Embryonic development – This is a chain of interconnected transformations, as a result of which a multicellular organism is formed from a single-celled zygote, capable of existing in the external environment. In embryogenesis, as part of ontogenesis, the processes of phylogenesis are also reflected. Phylogenesis- This is the historical development of the species from simple forms to complex ones. Ontogenesis– individual development of a particular organism. According to the biogenetic law, ontogeny is a short form of phylogeny, and therefore representatives of different classes of animals have common stages of embryonic development:
1. Fertilization and formation of the zygote;
2. Fragmentation of the zygote and formation of the blastula;
3. Gastrulation and the appearance of two germ layers (ectoderm and endoderm);
4. Differentiation of ecto- and endoderm with the appearance of the third germ layer - mesoderm, axial organs (notochord, neural tube and primary gut) and further processes of organogenesis and histogenesis (development of organs and tissues).
Fertilization – This is the process of mutual assimilation of the egg and sperm, in which a single-celled organism arises - a zygote, combining two hereditary information.
Zygote fragmentation – This is the repeated division of the zygote by mitosis without the growth of the resulting blastomeres. This is how the simplest multicellular organism is formed - blastula. We distinguish:
Complete, or holoblastic, fragmentation, in which the entire zygote is fragmented into blastomeres (lancelet, amphibians, mammals);
Incomplete, or meroblastic, if only part of the zygote (animal pole) undergoes cleavage (birds).
Complete crushing, in turn, happens:
Uniform - blastomeres of relatively equal size are formed (lancelet) with their synchronous division;
Uneven – during asynchronous division with the formation of blastomeres of different sizes and shapes (amphibians, mammals, birds).
Gastrulation– stage of formation of a two-layer embryo. Its superficial cell layer is called the outer germ layer - ectoderm, and its deep cell layer is called the inner germ layer - endoderm.
Types of gastrulation:
1. invagination – invagination of the blastomeres of the bottom of the blastula towards the roof (lancelet);
2. epiboly - fouling of the roof of the blastula of its marginal zones and bottom with rapidly dividing small blastomeres (amphibians);
3. delamination - separation of blastomeres and migration - movement of cells (birds, mammals).
Differentiation germ layers leads to the appearance of cells of different quality, giving rise to the rudiments of various tissues and organs. In all classes of animals, axial organs first appear - the neural tube, notochord, primary gut - and the third (middle in position) germ layer - the mesoderm.
Question 11. Features of embryonic development of mammals (formation of trophoblast and fetal membranes)
Features of mammalian embryogenesis are determined by the intrauterine nature of development, as a result of which:
1. The egg does not accumulate large reserves of yolk (oligolecithal type).
2. Fertilization is internal.
3. At the stage of complete uneven fragmentation of the zygote, early differentiation of blastomeres occurs. Some of them divide faster and are characterized by a light color and small size, while others are dark in color and large in size, since these blastomeres are delayed in dividing and fragment less frequently. Light blastomeres gradually envelop slowly dividing dark ones, resulting in the formation of a spherical blastula without a cavity ( morula). In the morula, dark blastomeres make up its internal contents in the form of a dense nodule of cells, which are later used to build the body of the embryo - this embryoblast.
Light blastomeres are located around the embryoblast in one layer. Their task is to absorb the secretion of the uterine glands (royal jelly) to ensure the nutritional processes of the embryo before the formation of the placental connection with the mother’s body. Therefore they form trophoblast.
4. The accumulation of royal jelly in the blastula pushes the embryoblast upward and makes it look like the discoblastula of birds. Now the embryo represents the germinal vesicle, or blastocyst. As a consequence, all further development processes in mammals repeat the already known paths characteristic of bird embryogenesis: gastrulation occurs through delamination and migration; the formation of axial organs and mesoderm occurs with the participation of the primitive streak and nodule, and the separation of the body and the formation of fetal membranes - the trunk and amniotic folds.
The trunk fold is formed as a result of the active proliferation of cells of all three germ layers in the zones bordering the germinal shield. The rapid growth of cells forces them to move inward and bend the leaves. As the trunk fold deepens, its diameter decreases, it increasingly isolates and rounds the embryo, simultaneously forming from the endoderm and the visceral layer of mesoderm the primary intestine and the yolk sac with the royal jelly enclosed in it.
The peripheral parts of the ectoderm and the parietal layer of mesoderm form an amniotic circular fold, the edges of which gradually move over the detached body and completely close over it. The fusion of the internal layers of the fold forms the internal water membrane - the amnion, the cavity of which is filled with amniotic fluid. Fusion of the outer layers of the amniotic fold ensures the formation of the outermost membrane of the fetus - the chorion (villous membrane).
Due to the blind protrusion through the umbilical canal of the ventral wall of the primary intestine, a middle membrane is formed - the allantois, in which a system of blood vessels (choroid) develops.
5. The outer shell - the chorion - has a particularly complex structure and forms multiple protrusions in the form of villi, with the help of which a close relationship is established with the mucous membrane of the uterus. The villi include areas of allantois with blood vessels that grow together with the chorion and trophoblast, the cells of which produce hormones to maintain the normal course of pregnancy.
6. The set of allantochorion villi and endometrial structures with which they interact form a special embryonic organ in mammals - the placenta. The placenta provides nutrition to the embryo, its gas exchange, removal of metabolic products, reliable protection from unfavorable factors of any etiology and hormonal regulation of development.
Ontogenesis call the totality of processes occurring in the body from the moment of formation of the zygote until death.
It is divided into two stages: embryonic And postembryonic.
Embryonic period The embryonic period is considered to be the period of embryonic development from the moment of formation of the zygote until exit from the egg membranes or birth; in the process of embryonic development, the embryo goes through the stages of crushing, gastrulation, primary organogenesis and further differentiation of organs and tissues. Crushed . Cleavage is the process of formation of a multicellular single-layer embryo - blastula. Fragmentation is characterized by: 1) cell division by mitosis with preservation of the diploid set of chromosomes; 2) very short mitotic cycle; 3) blastomeres are not differentiated, and hereditary information is not used in them; 4) blastomeres do not grow and subsequently become smaller; 5) the cytoplasm of the zygote does not mix or move.
Stages of embryo development.
1. The period of a one-cell embryo, or zygote, is short-term, occurring from the moment of fertilization until the beginning of egg fragmentation. 2. Crushing period. During this period, cell multiplication occurs. The resulting cells are called blastomeres. First, a bunch of blastomeres is formed, resembling a raspberry in shape - a morula, then a spherical single-layer blastula; the wall of the blastula is the blastoderm, the cavity is the blastocele. 3. Gastrulation. A single-layer embryo turns into a two-layer one - a gastrula, consisting of an outer germ layer - ectoderm and an inner one - endoderm. In vertebrates, already during gastrulation, the third germ layer, the mesoderm, appears. During evolution in chordates, the process of gastrulation became more complicated with the emergence of an axial complex of rudiments (the formation of the nervous system, axial skeleton and muscles) on the dorsal side of the embryo. 4. The period of separation of the main rudiments of organs and tissues and their further development. Simultaneously with these processes, the unification of parts into a single developing whole is intensified. From the ectoderm the epithelium of the skin, the nervous system and partly the sensory organs are formed, from the endoderm - the epithelium of the digestive canal and its glands; from mesoderm - muscles, epithelium of the genitourinary system and serous membranes, from mesenchyme - connective, cartilage and bone tissue, vascular system and blood.
