Nervous tissue is formed from one germ layer. Germinal leaves. Provisory organs of mammals

Not having in your disposal of early embryos human, showing some of the most important stages in 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 by 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.

Almost immediately they are located in the form of a hollow formation called the blastoderm vesicle. At one pole, a group of cells gathers, known as the inner cell mass. As soon as it is formed, cells begin to emerge from it, lining a small internal cavity - the primary intestine, or archenteron. These cells form the endoderm.

Ta part of the original group The cells from which the integument of the embryo and the outermost layer of its membranes are formed is called the ectoderm. Soon, between the first two germ layers, a third layer is formed, which is quite aptly called the mesoderm.

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 the lower animals - the ancestors of vertebrates. From the point of view of possible ontogenetic recapitulations, some facts quite allow for this.

Nervous system of embryos vertebrates arise from the ectoderm - a layer of cells with the help of which primitive organisms that do not yet have a nervous system are in contact with the external environment. The lining of the digestive tube of vertebrates is formed from the endoderm - a layer of cells that, in very primitive forms, lines their internal cavity similar to the gastrocoel.

Skeletal, muscular and circulatory systems originate in vertebrates almost exclusively from the mesoderm - a layer that is relatively inconspicuous in small, low-organized creatures, but whose role increases with their size and complexity due to the increase in their needs for supporting 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 ratios are basically the same in all vertebrate embryos strongly suggests a common origin and similar heredity in the various members of this vast group of animals.

One might think that in these germ layers for the first time, differences of different classes begin to be created over the general plan of the body structure, characteristic of all vertebrates.

The formation of embryonic leaflets the period ends when the main process of development is only an increase in the number of cells, and the period of differentiation and specialization of cells begins. Differentiation occurs in the germ layers before we can see signs of it with any of our microscopic methods. In a leaf that has a completely uniform appearance, localized groups of cells constantly arise with different potentialities for further development.

We have known this for a long time, for we can see how from the germ layer different structures emerge. At the same time, no visible changes are imperceptible 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, which we easily recognize as the rudiment of the definitive organ.

So, for example, if you cut from any site of Hensen's node a narrow transverse strip of the ectoderm of a twelve-hour embryo and grow it in tissue culture, then at a certain time specialized cellular elements of a type that is found only in the eye will be found, although the bud of the eye bubble of a chicken embryo does not appear before 30 hours of incubation. A strip taken from a different site, although it looks the same, when grown in culture does not form cells characteristic of the eye, but shows a different specialization.

Experiments show how early groups of cells with different potentialities for development are determined in the germ layers. As development progresses, these cell groups become more and more prominent. In some cases, they are isolated from the parent 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 that arose in this way, gradually definitive organs are formed. Therefore, the origin of various parts of the body in embryogenesis depends on the growth, subdivision and differentiation of the germ layers. This scheme shows us the general path along which the early processes discussed above develop. If we follow the process of development further, we will see that each normal division of the object is more or less clearly centered around a certain branch of this genealogical tree of germ layers.

Germ layers - groups of related cells that separate during gastrulation and give certain organs.

In all animals, two layers of cells separate during gastrulation - the ectoderm (outer layer) and the endoderm (inner layer). As shown in fig. 152, the frog immediately separates the third, intermediate layer - the mesoderm. Most other animals (except coelenterates) also have these three layers - ectoderm, endoderm and mesoderm, each of which originates from a certain group of blastomeres, so that all cells within one layer are relatives. These groups of cells that have a common origin and form certain organs and tissues of an adult animal are called germ layers.

In all animals, the same organs are obtained from the same germ layer. The ectoderm gives the outer coverings and the nervous system. Most of the digestive tract and digestive glands are formed from the endoderm (in vertebrates, the liver, pancreas, and lungs). The mesoderm forms the rest of the organs: muscles, lining of the secondary body cavity, organs of the circulatory, excretory and reproductive systems, in vertebrates and echinoderms - the internal skeleton. (It must be borne in mind that most organs of an adult animal include tissues originating from two or all three germ layers.) A very important conclusion follows from this: in all animals, the main organ systems have a common origin, and they can be compared. For example, the central nervous system has a common origin in the sense that in evolution it originates from the subcutaneous nerve plexus, similar to the nervous network of the hydra, and in ontogeny from the outer germ layer.

Previously, it was believed that there are clear boundaries between the germ layers: the cells of one leaf cannot pass into another. Gradually accumulated facts that do not fit into this scheme. For example, it has been shown that during regeneration in some animals, the entire body is restored from areas where the cells of one of the leaves are absent. Mesoderm cells have a different origin: after gastrulation, some of them are evicted from the ectoderm, and some from the endoderm. In the endoderm, most animals have a special section of the nervous system, which may also be laid down here. Some bones in vertebrates develop from cells of the neural crest, a derivative of the ectoderm. Cells of the nervous and muscular systems can be close relatives: in nematodes, the nerve and muscle cells can be "siblings". Nevertheless, all these facts do not refute the concept of germ layers, but only show that in this case there are a number of exceptions to the general rule.

The lancelet egg contains practically no yolk and therefore undergoes complete uniform crushing. A group of cells arises, which then move apart and form a hollow ball - the blastula. Depending on the size of the cells in the blastula, the animal (smaller cells) and vegetative (large cells) poles are distinguished. The cells of the vegetative pole begin to bulge into the cavity of the blastula, gradually completely displacing it. A 2-layer embryo is formed - gastrula, consisting of 2 germ layers: ectoderm (outer) and endoderm (inner); The cavity of the gastrula opens to the outside with an opening - the blastopore (or gastropore). Transverse sections of the gastrula show further differentiation of the germ layers.

Part of the ectoderm cells on the dorsal side of the embryo flattens, forming a longitudinal neural plate. Gradually, the neural plate forms a groove, and then folds into the neural tube; its cavity is first at the anterior end connected by an opening (neuropore) with the external environment, and at the posterior end by a neuro-intestinal canal with the intestinal cavity. Later, both connections are overgrown.

