. The villous chorion faces the uterine mucosa. And soon it establishes contacts with the mucous membrane of the uterus that are specific to various classes of mammals. Chorionic villi form the fetal part of the placenta. The second part is maternal. The structures of the uterine mucosa with which the chorionic villi come into contact will be different for different classes, so there are four types of placentas. At the same time, due to the protrusion of the posterior wall of the intestine into the extraembryonic cavity, the coelom is formed allantois. In mammals it does not reach much development. As the amnion grows, it compresses the yolk sac and allantois in the form of a cord. The walls of the yolk sac and allantois grow together. This is how the umbilical cord is formed. In their common wall, umbilical vessels are formed: two arteries and one vein. In mammals, as exemplified by the pig, the lumens of the allantois and yolk sac do not fuse together. They are visible in sections of the umbilical cord. In this case, the yolk sac is lined with squamous epithelium, and the allantois is lined with cubic epithelium. The walls of blood vessels have their own membranes. The umbilical cord fuses with the chorionic villi. Its vessels grow into the stroma of the villi. The blood of the fetus and mother do not mix.