Consequences of the influence of alcohol, nicotine, and drugs on the human embryo.
Systematic use of drugs, which include alcohol, and even nicotine, causes damage to germ cells - sperm and eggs. A child may be born with a delay in body length and weight, poorly developing physically, and predisposed to the development of any diseases. The stronger the drug used by parents, the more serious the changes in the children’s bodies can be. The use of these substances by women is especially dangerous.
2. The struggle for existence. Prerequisite for natural selection. Forms of the struggle for existence.
Struggle for existence – complex and diverse relationships of individuals within a species, between species and with unfavorable conditions of inanimate nature. Charles Darwin points out that the discrepancy between the possibility of species for unlimited reproduction and limited resources is the main reason for the struggle for existence. The struggle for existence is of three types:
Intraspecific
Interspecific
Combating abiotic factors
Developmental biology– a new direction of modern biology. This is the science of the patterns and mechanisms of ontogenesis.
Ontogenesis(Greek Ontos - being, genesis - development) - individual development of the organism.
It includes a set of successive morphological, physiological and biochemical transformations from birth to death.
Ontogenesis multicellular organisms are divided into two periods: embryonic (embryonic, gr. embruоn - embryo) and postembryonic (post-embryonic). In higher animals and humans, ontogenesis is divided into prenatal(before birth), and postnatal(after birth).
Embryonic, or prenatal Embryogenesis includes the development of an organism from fertilization of the egg to the release of the individual from the egg membranes or from the uterine cavity of the maternal organism.
The animal world has three most common types of ontogenesis: larval; non-larval; intrauterine.
Larval type of ontogenesis characterized by the development of the organism occurring with metamorphosis.
Non-larval type of ontogenesis characterized by the formation of the organism carried out in the egg.
Intrauterine Ontogenesis is determined by development within the maternal organism.
In humans, the body is up to 8 weeks old by the time the rudiments of organs are formed and is called an embryo, or fetus.
a fetus is an organism after the formation of the rudiments of organs and the body shape that a person has (8 weeks after the formation of the zygote).
Embryogenesis includes the following main stages (Fig. 5):
1. Fertilization and crushing of the egg.
Gastrulation and formation of germ layers.
3. Histogenesis and organogenesis. This is the formation of organs and tissues.
Fertilization represents the penetration of a sperm into an egg. in humans and mammals this occurs in the upper third of the fallopian tube.
After fertilization, a zygote is formed. She has genetic information from two parents and a diploid set of chromosomes (2 n). A fertilized egg (zygote) reproduces mitotically.
The early period of embryogenesis, i.e.
e. the development of a fertilized egg (zygote) is called crushing. The resulting cells are called blastomeres. Their development occurs through
successive mitotic divisions.
Fragmentation has a number of features: the mitotic cycle is characterized by a short duration, there are no pre- and postsynthetic phases, protein synthesis is repressed to a certain stage.
Since there is no postmitotic growth of germ cells, blastomeres decrease in size and, although their total number increases rapidly, the volume of the embryo does not change significantly in the early stages of development.
The nature of crushing depends on the type of egg cells and the amount of yolk in the oocyte. The following are distinguished: types of crushing :
1) Complete crushing (holoblastic) – uniform and uneven;
2) Incomplete cleavage (meroblastic) – discoidal and superficial.
With complete (holoblastic) fragmentation the zygote divides entirely.
Isolecithal and telolecithal eggs develop in this way.
With incomplete (meroblastic) fragmentation Only the part of the cytoplasm of the egg that does not have yolk inclusions divides.
incomplete crushing is discoidal and superficial.
In discoidal cleavage, segmentation occurs at the animal pole, while the vegetal pole of the egg remains intact. This method is typical for strongly telolecithal cells (for example, in birds).
Centrolecithal cells have superficial fragmentation. In this case, the entire peripheral zone of ovoplasm free from yolk is divided (for example, in insects).
The fragmentation of the zygote in humans and mammals is holoblastic and uniform.
the number of blastomeres increases in the wrong order, asynchronously. Fragmentation ends with the formation blastulas.
Blastula– it is a multicellular single-layer embryo. It has blastoderm.
This is the body wall that is formed by blastomeres. The blastocoel is the cavity of the blastula. There are different types of blastulas. With superficial crushing, the cavity is filled with yolk. This is periblastula. With discoidal cleavage, the germ cells are spread out in the form of a disk on the yolk. This is discoblastula.
In humans and mammals, crushing results in the formation of a blastocyst (germinal vesicle).
its walls are formed by trophoblast, a single layer of sharply flattened cells. The cavity of the blastocyst is filled with fluid. The blastula turns into gastrulu.
Gastrulation this is the directed movement of large groups of embryonic cells to the sites of formation of future organ systems.
As a result, three germ layers are formed. They consist of cells that differ in size, shape and other characteristics. In lower animals such as sponges and coelenterates, the gastrula consists of two layers of cells - the ectoderm (outer germ layer) and endoderm (inner germ layer).
All other higher phyla of animals have a three-layered gastrula. Then the third (middle) germ layer, the mesoderm, is formed.
From ectoderm The tissues of the nervous system develop, the outer covering of the skin - the epidermis and its derivatives (nails, hair, sebaceous and sweat glands), as well as tooth enamel, sensory cells of the organs of vision, hearing and smell, etc.
From endoderm epithelial tissue develops, lining the respiratory organs, partly the genitourinary and digestive systems, including the liver and pancreas.
Most numerous mesoderm derivatives– skeletal muscles, excretory organs and gonads; cartilage, bone and connective tissue.
Gastrula formation in various animals it is carried out in four ways: intussusception, immigration, delamination, epiboly .
A classic example of gastrulation by intussusception is the embryonic development of the lancelet.
In the blastula of the lancelet, a group of blastomeres begins to invaginate into the blastocoel. As a result, ectoderm and endoderm are formed. They form the cavity of the primary intestine - the gastrocoel. This cavity communicates with the external environment through an opening (blastopore). Then the mesoderm is formed in the form of paired outgrowths of the wall of the primary intestine (mesoderm pockets).
Further differentiation of the germ layers leads to the formation of the organs of the axial complex.
These are the neural tube, notochord and intestinal tube.
In humans, gastrulation occurs in two phases. First, a two-layer gastrula is formed by delamination of the embryoblast.
The second phase is the emergence of the middle germ layer and the appearance of the axial complex of primordia.