The ectoderm gradually overgrows the area that has differentiated into the neural tube, forming a continuous cover. Thus, the ectoderm gives rise to the integument of the body (the epidermal layer of the skin) and the CNS.

The endoderm also undergoes a process of differentiation. Its main part forms the intestinal tube, the cavity of which corresponds to the cavity of the gastrula. It is connected with the external environment at the posterior end by an anus, which breaks through at the site of a temporarily overgrown blastopore, and at the anterior end by a self-forming oral opening. Outgrowths appear in the walls of the intestinal tube, one of which is located on the dorsal side; later it laces off and turns into a longitudinally located unpaired chord that does not contain a cavity. In addition, paired "pockets" appear on the sides of the upper part of the intestinal wall. These paired outgrowths eventually separate from the walls of the intestine, retaining a cavity in themselves, which becomes the secondary cavity of the body (the whole). This process proceeds metamerically, and the common coelomic cavity is formed by the merging of the cavities of individual mesodermal segments. The tissue that makes up the walls of these outgrowths is the 3rd germ layer - the mesoderm. Thus, from the endoderm, the inner (epithelial) intestinal wall and notochord; mesoderm originates from it.

Subsequently, mesodermal outgrowths grow downward, surrounding the intestine, and at the same time differentiate into somites metamerically located along the chord and lateral plates covering the intestine. The walls of the somites give rise to metameric axial muscles and connective tissue that make up the inner layer of the skin (corium), the sheath of the notochord and neural tube, and the myosepta.

In vertebrates, a certain section of somites (sclerotome) forms the internal axial skeleton. From the walls of the side plates, the intestinal musculature arises, its connective tissue membrane, the peritoneum (mesenterium), 2 sheets of which cover the internal organs and “suspend” them to the walls of the body, as well as the walls of the blood vessels. A special section of the mesoderm, adjacent to the abdominal wall of the somites, gives rise to gonads. Thus, the mesoderm serves as a source of formation of the internal skeleton, muscles, connective tissue and circulatory system. In vertebrates, the excretory system is also formed from this germ layer: the excretory tubules of the kidneys (nephrons) are formed by a special section of the mesoderm at the junction of somites and lateral plates.

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germinalleaflets, germ layers, layers of the body of the embryo of multicellular animals and humans, formed in the process of gastrulation. Most organisms have three zonal layers: the outer one is the ectoderm, the inner one is the endoderm, and the middle one is the mesoderm. The exceptions are sponges and coelenterates, in which only two Z. l are formed. ≈ external and internal. Ectoderm derivatives perform integumentary, sensory and motor functions; from them, during the development of the embryo, the nervous system, the skin and the skin glands formed from it, hair, feathers, scales, nails, etc., the epithelium of the anterior and posterior sections of the digestive system, the connective tissue base of the skin, pigment cells and the visceral skeleton. The endoderm forms the lining of the intestinal cavity and provides nourishment to the embryo; from it arise the mucous membrane of the digestive system, the digestive glands, and the respiratory organs. The mesoderm provides a connection between the parts of the embryo and performs supporting and trophic functions; excretory organs, genitals, circulatory system, serous membranes are formed from it, lining the secondary cavity of the body (whole) and dressing internal organs, muscles; in vertebrates, the skeleton is also formed from the mesoderm. The eponymous Z. l. in different groups of organisms, along with similarities, they can have significant differences both in the method of formation and in structure, associated with the adaptation of the embryos to various conditions of development. See Embryonic Development.

The doctrine about Z. l. ≈ one of the largest generalizations of embryology ≈ has a long history and is associated with major biological teachings and discoveries, such as epigenesis, cell theory, Darwinism. K. F. Wolf, H. I. Pander, K. M. Baer, ​​R. Remak, E. Haeckel, O.

Gertwig, A. O. Kovalevsky and I. I. Mechnikov. The latest data from experimental embryology, including the results of intravital staining of different parts of the blastula wall and tracking their movements during gastrulation and neurulation, made it possible to determine, already at the blastula stage, the position of groups of cells from which various zona l cells will form in the future. and their derivatives, and create a map of future rudiments of organs and their systems. Experiments on transplantation and removal of material from different Z. l. at the stage of blastula and during the period of gastrulation, the properties of the material of different zona pelvis were elucidated. and their ability to differentiate: at first, sections of different Z. l. during transplantation, they can still replace each other, but by the end of gastrulation, they lose this ability.

T. A. Detlaf.

1.outdoor, ectoderm.

2.Internal, endoderm

3. Medium, mesoderm.

At the same time, a chord is formed from the mesoderm - a flexible skeletal cord located in the embryos of all vertebrates on the dorsal side. In vertebrates, the notochord is replaced by the vertebrae, and only in some lower vertebrates does its remnant remain between the vertebrae even in adulthood.

The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction.

From which germ layer does the notochord develop? neural tube?

At the same time, one germ influences the other, to a large extent determining the path of its development. In addition, the rate of growth and development of the embryo is affected by internal and external conditions.

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germ layers(embryonic layers, lat.folia embryonalia) - layers of the body of embryos of multicellular animals, formed during gastrulation and giving rise to various organs and tissues.

Organogenesis

The doctrine of germ layers, one of the main generalizations in embryology, has played a large role in the history of biology.

The formation of germ layers is the first sign of embryo differentiation. In most organisms, three germ layers are formed: the outer one is the ectoderm, the inner one is the endoderm and the middle one is the mesoderm. Derivatives of the ectoderm perform mainly integumentary and sensory functions, derivatives of the endoderm - the functions of nutrition and respiration, and derivatives of the mesoderm - connections between parts of the embryo, motor, support and trophic functions.

In 2000, Canadian embryologist Brian Keith Hall proposed that the neural crest be considered nothing more than a separate fourth germ layer. This interpretation quickly spread in the scientific literature.

In all animals, the same organs are obtained from the same germ layer. The ectoderm gives the outer coverings and the nervous system. Most of the digestive tract and digestive glands are formed from the endoderm (in vertebrates, the liver, pancreas, and lungs). The mesoderm forms the rest of the organs: muscles, lining of the secondary body cavity, organs of the circulatory, excretory and reproductive systems, in vertebrates and echinoderms - the internal skeleton.