Histogenesis and organogenesis.
Germ layers are the material from which the rudiments of certain tissues and organs are newly formed in all multicellular organisms .
The embryonic development of organisms is carried out with the participation of provisional (extraembryonic) - temporarily functioning organs that provide the necessary vital functions and connect the embryo with the environment.
in animals with a non-larval type of development (fish, reptiles, birds), eggs have a lot of yolk.
Their provisional body is yolk sac. He is the organ of nutrition and hematopoiesis of the embryo. The reduced yolk sac of mammals is part of placenta. In terrestrial animals (reptiles, birds, mammals) provisional authorities(Fig. 6) this is a water shell (amnion), allantois And serous membrane (chorion). In placental mammals, the chorion, together with the uterine mucosa, forms the placenta.
In human embryonic development there are 3 main critical periods:
Implantation (b – 7th day after conception) – implantation of the zygote into the wall of the uterus.
2. Placentation (end of the 2nd week of pregnancy) – the formation of a placenta in the embryo.
3. Perinatal period (childbirth) - the transition of the fetus from the aquatic to the air environment 9 months after conception.
Critical periods in the body of a newborn are associated with a sharp change in living conditions and a restructuring of the activities of all body systems (the nature of blood circulation, gas exchange, and nutrition changes).
Stages of embryogenesis
Embryogenesis (Greek embryon - embryo, genesis - development) is the early period of individual development of the organism from the moment of fertilization (conception) to birth, is the initial stage of ontogenesis (Greek ontos - being, genesis - development), the process of individual development of the organism from conception to of death.
The development of any organism begins as a result of the fusion of two sex cells (gametes), male and female.
All cells of the body, despite differences in structure and functions, are united by one thing - a single genetic information stored in the nucleus of each cell, a single double set of chromosomes (except for highly specialized blood cells - red blood cells, which do not have a nucleus).
That is, all somatic (soma - body) cells are diploid and contain a double set of chromosomes - 2 n, and only sex cells (gametes) formed in specialized gonads (testes and ovaries) contain a single set of chromosomes - 1 n.
When germ cells fuse, a cell is formed - a zygote, in which a double set of chromosomes is restored.
Recall that the nucleus of a human cell contains 46 chromosomes, respectively, sex cells have 23 chromosomes
The resulting zygote begins to divide. The first stage of zygote division is called cleavage, as a result of which the multicellular structure of the morula (mulberry) is formed.
The cytoplasm is distributed unevenly between the cells; the cells of the lower half of the morula are larger than the upper half. The volume of the morula is comparable to that of the zygote.
At the second stage of division, as a result of cell redistribution, a single-layer embryo is formed - a blastula, consisting of one layer of cells and a cavity (blastocoel).
Blastula cells vary in size.
At stage III, the cells of the lower pole seem to invaginate (invaginate) inward, and a two-layer embryo is formed - the gastrula, consisting of an outer layer of cells - ectoderm and an inner layer of cells - endoderm.
Very soon, between the I and II layers of cells, as a result of cell division, another layer of cells is formed, the middle one is the mesoderm, and the embryo becomes three-layered. This completes the gastrula stage.
From these three layers of cells (they are called germinal layers) the tissues and organs of the future organism are formed.
The integumentary and nervous tissue develops from the ectoderm, the skeleton, muscles, circulatory system, genitals, excretory organs from the mesoderm, and the respiratory and nutritional organs, liver, and pancreas from the endoderm. Many organs are formed from several germ layers.
Embryogenesis includes the processes from fertilization to birth.
The development of the human body begins after fertilization of the female reproductive cell - the egg (ovium) of the male - by a spermatozoon (spermatozoon, spermium).
The detailed study of the development of the human embryo (embryo) is the subject of embryology.
Here we will limit ourselves to only a general overview of the development of the embryo (embryogenesis), which is necessary for understanding the human physique.
The embryogenesis of all vertebrates, including humans, can be divided into three periods.
1.
Crushing: a fertilized egg, spermovium, or zygote is sequentially divided into cells (2,4,8,16 and so on) as a result of which a dense multicellular ball, morula, and then a single-layer vesicle - blastula, which contains a primary cavity in the middle, is formed. blastocoel.
The duration of this period is 7 days.
2.
Gastrulation consists of the transformation of a single-layer embryo into a two-, and later three-layer - gastrula. The first two layers of cells are called germ layers: the outer ectoderm and the inner endoderm (up to two weeks after fertilization), and the third, middle layer that appears later between them is called the middle germ layer - mesoderm.
The second important result of gastrulation in all chordates is the emergence of an axial complex of rudiments: on the dorsal (dorsal) side of the endoderm, the rudiment of the dorsal string, notochord, appears, and on its ventral (ventral) side - the rudiment of the intestinal endoderm; on the dorsal side of the embryo, along its midline, a neural plate stands out from the ectoderm - the rudiment of the nervous system, and the rest of the ectoderm goes to build the epidermis of the skin and is therefore called cutaneous ectoderm.
Subsequently, the embryo grows in length and turns into a cylindrical formation with a head (cranial) and caudal caudal ends.
This period lasts until the end of the third week after fertilization.
3. Organogenesis and histogenesis: the neural plate sinks under the ectoderm and turns into a neural tube, which consists of separate segments - neurotomes - and gives rise to the development of the nervous system. The mesodermal primordia are detached from the endoderm of the primary intestine and form a paired row of metamerically located sacs, which, growing on the sides of the body of the embryo, are each divided into two sections: the dorsal, which lies on the sides of the notochord and neural tube, and the ventral, which lies on the sides of the embryo. intestines.
The dorsal sections of the mesoderm form the primary segments of the body - somites, each of which in turn is divided into a sclerotome, which gives rise to the skeleton, and a myotome, from which muscles develop. A skin segment, the dermatome, is also distinguished from the somite (on its lateral side). The abdominal sections of the mesoderm, called splanchnotomes, form paired sacs that contain the secondary body cavity.
The intestinal endoderm, which remains after the separation of the notochord and mesoderm, forms the secondary gut - the basis for the development of internal organs.
Subsequently, all the organs of the body are laid down, the material for the construction of which is the three germ layers.
1. From the outer germ layer, ectoderm, develop:
A) epidermis of the skin and its derivatives (hair, nails, skin glands);
b) epithelium of the mucous membrane of the nose, mouth and anus;
V) nervous system and epithelium of sensory organs.
2. From the inner germ layer, the endoderm, the mucosal epithelium of most of the digestive tract with all the glandular structures belonging here, most of the respiratory organs, as well as the epithelium of the thyroid and thymus glands develops.
3.
From the middle germ layer, mesoderm, the musculature of the skeleton, the mesothelium of the membranes of the serous cavities with the rudiments of the gonads and kidneys develop.
In addition, from the dorsal segments of the mesoderm, embryonic connective tissue, mesenchyme, arises, which gives rise to all types of connective tissue, including cartilage and bone.
Since at first the mesenchyme carries nutrients to different parts of the embryo, performing a trophic function, then later blood, lymph, blood vessels, lymph nodes, and the spleen develop from it.
In addition to the development of the embryo itself, it is also necessary to take into account the formation of extra-embryonic parts, with the help of which the embryo receives the nutrients necessary for its life.
In the multicellular dense ball, there is an internal embryonic nodule, the embryoblast, and an outer layer of cells, which plays an important role in the nutrition of the embryo and is therefore called the trophoblast.
With the help of trophoblast, the embryo penetrates into the thickness of the uterine mucosa (implantation), and here the formation of a special organ begins, with the help of which the embryo is connected with the mother’s body and is nourished.
This organ is called the baby's place, litter, or placenta. Mammals that have a placenta are called placentals. Along with the formation of the placenta, there is a process of separation of the developing embryo from the extra-embryonic parts as a result of the appearance of the so-called trunk fold, which, protruding with a ridge towards the middle, seems to lace the body of the embryo from the extra-embryonic parts with a ring.
At the same time, however, the connection to the placenta is maintained through the umbilical stalk, which then turns into the umbilical cord. In the early stages of development, the vitelline duct passes through the latter, which connects the intestine with its protrusion into the extraembryonic area, the yolk sac. In vertebrates that do not have a placenta, the yolk sac contains the nutritional material of the egg - the yolk - and is an important organ through which the embryo is nourished.
In humans, although the yolk sac appears, it does not play a significant role in the development of the embryo and, after absorption of its contents, is gradually reduced.
The umbilical cord also contains umbilical (placental) vessels, through which blood flows from the placenta to the body of the fetus and back. They develop from the mesoderm of the urinary sac, or allantois, which protrudes from the ventral wall of the intestine and exits the body of the embryo through the umbilical opening into the extraembryonic part. In humans, from the part of the allantois, which is contained in the middle of the body of the embryo, part of the bladder is formed, and from its vessels the umbilical blood vessels are formed.
The developing embryo is covered with two germinal membranes. The inner membrane, the amnion, forms a voluminous sac, which is filled with protein fluid and forms a liquid environment for the embryo, through which the sac is called the aqueous membrane.
The entire embryo, along with the amniotic and yolk sacs, is surrounded by an outer membrane (which also includes the trophoblast). This membrane, having villi, is called villous, or chorion.
The chorion performs trophic, respiratory, excretory and barrier functions.
Embryogenesis, according to the nature of the processes occurring in the embryo, is divided into three periods:
1) crushing period;
2) period of gastrulation;
3) the period of histogenesis (tissue formation), organogenesis (organ formation), systemogenesis (formation of functional systems of the body).
Splitting up.
The lifespan of a new organism in the form of one cell (zygote) lasts in different animals from several minutes to several hours and even days, and then fragmentation begins.
Cleavage is the process of mitotic division of the zygote into daughter cells (blastomeres). Cleavage differs from ordinary mitotic division in the following ways:
- blastomeres do not reach the original size of the zygote;
2) blastomeres do not diverge, although they are independent cells.
The following types of crushing are distinguished:
1) complete, incomplete;
2) uniform, uneven;
3) synchronous, asynchronous.
The eggs and the zygotes formed after their fertilization, containing a small amount of lecithin (oligolecithal), evenly distributed in the cytoplasm (isolecithal), are completely divided into two daughter cells (blastomeres) of equal size, which are then simultaneously (synchronously) divided again into blastomeres.
This type of crushing is complete, uniform and synchronous. Eggs and zygotes containing a moderate amount of yolk are also crushed completely, but the resulting blastomeres have different sizes and are crushed non-simultaneously - the crushing is complete, uneven, asynchronous. As a result of fragmentation, an accumulation of blastomeres is first formed, and the embryo in this form is called a morula. Then fluid accumulates between the blastomeres, which pushes the blastomeres to the periphery, and a cavity filled with fluid is formed in the center.
At this stage of development, the embryo is called blastula.
The blastula consists of:
1) blastoderm - shells of blastomeres;
2) blastocoel - a cavity filled with liquid.
The human blastula is a blastocyst.
After the formation of the blastula, the second stage of embryogenesis begins - gastrulation.
Gastrulation- the process of formation of germ layers, formed through the reproduction and movement of cells. The process of gastrulation occurs differently in different animals.
The following methods of gastrulation are distinguished:
- delamination (splitting of a cluster of blastomeres into plates);
2) immigration (movement of cells inside the developing embryo);
3) intussusception (invagination of a layer of cells into the embryo);
4) epiboly (overgrowth of slowly dividing blastomeres with rapidly dividing ones with the formation of an outer layer of cells).
As a result of gastrulation, three germ layers are formed in the embryo of any animal species:
1) ectoderm (outer germ layer);
2) endoderm (inner germ layer);
3) mesoderm (middle germ layer).
Each germ layer is a separate layer of cells.
Between the sheets there are initially slit-like spaces into which process cells soon migrate, collectively forming the germinal mesenchyme (some authors consider it as the fourth germ layer). Germinal mesenchyme is formed by the eviction of cells
from all three germ layers, mainly from the mesoderm.
The embryo, consisting of three germ layers and mesenchyme, is called gastrula.
The process of gastrulation in the embryos of different animals differs significantly both in methods and in time. The germ layers and mesenchyme formed after gastrulation contain presumptive tissue rudiments. After this, the third stage of embryogenesis begins - histo- and organogenesis.
Histo- and organogenesis(or differentiation of germ layers) is the process of transformation of tissue primordia into tissues and organs, and then the formation of functional
body systems.
The basis of histo- and organogenesis are the following processes: mitotic division (proliferation), induction, determination, growth, migration and differentiation of cells.
As a result of these processes, axial rudiments of organ complexes (notochord, neural tube, intestinal tube, mesodermal complexes) are first formed. At the same time, various tissues are gradually formed, and from the combination of tissues, anatomical organs are formed and developed, united into functional systems - digestive, respiratory, reproductive, etc. At the initial stage of histo- and organogenesis, the embryo is called an embryo, which later turns into a fetus.
At present, it has not been definitively established how cells completely different in morphology and function are formed from one cell (zygote), and subsequently from identical germ layers, and from them tissues are formed (from ectoderm
epithelial tissues, horny scales, nerve cells and glial cells).
Presumably, genetic mechanisms play a leading role in these transformations.
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Without having in your disposal of early embryos humans, showing some of the most important stages of the formation of germ layers, we tried to trace their formation in other mammals. The most noticeable feature of early development is the formation of many cells from a single fertilized egg through successive mitoses. Even more important is the fact that even during the early phases of rapid proliferation, the cells thus formed do not remain an unorganized mass.
Video: Embryogenesis: Development of the embryo
Almost immediately they are located in the form of a hollow formation called a blastoderm vesicle.