Most organs of an adult animal include tissues originating from two or all three germ layers. A very important conclusion follows from this: in all animals, the main organ systems have a common origin, and they can be compared. For example, the central nervous system has a common origin in the sense that in evolution it originates from the subcutaneous nerve plexus, similar to the nervous network of the hydra, and in ontogeny from the outer germ layer.

see also

Notes

Organogenesis- a set of processes of differentiation and change in the shape of parts of the body based on the implementation of genetic information.

organogenesis, consisting in the formation of individual organs, constitute the main content of the embryonic period.

• continue in the larval and end in the juvenile period
• differ in the most complex and diverse morphogenetic transformations
• A necessary prerequisite for the transition to organogenesis is the achievement by the embryo of the gastrula stage, namely the formation of germ layers.

Occupying a certain position in relation to each other, the germ layers, by contacting and interacting, provide such relationships between different cell groups that stimulate their development in a certain direction. This so-called embryonic induction - the most important consequence of the interaction between the germ layers.

During organogenesis:

• changes in the shape, structure and chemical composition of cells
• cell groups are isolated, representing the rudiments of future organs.
• A certain form of organs gradually develops, spatial and functional connections are established between them.
• The processes of morphogenesis are accompanied by differentiation of tissues and cells, as well as selective and uneven growth of individual organs and parts of the body.

The very beginning of organogenesis is called the period of neurulation.

Neurulation covers the processes from the appearance of the first signs of the formation of the neural plate to its closure in the neural tube.

In parallel formed notochord and secondary gut , and the mesoderm lying on the sides of the chord splits in the craniocaudal direction into segmented paired structures - somites .

The nervous system of vertebrates, including humans, is characterized by the stability of the main structural plan throughout the evolutionary history of the subtype. In the formation of the neural tube, all chordates have much in common. Initially, the unspecialized dorsal ectoderm, responding to the induction action from the chordomesoderm, turns into neural plate, submitted neuroepithelial cells.

The neural plate does not remain flattened for long. Soon, its lateral edges rise, forming neural folds , which lie on both sides of a shallow longitudinal neural groove . The edges of the neural folds then close, forming a closed neural tube with a channel inside - neuroceleme . First of all, the closure of the neural folds occurs at the level of the beginning of the spinal cord, and then spreads in the head and tail directions.

It has been shown that microtubules and microfilaments of neuroepithelial cells play an important role in the morphogenesis of the neural tube. The destruction of cellular structures by colchicine and cytochalasin B leads to the fact that the neural plate remains open. Non-closure of the neural folds leads to congenital malformations of the neural tube.

After the closure of the neural folds, the cells that were originally located between the neural plate and the future skin ectoderm form neural crest. Neural crest cells are distinguished by their ability to migrate extensively but in a highly regulated fashion throughout the body and form two main streams. The cells of one of them - superficial - are included in the epidermis or dermis of the skin, where they differentiate into pigment cells. Another stream migrates in the abdominal direction, forms sensitive spinal ganglia, sympathetic ganglions, adrenal medulla, parasympathetic ganglia. Cells from the cranial neural crest give rise to both nerve cells and a number of other structures, such as gill cartilage, some covering bones of the skull.

The mesoderm, which occupies a place on the sides of the notochord and extends further between the skin ectoderm and the endoderm of the secondary intestine, is divided into dorsal and ventral regions. The dorsal part is segmented and presented in pairs. somites. The laying of somites goes from the head to the tail end. The ventral part of the mesoderm, which looks like a thin layer of cells, is called side plate. The somites are connected to the lateral plate by the intermediate mesoderm in the form of segmented somite legs.

All areas of the mesoderm gradually differentiate. At the beginning of formation, somites have a configuration characteristic of an epithelium with a cavity inside. Under the induction effect coming from the notochord and neural tube, the ventromedial parts of the somites - sclerotomes - turn into secondary mesenchyme, are evicted from the somite and surround the notochord and the ventral part of the neural tube. In the end, vertebrae, ribs and shoulder blades are formed from them.

The dorsolateral part of the somites on the inside forms myotomes , from which the striated skeletal muscles of the body and limbs will develop. The outer dorsolateral part of the somites forms dermatomes, which give rise to the inner layer of the skin - the dermis. From the region of the legs of somites with rudiments nephrotome and gonotomy excretory organs and sex glands are formed.

The right and left non-segmented lateral plates split into two sheets that limit the secondary body cavity - in general. The inner leaf adjacent to the endoderm is called visceral. It surrounds the intestine from all sides and forms the mesentery, covers the pulmonary parenchyma and the heart muscle. The outer sheet of the lateral plate is adjacent to the ectoderm and is called parietal. In the future, it forms the outer sheets of the peritoneum, pleura and pericardium.

The endoderm in all embryos ultimately forms the epithelium of the secondary gut and many of its derivatives. The secondary gut itself is always located under the chord.

Thus, in the process of neurulation, a complex axial organs neural tube - chord - gut, which are a characteristic feature of the organization of the body of all chordates. The same origin, development and mutual arrangement of the axial organs reveal their complete homology and evolutionary continuity.

Ectoderm, mesoderm and endoderm in the course of further development, interacting with each other, participate in the formation of certain organs. The emergence of the rudiment of an organ is associated with local changes in a certain area of ​​the corresponding germ layer. From the ectoderm the skin epidermis and its derivatives (feather, hair, nails, skin and mammary glands), components of the organs of vision, hearing, smell, oral cavity epithelium, and tooth enamel develop.

2.7. Derivatives of the germ layers

The ectodermal derivatives are the neural tube, the neural crest, and all the nerve cells derived from them.

Endoderm derivatives are the epithelium of the stomach and intestines, liver cells, secreting cells of the pancreas, intestinal and gastric glands. The anterior part of the embryonic gut forms the epithelium of the lungs and airways, as well as the secreting cells of the anterior and middle lobes of the pituitary, thyroid and parathyroid glands.

mesoderm forms skeletal muscles, dermis of the skin, organs of the excretory and reproductive systems, cardiovascular system, lymphatic system, pleura, peritoneum and pericardium. From the mesenchyme, which has a mixed origin due to the cells of the three germ layers, all types of connective tissue, smooth muscles, blood and lymph develop.