At one pole, a group of cells known as the inner cell mass gathers. As soon as it is formed, cells begin to emerge from it, lining a small internal cavity - the primary gut, or archenteron. From these cells the endoderm is formed.
Ta part of the original group The cells from which the integuments of the embryo and the outermost layer of its membranes are formed are called ectoderm.
Soon, between the first two germ layers, a third layer is formed, called, quite aptly, mesoderm.
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Germ layers are of interest to the embryologist from several points of view.
The simple structure of the embryo, when it first contains one, then two and finally three primary layers of cells, is a reflection of the phylogenetic changes that took place in lower animals - the ancestors of vertebrates. From the point of view of possible ontogenetic recapitulations, some facts fully allow this.
Nervous system of embryos vertebrates arise from the ectoderm - a layer of cells through which primitive organisms that do not yet have a nervous system are in contact with the external environment.
The lining of the vertebrate digestive tube is formed from the endoderm, a layer of cells that in very primitive forms lines their gastrocoel-like internal cavity.
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Skeletal, muscular and circulatory systems originate in vertebrates almost exclusively from the mesoderm - a layer that is relatively unnoticeable in small, low-organized creatures, but whose role increases as their size and complexity increase due to their increasing needs for support and circulatory systems.
Along with the possibility interpretation of germ layers from the point of view of their phylogenetic significance, it is also important for us to establish the role they play in individual development.
The germ layers are the first organized groups of cells in the embryo, which are clearly distinguished from each other by their features and relationships. The fact that these relationships are essentially the same in all vertebrate embryos strongly suggests a common origin and similar heredity in the various members of this huge group of animals.
One might think that in these germ layers For the first time, differences between different classes begin to be created over the general plan of body structure, characteristic of all vertebrates.
Formation of embryonic leaflets The period when the main process of development is only an increase in the number of cells ends, and the period of differentiation and specialization of cells begins.
Differentiation occurs in the germ layers before we can see its signs using any of our microscopic techniques. In the leaf, which has a completely homogeneous appearance, localized groups of cells with different potencies for further development constantly appear.
We have known about this for a long time, for we can see how from the germ layer various structures arise. At the same time, no visible changes are noticeable in the germ layer due to which they arise.
Recent experimental studies indicate how early this invisible differentiation precedes the visible morphological localization of cell groups that we easily recognize as the rudiment of the definitive organ.
So, for example, if you cut from any place of Hensen's node a narrow transverse strip of the ectoderm of a twelve-hour embryo and grown in tissue culture, then at a certain time specialized cellular elements of a type that is found only in the eye will be discovered, although the rudiment of the optic vesicle of a chick embryo does not appear before 30 hours of incubation.
A strip taken from another area, although it appears the same, when grown in culture does not form cells characteristic of the eye, but exhibits a different specialization.
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Experiments show how early in the germ layers groups of cells with different potencies for development are determined.
As development progresses, these cell groups become more and more prominent. In some cases, they are separated from the mother leaf by protrusion, in other cases - by migration of individual cells, which later accumulate somewhere in a new place.
From the primary groups of cells thus formed, gradually definitive organs are formed.
Therefore, the origin of various parts of the body in embryogenesis depends on the growth, division and differentiation of the germ layers. This diagram shows us the general path along which the early processes discussed above develop. If we follow the process of development further, we see that each normal division of an object is more or less clearly centered around a certain branch of this family tree of germ layers.
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Main article: Sexual reproduction
Fertilization
Human life begins from the moment of fusion in the mother’s body of two sex cells - an egg and a sperm, and one new cell is formed, that is, a new organism. Each of the female and male germ cells contains 23 pairs of chromosomes, 22 of which transmit the hereditary characteristics of the father and mother to the fetus.
In both of these germ cells there are about 100 thousand genes that determine the structural and functional characteristics of the newly formed organism.
The gender of the unborn child depends on the 23rd pair of chromosomes of the female and male germ cells. The 23rd pair of chromosomes of the female germ cell is designated as X-X (XX), and the 23rd pair of chromosomes of the male germ cell is designated X-Y (XY).
If the X chromosome of a male cell merges with a female cell, a girl is born, and when the Y chromosome of a male cell merges with a female cell, a boy is born.
Thus, the sex of the unborn child depends on the father’s reproductive cell, but not on his will or desire.
The female and male reproductive cells, merging in the fallopian tube, form one cell, that is, a new organism that has 46 pairs of chromosomes. As soon as such a cell is formed, it begins to multiply by division within one week, while gradually moving towards the uterus. Once in the uterine cavity, it attaches to its wall and continues its development in the form of an embryo, or fetus.
Fetal development
A new organism that arises in the womb develops in the oviduct in the first week of its life and, starting from the second week, its development proceeds in the uterine cavity and continues for 9 months.
And all this time the fetus is nourished by the blood of the mother’s body. From the 23rd day of development of the embryo, its heart and systemic circulation begin to function. But his lungs and pulmonary circulation do not work during the period of embryonic development, and the fetus is provided with oxygen through the umbilical vessels at the expense of the maternal body.
As soon as the baby is born, the umbilical cord is cut and separated from the mother's body. From this moment, his lungs and pulmonary circulation begin to function.
Afterbirth
From the outer part of the embryo in the uterine cavity, a special tissue is formed, rich in blood vessels and consisting of special cells - the so-called afterbirth, with the help of which the embryo is attached to the wall of the uterus (Fig.
82). The umbilical cord is formed from its vessels, through the arteries and veins of which the fetus is connected to the vessels of the mother’s body. The afterbirth provides nutrition to the fetus and, in addition, protects it from the effects of harmful chemicals and microbes that have entered the mother’s body.
Damage to the placenta and its detachment from the uterine wall poses a danger to the fetus. Material from the site http://wiki-med.com
Amnion
The fetus is surrounded by a thin membrane (amnion), the internal cavity of which is filled with amniotic fluid.
This fluid plays an important role in metabolic processes in the fetus’s body, in protecting it from adverse external influences and facilitating its free movement (Fig. 83).
Layers of the embryo
In the third week of intrauterine life, the cells of the embryo form three layers. The outer one is called ectoderm, the middle one is mesoderm and the inner one is called endoderm.
Each of them gives rise to different tissues and organs of the embryo.
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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.
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 shell is follicular cells (see Fig. 37). 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.
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 dark cells form the embryoblast.
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 looks like 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 disc 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). It is believed that one of the features of the development of mammals is that during the isolecithal egg 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 parts. 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. Diagram of placentas:
A- epitheliochorial; b- desmochorial; V- endotheliochorionic; 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.
Mammals and humans are characterized by the intrauterine development of embryos in a special organ - the uterus, where the embryo is supplied with the nutrients and oxygen necessary for its development at the expense of the mother's body; fertilization is internal. The eggs again (secondarily in the evolution of chordates) become microscopic and contain very little yolk.
Ovum type
The eggs of mammals, in comparison with those of the lancelet, are called secondary isolecithal.
Crushing type
The fragmentation is complete, as in the lancelet and lower vertebrates, but uneven and asynchronous. Blastomers are formed of different sizes. There is no rightness in increasing their number. During the crushing process, a compact embryo appears - a morula, which is a dense accumulation of blastomeres. Already at the beginning of fragmentation, two types of blastomeres become distinguishable: smaller light ones and larger dark ones. Light blastomeres are located closer to the surface of the embryo and, due to faster division, overgrow a cluster of dark ones. Light blastomeres received the name “ trophoblast", dark - " embryoblast».
As the number of blastomeres increases, a cavity filled with protein fluid appears between the trophoblast and embryoblast, and a blastula is formed, called blastocyst. A group of embryoblast cells first adjoins the trophoblast in one of the areas and takes the form of an embryonic shield.
Further development shows that there is a certain analogy in the structure of the blastocyst of mammals and humans and the discoblastula of reptiles and birds. The germinal shield of the blastocyst corresponds to the blastodisc in the blastula of birds. The fundamental difference between the blastocyst and the discoblastula of reptiles and birds is the appearance of a new structure - the trophoblast. The trophoblast consists of cells of the extra-embryonic ectoderm, the embryoblast is the cells from which some extra-embryonic parts and the body of the embryo will develop.
Gastrulation
In mammals, gastrulation occurs in the same way as in birds, with the difference that a layer of trophoblast constantly exists around the mammalian embryo. In the first phase of gastrulation, two germ layers are formed - the epiblast and the hypoblast by delamination of the material of the germinal shield.
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3.1 Neonatal period
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Age periods of human development
3.2 Breast period
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Age periods of human development
3.6 Adolescence
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Watching a honey bee
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The annual cycle of a bee colony is characterized by a significant renewal of wintered bees by their new generation. This process begins with the queen's first laying of eggs. Initially, the queen lays 20-30 eggs per day...
Scientific revolutions of the twentieth century
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General biology
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Individual development of an organism, or ontogenesis, is a set of successive morphological, physiological and biochemical transformations undergone by the organism from the moment of its inception to death...
1.2.1 Prenatal period
The behavior of the embryo is in many respects the basis of the entire process of behavioral development in ontogenesis. In both invertebrates and vertebrates it has been established...
Sexual behavior of dogs in ontogenesis
1.2.2 Postnatal period
The prenatal (also known as embryonic or intrauterine) period of animal development ends with childbirth. After the moment of birth, the postnatal (also known as post-uterine or post-embryonic) period begins...
Sexual behavior of dogs in ontogenesis
1.2.4 Juvenile period
After four months, the baby begins a new period of ontogenesis - juvenile, or as it is otherwise called, teenage or pre-adult, i.e. preceding adulthood. It continues until puberty...
Reproduction
2.2 Postembryonic period of development
At the moment of birth or the release of the organism from the egg shells, the embryonic period ends and the postembryonic period of development begins. Postembryonic development can be direct or accompanied by transformation (metamorphosis)...
Evolution of groups of organisms
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Biology Periodization and early embryonic development
EMBRYONAL DEVELOPMENT OF MAMMALS AND HUMANS
The study of prenatal and, in particular, embryonic development of humans is very important, as it helps to better understand the relationships between organs and the mechanisms of the occurrence of congenital malformations. There are common features in the embryonic development of different mammalian species, but there are also differences. In all placentals, for example, the processes of early embryogenesis differ significantly from those previously described in other vertebrates. At the same time, among placentals there are interspecific features.
Splitting up The human zygote is characterized by the following features. The plane of the first division passes through the poles of the egg, ᴛ.ᴇ. , like other vertebrates, is a meridian.
Embryogenesis of mammals
In this case, one of the resulting blastomeres turns out to be larger than the other, which indicates uneven division. The first two blastomeres enter the next division asynchronously. The furrow runs along the meridian and perpendicular to the first furrow. However, the stage of three blastomeres occurs. During division of the smaller blastomere, the pair of resulting smaller blastomeres rotates by 90° so that the plane of the division furrow is perpendicular to the first two furrows. A similar arrangement of blastomeres at the 4-cell stage has been described in the mouse, rabbit, mink and monkey (Fig. 7.15). Due to asynchronous cleavage, there are stages with an odd number of blastomeres - 5, 7, 9.
Rice. 7.15. Early stages of cleavage of the rabbit zygote:
I-plane of the first crushing furrow. IIa - the plane of the second cleavage furrow of one of the first two blastomeres, IIb - plane of the second cleavage furrow of the second of the first two blastomeres
As a result of fragmentation, an accumulation of blastomeres is formed - morula. Superficially located blastomeres form a cell layer, and blastomeres lying inside the morula are grouped into a central cellular nodule. At approximately the stage of 58 blastomeres, fluid appears inside the morula, a cavity (blastocoel) is formed and the embryo turns into a blastocyst.
IN blastocyst distinguish between the outer layer of cells (trophoblast) and the inner cell mass (germinal nodule, or embryoblast). The inner cell mass is pushed by fluid to one of the poles of the blastocyst. Later from trophoblast the outer fruit membrane, the chorion, will develop, and from embryoblast - the embryo itself and some extra-embryonic organs. It has been shown that the embryo itself develops from a very small number of cells of the germinal node.
The crushing stage occurs under the shell radiata. In Fig. Figure 7.16 shows the early stages of human embryogenesis, indicating where the embryo is located in the maternal body. The fragmentation of the human zygote and the emergence of blastocytes are schematically presented in Fig. 7.17 and 7.18.
Rice. 7.16. Ovulation, fertilization and human embryo
on the 1st week of development:
1 -ovary, 2- second order oocyte (ovulation), 3 -oviduct, 4- fertilization, 5- zygote 6- embryo at the stage of two blastomeres, 7-embryo at the stage of four blastomeres, 8- embryo at the stage of eight blastomeres, 9 -morula. 10, 11 -blastodista 12- posterior wall of the uterus
Rice. 7.17. Fragmentation of a human zygote.
A- two blastomeres; B- three blastomeres; IN- four blastomeres; G- morula; D- morula section; E, F- section of early and late blastocyst:
1 -embryoblast, 2- trophoblast, 3- blastocoel
Approximately on the 6-7th day after fertilization, the embryo is already 2-3 days old. floating freely in the uterine cavity, ready for implantation, ᴛ.ᴇ. to immersion in its mucous membrane. The radiant shell is destroyed. Having come into contact with maternal tissues, trophoblast cells quickly multiply and destroy the uterine mucosa. Οʜᴎ form two layers: the inner one, called the cytotrophoblast, since it retains the cellular structure, and the outer one, called the syncytiotrophoblast, since it is a syncytium. In Fig. Figure 7.19 shows a human embryo in the process of implantation.
Rice. 7.18. Blastocyst of a human embryo (section):
1- embryoblast, 2- trophoblast, 3- blastocoel
Rice. 7.19. Successive stages of implantation and development
human embryo at the end of the 1st and 2nd week.