The rudiment of a particular organ is initially formed from a certain germ layer, but then the organ becomes more complex and, as a result, two or three germ layers take part in its formation.

25. Provisional organs of vertebrate embryos, their functions. Animal groups: anamnia and amniotes.

Provisional or provisional bodies- organs that are formed in the embryogenesis of a number of representatives of vertebrates to provide vital functions, such as respiration, nutrition, excretion, movement, etc.

Provisory organs of birds:

1) Yolk sac- trophic function, the formation of blood cells and vessel walls, the formation of primary germ cells - gonoblasts. The wall of the yolk sac is formed by the extraembryonic endoderm and the visceral layer of the extraembryonic mesoderm.

2) Amnion creates an internal environment in which the development of the embryo occurs. It produces a liquid that occupies the space between the shell and the body of the embryo and performs trophic and protective functions. It consists of the extra-embryonic ectoderm and the parietal sheet of the extra-embryonic mesoderm.

3) Serous membrane is a provisional respiratory organ and is involved in the supply of oxygen to the embryo. It consists of the extra-embryonic ectoderm and the parietal sheet of the extra-embryonic mesoderm.

4) Allantois participates in the release of metabolic products of the embryo and in gas exchange. The wall consists of the extra-embryonic endoderm and the visceral layer of the extra-embryonic mesoderm.

Provisional organs of mammals:

1) Yolk sac is formed as a result of the formation of the trunk fold, which divides the endoderm into intestinal and yolk. It is involved in the absorption and transport of nutrients from the uterus. Primary sex cells are localized in the yolk sac, blood stem cells and primary blood vessels are formed.

2) Amnion It is formed from a fold of the extra-embryonic ectoderm and a parietal sheet of the extra-embryonic mesoderm. The function of the amnion is to create and maintain the fluid environment in which the embryo develops. The embryo experiences the same pressure from all sides, which protects it from mechanical damage.

3) Allantois It is formed as a small finger-shaped outgrowth of the posterior part of the primary intestine, growing into the amniotic stalk. Its wall is formed by the extraembryonic endoderm and the visceral sheet of the extraembryonic mesoderm. Its function is associated with the release of metabolic products, but only before the formation of the placenta. In addition, it is a "vector" along which vessels grow from the embryo to the chorion.

4) Chorion develops from the trophoblast (extra-embryonic ectoderm) and extra-embryonic mesoderm. Numerous outgrowths - villi appear on the outer surface of the chorion. After the introduction of the embryo into the uterine mucosa, on the one hand, the chorion villi grow strongly, forming a branched chorion, which, together with the uterine mucosa, forms a new organ - placenta. This organ is necessary to supply the fetus with nutrients and oxygen and to remove end products of metabolism from the fetus.

Anamnii- lower vertebrates that do not have embryonic membranes. (there is no germinal membrane - amnion and germinal organ - allantois).

Amniotes– higher vertebrates, which are characterized by the presence of embryonic membranes (amnion and serosa) that form the amniotic cavity.

26. Placenta, its role. Types of placenta. Human placenta.

Placenta - close overlap or fusion of the embryonic membranes with the tissues of the parent organism. (or: placenta - the area where the tissues of the chorion and the uterine mucosa are closely adjacent.)

The placenta is necessary to supply the fetus with nutrients and oxygen and to remove metabolic end products from the fetus.

Types of placenta:

1) Epitheliochorial - found in horses, pigs, cetaceans. When the placenta is formed, the chorionic villi grow into the openings of the uterine glands and come into contact with the intact epithelium of these glands.

2) Syndesmochorial - found in cows, sheep, deer. When the placenta is formed, chorionic villi destroy the epithelium of the uterine glands and come into contact with the underlying connective tissue of the endometrium.

3) Endotheliochorial - found in cats, dogs, seals. When the placenta is formed, the chorionic villi grow to the blood vessels and contact directly with them.

4) Hemochorial - occurs in humans, primates, rodents and hares. During the formation of the placenta, the chorionic villi also destroy the walls of the vessels of the uterus and come into contact with maternal blood, washing the lacunae with it.

In humans, the chorionic villi are actually washed by the blood and lymph of the mother's body. As pregnancy develops, the villi increase in size, branch out, but the blood of the fetus is isolated from the mother's blood from the very beginning to the end by the placental barrier.

The placental barrier is from trophoblast, connective tissue and endothelium of fetal vessels. This barrier is permeable to water, electrolytes, nutrients and dissimilation products, as well as to fetal erythrocyte antigens and maternal antibodies, toxic substances and hormones. The cells of the placenta produce 4 hormones, incl. chorionic gonadotropin, which is found in the urine of a pregnant woman from 2-3 weeks of pregnancy.

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Organs that develop from germ layers.

1.outdoor, ectoderm. Organs and parts of the embryo. Neural plate, neural tube, outer layer of the skin, organs of hearing.

2.Internal, endoderm. Organs and parts of the embryo. Intestines, lungs, liver, pancreas.

3. Medium, mesoderm. Organs and parts of the embryo. Notochord, cartilage and bone skeleton, muscles, kidneys, blood vessels.

At the same time, a chord is formed from the mesoderm - a flexible skeletal cord located in the embryos of all vertebrates on the dorsal side.

51. Development of germ layers and main systems on the example of the lancelet

In vertebrates, the notochord is replaced by the vertebrae, and only in some lower vertebrates does its remnant remain between the vertebrae even in adulthood.

From the ectoderm, located above the chord itself, the neural plate is formed. Later, 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 intestines form the axial complex of the organs of the embryo, 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. At the same time, one germ influences the other, to a large extent determining the path of its development. In addition, the rate of growth and development of the embryo is affected by internal and external conditions.

The 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 vertebrates 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 a teenager and an adult.

The germ layers are dynamic accumulations of cells that are naturally formed in embryogenesis by a certain spatial arrangement.

The first who drew attention to the emergence of organs from germ layers, or layers, was K. F. Wolf (1759). Studying the development of the chicken, he showed that germ layers arise from the "unorganized, structureless" mass of the egg, which then give rise to individual organs. KF Wolf distinguished between the nervous and intestinal layers, from which the corresponding organs develop. Subsequently, X. Pander (1817), a follower of K. F. Wolf, also described the presence of germ layers in the chicken embryo. K. M. Baer (1828) discovered the presence of germ layers in other animals, in connection with which he extended the concept of germ layers to all vertebrates. So, K. M. Baer distinguished primary germ layers, calling them animal and vegetative, from which later, in the process of embryonic development, secondary germ layers arise, giving rise to certain organs.

The description of the germ layers greatly facilitated the study of the features of the embryonic development of organisms and made it possible to establish phylogenetic relationships between animals, which seemed to be very distant in a systematic sense. This was brilliantly demonstrated by A. O. Kovalevsky (1865, 1871), who is rightfully considered the founder of the modern theory of germ layers. A. O. Kovalevsky, on the basis of extensive comparative embryological comparisons, showed that almost all multicellular organisms pass through the two-layer stage of development. He proved the similarity of the germ layers in different animals, not only in origin, but also in the derivatives of the germ layers.

However, there are a number of exceptions to the germ layer theory. According to this theory, the notochord develops from the endoderm, the nervous system from the ectoderm, and the muscle tissue from the mesoderm. However, in reptiles, birds, and mammals, the notochord develops from the mesoderm, which arises from the ectoderm. In ascidians, certain groups of blastomeres simultaneously give rise to both the notochord and the nervous system, i.e., organs that, according to the theory of germ layers, originate from various germ layers. The smooth muscle tissue of the iris of the eye, the muscles of the hair follicles of the skin of mammals does not develop from the mesoderm, as required by the theory of germ layers, but from the ectoderm.

Thus, the theory of germ layers is the largest morphological generalization in the history of embryology. Thanks to her, a new direction in embryology arose, namely evolutionary embryology, which showed that the germ layers present in the vast majority of animals are one of the evidences of the common origin and unity of the entire animal world.

GERMAN LEAF DERIVATIVES
From the moment the germ layers appear, their cellular material is specialized in the direction of the formation of certain embryonic rudiments, as well as a wide range of tissues and organs. Already at the stage of formation of germ layers, differences in their cellular composition are observed. So, ectoderm cells are always smaller in size, more regular in shape and divide faster than endoderm cells. The differences arising in the process of embryonic development in the initially homogeneous material, as well as between the cells of the germ layers, are called differentiation . This is the final stage of embryogenesis.

Outer germ layer or ectoderm , in the process of development gives such embryonic rudiments as the neural tube, ganglionic plate, skin ectoderm and extraembryonic ectoderm. From these embryonic rudiments, the following tissues and organs arise. The neural tube gives rise to neurons and macroglia (cells in the brain that fill the spaces between nerve cells - neurons - and capillaries surrounding them) of the brain and spinal cord, the tail muscles of amphibian embryos, and the retina of the eye. From the ganglionic plate arise neurons and macroglia of the ganglia of the somatic and autonomic nervous system, macroglia of nerves and nerve endings, chromatophores of lower vertebrates, birds and mammals, chromaffin cells, the adrenal medulla, skeletal anlages of the jaw, hyoid, gill arches, cartilage of the larynx, as well as ectomesenchyme . The neurons and macroglia of some ganglia, or nerve ganglions, of the head develop from the placodes, as well as the organs of balance, hearing, and the lens of the eye. The skin ectoderm gives rise to the epidermis of the skin and its derivatives - the glands of the skin, hairline, nails, etc., the epithelium of the mucous membrane of the vestibule of the oral cavity, vagina, rectum and their glands, as well as tooth enamel. In addition, the muscle fibers of the hair follicles of the skin and the iris of the eye develop from the skin ectoderm. From the extraembryonic ectoderm, the epithelium of the amnion, chorion, and umbilical cord arises, and in the embryos of reptiles and birds, the epithelium of the serous membrane.

inner germ layer or endoderm , in development it forms such embryonic rudiments as the intestinal and yolk endoderm. From these embryonic rudiments, the following tissues and organs develop. The intestinal endoderm is the starting point for the formation of the epithelium of the gastrointestinal tract and glands - the glandular part of the liver, pancreas, salivary glands, as well as the epithelium of the respiratory organs and their glands. The yolk endoderm differentiates into the yolk sac epithelium. The extraembryonic endoderm develops into the corresponding sheath of the yolk sac.

middle germ layer or mesoderm , in the process of development, it gives such embryonic rudiments as the chordal rudiment, somites and their derivatives in the form of a dermatome, myotome and sclerotome (scleros - solid), as well as embryonic connective tissue, or mesenchyme. In addition, the mesoderm forms the nephrotome, mesonephric, or wolfian, channels; müllerian, or paramesonephric, canals; splanchnotome; mesenchyme escaping from the splanchnotome; extraembryonic mesoderm. From the notochordal rudiment in non-cranial, cyclostomes, whole-headed, sturgeons and lungfish, a notochord develops, which in the listed groups of animals persists for life, and in vertebrates it is replaced by skeletal tissues. The dermatome gives the connective tissue basis of the skin, the myotome gives the striated muscle tissue of the skeletal type, and the sclerotome forms the skeletal tissues - cartilage and bone. Nephrotomes give rise to the epithelium of the kidney, urinary tract, and wolfian channels give rise to the epithelium of the vas deferens. Müllerian canals form the epithelium of the oviduct, uterus, and the primary epithelial lining of the vagina. From the splanchnotome develops the coelomic epithelium, or mesothelium, the cortical layer of the adrenal glands, the muscle tissue of the heart and the follicular epithelium of the gonads. The mesenchyme, which is evicted from the splanchnotome, differentiates into blood cells, connective tissue, vessels, smooth muscle tissue of hollow internal organs and vessels. The extraembryonic mesoderm gives rise to the connective tissue basis of the chorion, amnion, and yolk sac.