A - blastocyst; B - blastocyst at the very beginning of implantation (7th day of development); IN - partially implanted blastocyst (8th day of development); G - embryo on the 9-10th day of development; D- embryo on the 13th day of development:
1 -embryoblast, 2- blastocoel, 3- trophoblast, 4- amnion cavity,
5 -hypoblast, b- synnitiotrophoblast, 7-cytotrophoblast, 8 -epiblast,
9-amnion, 10- trophoblast lacuna 11- uterine epithelium, 12- body leg,
13 - allantois kidney, 14- yolk sac, 15- extraembryonic coelom, 16- chorionic villus, 17- primary yolk sac 18- secondary yolk sac
Rice. 7.19. Continuation
Rice. 7.20. Development of the human embryo at the primitive streak stage
(15-17th day).
A - top view of the embryo (amnion removed); B - longitudinal section; IN - cross section through the primitive streak:
1 - Hensen's node, 2- primitive streak, 3- chord, 4- prechordal plate, 5- amnion, 6- yolk sac, 7-ectoderm. 8- mesoderm, 9- endoderm
Gastrulation in mammals it is closely related to other embryonic transformations. Simultaneously with the division of the trophoblast into two layers, the embryonic nodule becomes flattened and turns into a two-layer embryonic shield. Bottom layer of shield - hypoblast, or primary endoderm, according to most authors, is formed by delamination of the inner cell mass, approximately as it occurs in the germinal disc of birds. Primary endoderm is completely spent on the formation of extraembryonic endoderm. Lining the cavity of the trophoblast, it together with it forms the primary yolk sac of mammals.
Upper cell layer - epiblast - is the source of future ectoderm, mesoderm and secondary endoderm. On the 3rd week, the epiblast forms primitive streak, the development of which is accompanied by almost the same movements of cell masses as during the formation of the primary streak of birds (Fig. 7.20). At the head end of the primitive streak, Hensen's node And primary fossa, homologous to the dorsal lip of the blastopore of other vertebrates. Cells that move in the region of the primary fossa are directed under the epiblast towards the prechordal plate.
Prechordal plate is located at the head end of the embryo and marks the site of the future oropharyngeal membrane. Cells moving along the central axis form the rudiment of the notochord and mesoderm and make up chordomesodermal process. Hensen's node gradually shifts to the caudal end of the embryo, the primary streak shortens, and the notochord primordium lengthens. On the sides of the chordomesodermal process, mesodermal plates are formed, which expand in both directions. Below is a general diagram (7.2) of some processes of early embryonic development.
By the end of the 3rd week, a neural plate. It consists of tall cylindrical cells. In the center of the neural plate a deflection is formed in the form nerve groove, and on its sides rise nerve folds. This is the beginning of neurulation. In the middle part of the embryo, the closure of the neural folds occurs - a neural tube. The closure then spreads in the head and tail directions. The neural tube and adjacent areas of the ectoderm, from which it subsequently develops neural crest, completely submerged and separated from the ectoderm that grows together above them (see Fig. 7.9). The strip of cells lying under the neural tube turns into a notochord. On the sides of the notochord and neural tube in the middle part of the embryo, segments of the dorsal mesoderm appear - somites. By the end of the 4th week they spread to the head and tail ends, reaching approximately 40 pairs.
The beginning of the formation of the primary intestine, the anlage of the heart and the vascular network of the yolk sac dates back to this time. In Fig. 7.21 shows the ratio of the sizes of the embryo and extra-embryonic organs on the 21st day of development. In more detail, the separation of the body of the embryo from the embryonic membranes and the formation of organs can be seen in Fig. 7.22, which shows not only the general view of the embryo, but also the plans of the sections. Noteworthy is the rapid (in 7 days of the 4th week) formation of the embryo in the form of an elongated and curved body, raised and cut off from the yolk sac by trunk folds. During this time, all the somites, four pairs of gill arches, the heart tube, the kidneys of the limbs, the midgut, as well as the “pockets” of the foregut and hindgut are formed.
Scheme 7.2. Differentiation of mammalian germ layers
Rice. 7.21. Human embryo and extra-embryonic organs on the 21st day of development:
1 -amnion, 2- embryo, 3- chorion, 4- tertiary villus, 5- maternal blood, 6- yolk sac
In the next four weeks of embryonic development, all major organs are formed. Violation of the development process during this period leads to the most severe and multiple congenital malformations.
As noted above, the development of extraembryonic provisional organs in mammals and humans has its own characteristics. These organs are formed very early, simultaneously with gastrulation, and somewhat differently than in other amniotes. The beginning of the development of the chorion and amnion occurs on the 7-8th day, ᴛ.ᴇ. coincides with the beginning of implantation.
Chorion arises from the trophoblast, which has already divided into cytotrophoblast and syncytiotrophoblast. The latter, under the influence of contact with the uterine mucosa, grows and destroys it. By the end of the 2nd week, primary chorionic villi are formed in the form of an accumulation of epithelial cytotrophoblast cells. At the beginning of the 3rd week, mesodermal mesenchyme grows into them and secondary villi appear, and when, by the end of the 3rd week, blood vessels appear inside the connective tissue core, they are called tertiary villi. The area where the chorion tissue and the uterine mucosa are closely adjacent is called placenta.
Rice. 7.22. Development of the human embryo at the 4th week.
A 1B 1IN 1- general form; A 2B 2IN 2 - longitudinal section; A 3B 3IN 3 - cross section; A 1A 2A 3 - 22 days; B 1B 2B 3 - 24 days; IN 1IN 2B 3 - 28 days:
1 - level of cross-section, 2- oropharyngeal membrane, 3- brain, 4- cloacal membrane, 5- yolk sac, 6-amnion, 7-somites, 8- neural tube, 9-chord, 10- paired anlages of the abdominal aorta, 11 - cardiac protuberance, 12- heart, 13- head trunk fold. 14- caudal trunk fold, IS- body leg, 16- allantois, 17- lateral trunk folds, 18 - neural crest, 19 - dorsal aorta 20 - midgut, 21 - gill arches, 22- forelimb kidney, 23- kidney of the hind limb. 24- tail, 25- pericardium, 26- hindgut pocket, 27-umbilical cord, 28- foregut pocket, 29- dorsal mesentery, 30- dorsal root ganglion, 31 - intraembryonic coelom
In humans, as in other primates, the vessels of the maternal part of the placenta lose their continuity and the chorionic villi are actually washed by the blood and lymph of the maternal body. This placenta is usually called hemochorial. As pregnancy progresses, the villi increase in size and branch, but the fetal blood from the very beginning to the end is isolated from the maternal blood by the placental barrier.
Placental barrier consists of trophoblast, connective tissue and fetal vascular endothelium. This barrier is permeable to water, electrolytes, nutrients and dissimilation products, as well as to fetal red blood cell antigens and maternal antibodies, toxic substances and hormones. The cells of the placenta produce four hormones, including human chorionic hormone, which is found in the urine of a pregnant woman from the 2-3rd week of pregnancy.
Amnion occurs by divergence of epiblast cells of the inner cell mass. The human amnion is called schizamnion(see Fig. 7.19) in contrast to the pleuramnion of birds and some mammals. The amniotic cavity is not always limited by the epiblast cells and partly by the trophoblast area. Then the side walls of the epiblast form folds directed upward, which subsequently grow together. The cavity is completely lined with epiblastic (ectodermal) cells. Outside, the amniotic ectoderm is surrounded by extraembryonic mesodermal cells.
yolk sac, appears when a thin layer of hypoblast separates from the inner cell mass and its extraembryonic endodermal cells, moving, line the surface of the trophoblast from the inside. The resulting primary yolk sac collapses on the 12th-13th day and transforms into a secondary yolk sac associated with the embryo. Endodermal cells are overgrown with extraembryonic mesoderm on the outside. The fate and functions of the yolk sac have been described previously.
Allantois arises in the human embryo, as in other amniotes, in the form of a pocket in the ventral wall of the hindgut, but its endodermal cavity remains a rudimentary structure. Nevertheless, an abundant network of vessels develops in its walls, connecting with the main blood vessels of the embryo. The allantois mesoderm connects with the chorion mesoderm, giving blood vessels to it. This is how the vascularization of this chorioallantoic placenta occurs.
When comparing the formation, structure and functions of the provisional organs of mammals with similar organs of other amniotes, attention is paid to the manifestations of heterochrony, the intensification of some functions and the weakening of others, and the expansion of functions. However, in the evolution of provisional organs the same methods of phylogenetic transformations of organs are manifested as in the permanent organs of animals.
Some stages and timing of organ development in human embryos are presented in Table. 7.2.
Development of the mammalian embryo
Development in mammals has the same stages as embryogenesis in birds, but there are differences that relate to the early stages of embryogenesis, especially gastrulation.
Provisional organs have their own structure and functions. At the beginning, the formation of provisional organs occurs, conditions for development are created, then gastrulation occurs. This is due to the complex structure of mammals. Embryogenesis is long, there is no larval stage, it occurs in utero at the expense of the mother’s body. The egg is secondary isoleucital, fertilization occurs in the proximal genital tract. The crushing is complete, uneven, asynchronous. The difference is revealed at the stage of two blastomeres (dark and light). Light blastomeres divide faster than dark ones. Dark blastomeres are located in the center of the embryo and form the embryoblast. The light ones grow around the dark ones, and a germinal nodule is formed. Light blastomeres form trophoblast− provisional organ formed from extraembryonic ectoderm. It will perform a trophic function. The trophoblast absorbs mucus from the genital tract, which is used to nourish the embryo. Fluid accumulates inside the embryo and a cavity forms, forming a germinal vesicle (blastula). The cavity increases, the volume of fluid increases, which pushes the embryoblast upward.
Early gastrulation - delamination with the formation of a two-layer embryo. The inner layer contains endoderm material, and the outer layer contains ecto and mesoderm. The trophoblast above the embryoblast is resorbed and its place is taken by the outer layer of the embryo.
The late period of gastrulation proceeds in the same way as in birds.
In the outer layer, the germinal shield is distinguished, the blastomeres proliferate, and the primitive streak is formed. The primary nodule, the presumptive material of the notochord, neural plate, mesoderm, notochord and neural tube are formed, a three-layered embryo with a complex of axial organs is formed. A trunk fold is formed, which separates the embryonic material from the non-embryonic material, the amnion is formed, which contains an aqueous environment for development; the yolk sac (without yolk) loses its trophic function. Its main function is hematopoietic (blood stem cells are deposited in its wall). Reproductive function (primary germ cells) is also present.
From the caudal part of the intestinal tube, allontosi is formed, which does not perform an excretory function, but serves as a guide for growing blood vessels.
The trophoblast forms villi, and parietal mesoderm grows towards it. Grows into the villi. Blood vessels are formed in the mesoderm.
DEVELOPMENT OF MAMMALS
The trophoblast turns into chorion. Chorionic villi penetrate the uterine mucosa and form the placenta with it.
Peculiarities:
Early trophoblast release.
●Trophoblast transformation into chorion and placenta.
●The placenta is a provisional organ that has its own evolution. Types of placentas
●Epitheliochorial (diffuse) - in horses, cows. The chorionic villi deepen into small pits in the epithelium of the uterine mucosa.
●Desmochorial (cotyledonous) - in ruminants. The villi are embedded in the underlying connective tissue.
●Endotheliochorial (girdle) - in carnivores. The villi penetrate deeply into the mucosa and reach the walls of the blood capillaries. There is postpartum bleeding.
●Hemochorial (discoidal). The villi penetrate deeply into the mucous membrane of the uterus and into the lumen of blood vessels. Maternal blood washes the villi.
The placenta takes on the functions of all provisional organs:
● Trophic - the placenta absorbs simple proteins from the mother’s blood, from which it synthesizes complex proteins that enter the developing organism and are used to build its tissues and organs.
● Respiratory function
● Protective function, including the function of immunobiological protection.
● Hormonal function
● Regulates the development of the fetus, maintains pregnancy, prepares the mother’s body for feeding.
● Excretory
Organs formed from germ layers.
1.Outer, ectoderm. Organs and parts of the embryo. Neural plate, neural tube, outer layer of skin, hearing organs.
2.Internal, endoderm. Organs and parts of the embryo. Intestines, lungs, liver, pancreas.
3. Medium, mesoderm. Organs and parts of the embryo. Notochord, cartilaginous and bony skeleton, muscles, kidneys, blood vessels.
At the same time, the notochord is formed from the mesoderm - a flexible skeletal cord located on the dorsal side of the embryos of all vertebrates.
8. Embryonic development of animals
In vertebrates, the notochord is replaced by the spine, and only in some lower vertebrates are its remains preserved between the vertebrae even in adulthood.
The neural plate is formed from the ectoderm located above the notochord itself. Subsequently, the lateral edges of the plate rise, and its central part descends, forming the neural groove. Gradually, the upper edges of these folds close, and the groove turns into a neural tube lying under the ectoderm - the rudiment of the central nervous system.
The neural tube, notochord and intestine create the axial complex of embryonic organs, which determines the bilateral symmetry of the body.
The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. In this case, one rudiment influences the other, largely determining the path of its development. In addition, the rate of growth and development of the embryo is affected by internal and external conditions.
Interaction of parts of the embryo in the process of embryonic development - the basis of its integrity. The similarity of the initial stages of development of the embryos of vertebrate animals is proof of their relationship.
High sensitivity of the embryo to environmental factors. The harmful effects of alcohol, drugs, smoking on the development of the fetus, on adolescents and adults.