Provisional organs of vertebrate embryos or embryonic membranes. The relationship between mother and fetus. Influence of bad habits of parents (drinking alcohol, etc.) on the development of the fetus.

provisional, or temporary, organs are formed in the embryogenesis of a number of representatives of vertebrates to provide vital functions, such as respiration, nutrition, excretion, movement, etc. The underdeveloped organs of the embryo itself are not yet able to function as intended, although they necessarily play some role in the system of a developing integral organism. As soon as the embryo reaches the necessary degree of maturity, when most of the organs are capable of performing vital functions, the temporary organs are resorbed or discarded.

The time of formation of provisional organs depends on what reserves of nutrients have been accumulated in the egg and in what environmental conditions the embryo develops. In tailless amphibians, for example, due to the sufficient amount of yolk in the egg and the fact that development takes place in water, the embryo carries out gas exchange and releases dissimilation products directly through the egg membranes and reaches the tadpole stage. At this stage, provisional organs of respiration (gills), digestion and movement adapted to an aquatic lifestyle are formed. The listed larval organs enable the tadpole to continue its development. Upon reaching the state of morphological and functional maturity of the organs of the adult type, temporary organs disappear in the process of metamorphosis.

There is much in common in the structure and functions of the provisional organs of various amniotes. Characterizing in the most general form the provisional organs of the embryos of higher vertebrates, also called germinal membranes, it should be noted that they all develop from the cellular material of already formed germ layers. Some features are present in the development of the embryonic membranes of placental mammals.

Amnion is an ectodermal sac containing the embryo and filled with amniotic fluid. The amniotic membrane is specialized for the secretion and absorption of the amniotic fluid surrounding the fetus. Amnion plays a primary role in protecting the embryo from drying out and from mechanical damage, creating for it the most favorable and natural aquatic environment. The amnion also has a mesodermal layer from the extraembryonic somatopleura, which gives rise to smooth muscle fibers. The contractions of these muscles cause the amnion to pulsate, and the slow oscillatory movements communicated to the embryo in this process apparently help to ensure that its growing parts do not interfere with each other.

Chorion(serosa) - the outermost germinal membrane adjacent to the shell or maternal tissues, arising, like the amnion, from the ectoderm and somatopleura. The chorion serves for the exchange between the embryo and the environment. In oviparous species, its main function is respiratory gas exchange; in mammals, it performs much more extensive functions, participating in addition to respiration in nutrition, excretion, filtration, and the synthesis of substances, such as hormones.

Yolk sac is of endodermal origin, covered by visceral mesoderm and directly connected to the intestinal tube of the embryo. In embryos with a large amount of yolk, it takes part in nutrition. In birds, for example, in the splanchnopleura of the yolk sac, a vascular network develops. The yolk does not pass through the yolk duct, which connects the sac to the intestine. First, it is converted into a soluble form by the action of digestive enzymes produced by the endodermal cells of the sac wall. Then it enters the vessels and spreads with blood throughout the body of the embryo. Mammals do not have yolk reserves and the preservation of the yolk sac may be associated with important secondary functions. The endoderm of the yolk sac serves as the site of the formation of primary germ cells, the mesoderm gives the blood cells of the embryo. In addition, the yolk sac of mammals is filled with a liquid characterized by a high concentration of amino acids and glucose, which indicates the possibility of protein metabolism in the yolk sac. The fate of the yolk sac in different animals is somewhat different. In birds, by the end of the incubation period, the remnants of the yolk sac are already inside the embryo, after which it quickly disappears and completely resolves by the end of the 6th day after hatching. In mammals, the yolk sac is developed in different ways. In predators, it is relatively large, with a highly developed network of vessels, while in primates it quickly shrinks and disappears without a trace before childbirth.

Allantois develops somewhat later than other extra-embryonic organs. It is a sac-like outgrowth of the ventral wall of the hindgut. Therefore, it is formed by the endoderm on the inside and the splanchnopleura on the outside. In reptiles and birds, the allantois quickly grows to the chorion and performs several functions. First of all, it is a reservoir for urea and uric acid, which are the end products of the metabolism of nitrogen-containing organic substances. The allantois has a well-developed vascular network, due to which, together with the chorion, it participates in gas exchange. When hatching, the outer part of the allantois is discarded, and the inner part is preserved in the form of a bladder. In many mammals, the allantois is also well developed and, together with the chorion, forms the chorioallantoic placenta. Term placenta means close overlap or fusion of the germinal membranes with the tissues of the parent organism. In primates and some other mammals, the endodermal part of the allantois is rudimentary, and the mesodermal cells form a dense cord extending from the cloacal region to the chorion. Vessels grow along the allantois mesoderm to the chorion, through which the placenta performs excretory, respiratory and nutritional functions.

Vertebrates have a special embryonic germ called the neural crest (it is located next to the neural tube). From the cells of the neural crest, a surprising number of different structures form, from some ganglions to most of the skull. Many modern scientists consider the neural crest to be the fourth germ layer, along with the ectoderm, endoderm, and mesoderm. The closest relatives of vertebrates - tunicates - have a group of germ cells, similar in properties to the neural crest, which differentiates into pigment cells of the integument. Probably, this group of cells has also been preserved in vertebrates, having significantly expanded the set of their differentiation pathways. In addition, new regulatory genes with neural crest-specific expression have appeared in vertebrates; this was facilitated by the fact that genome-wide duplication occurred in their evolution. Thus, two unique features of the vertebrate subtype - a genome-wide duplication and the presence of a "fourth germ layer" - are most likely related.

Is it possible to reduce the device of all animals to a single scheme? There is no simple answer to this question. It all depends on the detail of the required circuit and how exactly we are going to use it. Nevertheless, the question of whether animals have a “single structural plan” was considered in classical zoology as the most important, and there were grandiose disputes between supporters of different answers to it (see, for example: B. Zhukov, 2011. The dispute between two truths). Indeed, this question is important, if only because any science seeks to describe its objects according to a common template for all, and a “single building plan” could just provide such a template.

In the middle of the 19th century, embryology gave evolutionary science a valuable generalization that made it possible, at least, to compare arbitrarily different animals among themselves. It has been found that the embryo of any (or almost any) animal, having reached a certain stage, divides into stable layers of cells called germ layers. There are three germ layers: ectoderm (outer), endoderm (inner) and mesoderm (middle). From the ectoderm, the skin (epidermis) and the nervous system are formed. From the endoderm, the intestines are formed - more precisely, the digestive tract - and organs that develop as its outgrowths, such as the liver. From the mesoderm, as a rule, the musculoskeletal, circulatory and excretory systems are formed.

Some animals (for example, hydroid polyps, which include freshwater hydra) have ectoderm and endoderm, but no mesoderm. Bilaterally symmetrical animals, to which we also belong, have all three germ layers. Animals with two germ layers are called bilayer (diploblasts), animals with three germ layers are called three-layer (triploblasts).

The author of the well-known course in general embryology, B.P. Tokin, called the theory of germ layers "the largest morphological generalization in the entire history of embryology." By the turn of the 19th and 20th centuries, this theory had become generally accepted. Moreover, a peculiar idea of ​​the "holiness" of the germ layers, the boundaries of which were considered unshakable, has developed. If an organ is formed from one germ layer, it can never, in any organism, be formed from another.

But, as often happens, wildlife turned out to be more voluminous than academic schemes. In this case, it turned out quickly. In 1893, the American embryologist Julia Platt discovered that some cartilages of the branchial apparatus of vertebrates do not develop from the mesoderm (as one would expect from the classical germ layer theory), but from the ectoderm. Julia Platt has done a whole series of work on tracing the fate of the ectodermal cells that make up cartilage. Her findings have been confirmed by several other embryologists. But this discovery did not find wide recognition, mainly because of purely dogmatic doubts: cartilage is “supposed” to develop from the mesoderm, which means that they cannot develop from the ectoderm, and that’s it! Julia Platt did not even get a permanent position at the university, after which she decided to leave science altogether. She took up social work, became a prominent politician in the state of California, did a lot for conservation, so humanity as a whole here may not have suffered. But the special origin of the gill cartilages became a generally accepted fact only in the late 1940s, after very subtle experiments by the Swedish embryologist Sven Hörstadius, the results of which were already difficult to doubt.

It would seem, what is the significance for our worldview of the question of which germ cells form the gill arches of a newt or a shark? Isn't this a trifle? No, not a trifle. Pulling, as if by a thread, the data of Platt and Hirstadius, we find ourselves facing a serious macroevolutionary problem.

We already know that the ectoderm is the outermost of the three germ layers. In vertebrates, it is divided into two parts: (1) the integumentary ectoderm and (2) the neuroectoderm. The epidermis is formed from the integumentary ectoderm, and the central nervous system is formed from the neuroectoderm. The integumentary ectoderm naturally covers the body of the future animal from the outside. As for the neuroectoderm, it is first located on the future back neural plate, which then sinks, folds and closes in neural tube. This tube becomes the central nervous system, that is, the brain (spinal and brain).

At the very border of the neuroectoderm and the integumentary ectoderm in vertebrates is a group of cells called nervous roller, or neural crest. Neural crest cells are not part of either the neural tube or the epidermis. But they are able to spread throughout the body, migrating, like amoeba, with the help of pseudopods. It was the fate of the neural crest cells that Julia Platt studied. Indeed, numerous structures are formed from them, far from being only nervous. Sven Herstadius once showed that if the neural crest in the anterior third of the body is microsurgically removed from the embryo of a caudate amphibian, then the back of the head develops normally, the ear capsules develop normally - and the rest of the skull simply does not exist. Neither the major part of the braincase, nor the capsule of the olfactory organs, nor the jaw develop without the contribution of neural crest cells (Fig. 2).

Here is a list (certainly incomplete) of neural crest derivatives in vertebrates:

• The nerve ganglions of the dorsal roots of the spinal nerves (often referred to simply as the spinal ganglia).
• Nerve nodes of the autonomic nervous system (sympathetic, parasympathetic and metasympathetic).
• The medulla of the adrenal glands.
• Schwann cells, which form the sheath of the processes of neurons.
• Inner lining (endothelium) and smooth muscle layer of some vessels, including the aorta.
• Ciliary muscles that constrict and dilate the pupil.
• Odontoblasts are cells that secrete dentin, the hard substance of teeth.
• Pigment cells of the integument: erythrophores (red), xanthophores (yellow), iridophores (reflective), melanophores and melanocytes (black).
• Part of adipocytes - cells of adipose tissue.
• Parafollicular thyroid cells that secrete the hormone calcitonin.
• Cartilages and bones of the skull, primarily its visceral (pharyngeal) section, which includes not only the gill arches, but also the jaws.

Rich list, isn't it? Well, the spinal ganglia are not surprising: they are located just about the place of the neural crest, the cells of which in this case do not even have to migrate. Vegetative ganglia - also nothing surprising. They are located much farther from the spinal cord, but, after all, they are part of the nervous system. And the adrenal medulla is actually a vegetative ganglion, only transformed. And Schwann cells are part of the nervous tissue. But further down the list are structures that have nothing to do with the nervous system, moreover, they are diverse and numerous. A person also has diseases caused by anomalies in the derivatives of the neural crest - neurocristopathy.

The last item on the list is extremely important: the skull! From the neural crest, in fact, most of it is formed (except for the auditory region and the back of the head). Meanwhile, the rest of the skeleton - the spine, the skeleton of the limbs - is formed from the mesoderm. The classical concept, according to which organs of the same type should not develop from different germ layers, clearly failed here.

Another important point: the entire list of derivatives of the neural crest does not apply to chordates, namely to vertebrates. In addition to vertebrates, the chordate type includes two more modern groups of animals: tunicates and lancelets. So they have a neural crest is not expressed. This is a unique feature of the vertebrate subtype.

What is the neural crest? If this is part of the ectoderm (as was believed in the time of Julia Platt), then some is too unusual. In 2000, Canadian embryologist Brian Keith Hall proposed that the neural crest be considered nothing more than a separate fourth germ layer. This interpretation quickly spread in the scientific literature, where the neural crest is now generally a popular topic. It turns out that vertebrates are the only four-layered animals (quadroblasts).

The fourth germ layer is just as important a feature of vertebrates as, for example, the whole genome duplication that occurred at the beginning of their evolution (see, for example: Vertebrates owe their heart to whole genome duplication, "Elements", 06/17/2013). But how did it come about? American biologists William A. Muñoz and Paul A. Trainor published an article on the current state of this problem (Fig. 1). Paul Traynor is a prominent vertebrate embryologist who has specialized in the neural crest for many years, so the review signed by him definitely deserves attention.

According to modern data, the branch leading to the lancelet was the first to depart from the evolutionary tree of chordates (see, for example: The reason for the peculiarities of the tunicate genome is the determinism of their embryonic development, "Elements", 06/01/2014). Tunicates and vertebrates are closer relatives; together they form a group called Olfactores ("animals with an organ of smell"). Since the lancelet represents a more ancient branch, then more ancient signs can be expected from it. Indeed, no close analogues of neural crest cells have been found in the lancelet. Most of the organs and tissues that in vertebrates are formed from the material of the neural crest are simply not in his body. There is one major exception: the fibers of the sensory spinal nerves of the lancelet are surrounded by accessory (glial) cells, very similar to vertebrate Schwann cells. Schwann cells are the most important derivatives of the neural crest. But their counterparts in the lancelet are formed from the usual neuroectoderm, that is, from the material of the neural tube. This example only confirms that the lancelet has no neural crest.

With shellers, the situation is more complicated and interesting. Ascidia Ciona intestinalis(a quite typical and well-studied tunicate) there are analogues of neural crest derivatives - these are pigment cells containing melanin. And their embryonic source is located just “where it is needed”: on the border of the neural plate and the integumentary ectoderm. Features of the individual development of ascidia make it possible to trace the fate of these cells very accurately. Before taking their place in the integument, they make a long migration (sometimes through the loose mesoderm, and sometimes between the mesoderm and the epidermis); all this is very similar to the behavior of the cells of a typical neural crest. Moreover, the precursors of ascidian pigment cells express the HNK-1 antigen, which is specific for neural crest cells of vertebrates, up to birds and mammals.

The "neural crest" of the ascidian comes from a specific blastomere (that is, from a specific cell of the early embryo; a map of early development has been compiled for the ascidian, where all blastomeres are numbered). Interestingly, not all descendants of this blastomere become pigment cells. Some of them are part of the mesoderm and can, for example, become blood cells or muscles of the body wall. The connection between the neural crest and the mesoderm has not yet been studied in sufficient detail, but it is certainly not accidental. It seems that here we touched on a rather subtle and deep evolutionary mechanism. In most animals, pigment cells develop precisely from the mesoderm. Most likely, this was the case with the ancestors of ascidia. Then, in the process of evolution of chordates, the emerging neural crest “intercepted” the path of differentiation of pigment cells from the mesoderm, starting to form them from itself. In vertebrates, this process continued: the neural crest “intercepted” the differentiation pathways of such traditionally mesodermal tissues as cartilage, bone, adipose tissue, and smooth muscles, and in all these cases, only partially.

This is how metorisis could manifest itself - the process of changing the boundaries of the germ layers, when one of them partially replaces the other. This concept was introduced in 1908 by a professor at St. Petersburg (later Petrograd) University, Academician Vladimir Mikhailovich Shimkevich. But Shimkevich did not know that a whole new germ layer could be formed through metorisis. In vertebrates, it turns out that this is exactly what happened. This is what makes their building plan unique.

The skeletal tissue that in all animals known to us develops exclusively from the neural crest is dentin. Fortunately, dentin is very hard, and it is perfectly preserved as a fossil. For example, we know that representatives of one of the most ancient groups of jawless vertebrates - Pteraspidomorphi - were literally clad in dentine armor (Fig. 3). Apparently, this can be regarded as documentary evidence that their neural crest was already fully developed. But most likely, it arose even earlier.

There remains one more intriguing question. Are two unique vertebrate traits related: the fourth germ layer and the genome-wide duplication?

Yes, there is likely to be such a connection. This can be shown in some examples, despite the fact that the system of genes that control the development of the neural crest is not yet very fully understood. It is generally accepted that two successive whole-genome duplication events (WGD) occurred at the beginning of vertebrate evolution. Duplication, that is, doubling of the entire genome, cannot but lead to the appearance of additional copies of genes, including those that control individual development. An example of such a gene is the gene FoxD belonging to a large gene family Fox. The lancelet has only one gene. The area of ​​its expression includes some parts of the neural tube, as well as the axial mesoderm. The ascidian gene FoxD also one, since there was no genome-wide duplication in tunicates. But the sea squirt, unlike the lancelet, has the rudiment of the neural crest. Gene FoxD expressed in it too. And in vertebrate genes FoxD becomes several, and in the cells of the neural crest only one of them is expressed - the gene FoxD3. This is the separation of functions typical of the consequences of duplication. There is an idea that any duplication in itself "encourages" new copies of the gene to share tasks among themselves, if possible, so that there are no failures in the gene network due to duplication (see Conflict between copies of a duplicated gene leads to excessive complication of gene-regulatory networks, "Elements", 10.10.2013).

On the other hand, it can be said that duplication gave the vertebrate genome additional degrees of freedom, which were useful, in particular, in the creation of a new germ layer. After all, the ascidian does not have such a variety of neural crest derivatives even remotely; in them it is an ordinary small primordium, which ensures the formation of a single type of cell. In vertebrates, this germ has gone berserk, taking over a huge number of different differentiation pathways along with the cell types to which these pathways lead. And the increase in the number of genes clearly served as a prerequisite here.

In the light of these data, the old naive notion that vertebrates are more complex than all other animals begins, oddly enough, to look true. Genome-wide duplication and a new germ layer are significant objective indicators of complexity. Another similar indicator can be, for example, the number of regulatory miRNAs (see The complication of the organism in ancient animals was associated with the emergence of new regulatory molecules, "Elements", 04.10.2010). But the neural crest example is even brighter.