Germinal leaf and organs. Germ sheets: formation, brief description, functions. From which germ layer does the notochord develop? neural tube

germ layers(lat. embryonic folia), germ layers, layers of the body of the embryo of multicellular animals, formed during gastrulation and giving rise to various organs and tissues. In most organisms, three germ layers are formed: the outer one is the ectoderm, the inner one is the endoderm and the middle mesoderm.

Derivatives of the ectoderm perform mainly integumentary and sensitive 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.

The same germ layer in representatives of different classes of vertebrates has the same properties, i.e. germ layers are homologous formations and their presence confirms the position of the unity of the origin of the animal world. Germ layers are formed in embryos of all major classes of vertebrates, i.e. are universally distributed.

The germ layer is a layer of cells that occupies a certain position. But it cannot be considered only from topographic positions. The germ layer is a collection of cells that have certain developmental tendencies. A clearly defined, albeit rather wide, range of developmental potentials is finally determined (determined) by the end of gastrulation. Thus, each germ layer develops in a given direction, takes part in the emergence of the rudiments of certain organs. Throughout the animal kingdom, individual organs and tissues originate from the same germ layer. From the ectoderm, the neural tube and integumentary epithelium are formed, from the endoderm - the intestinal epithelium, from the mesoderm - muscle and connective tissue, the epithelium of the kidneys, gonads, and serous cavities. From the mesoderm and the cranial part of the ectoderm cells are evicted, which fill the space between the sheets and form the mesenchyme. Mesenchymal cells form syncytium: they are connected to each other by cytoplasmic processes. The mesenchyme forms the connective tissue. Each individual germ layer is not an autonomous formation, it is part of the whole. The germ layers are able to differentiate only by interacting with each other and being under the influence of the integrating influences of the embryo as a whole. A good illustration of such interaction and mutual influence are experiments on early gastrulae of amphibians, according to which the cellular material of the ecto-, ento- and mesoderm can be forced to radically change the path of its development, to participate in the formation of organs that are completely uncharacteristic of this leaf. This suggests that, at the beginning of gastrulation, the fate of the cellular material of each germ layer, strictly speaking, is not yet predetermined. The development and differentiation of each leaf, their organogenetic specificity, is due to the mutual influence of the parts of the whole embryo and is possible only with normal integration.

62. Histo- and organogenesis. The process of neurulation. Axial organs and their formation. mesoderm differentiation. Derivative organs of vertebrate embryos.

Histogenesis(from other Greek ἱστός - tissue + γένεσις - education, development) - a set of processes leading to the formation and restoration of tissues in the course of individual development (ontogenesis). One or another germ layer is involved in the formation of a certain type of tissue. For example, muscle tissue develops from mesoderm, nervous tissue from ectoderm, etc. In some cases, tissues of the same type may have a different origin, for example, the epithelium of the skin is ectodermal, and the absorbent intestinal epithelium is endodermal in origin.

Organogenesis- the last stage of embryonic individual development, which is preceded by fertilization, crushing, blastulation and gastrulation.

In organogenesis, neurulation, histogenesis and organogenesis.

In the process of neurulation, a neurula is formed, in which the mesoderm is laid, consisting of three germ layers (the third layer of the mesoderm splits into segmented paired structures - somites) and the axial complex of organs - the neural tube, chord and intestine. The cells of the axial complex of organs mutually influence each other. Such mutual influence called embryonic induction.

In the process of histogenesis, body tissues are formed. From the ectoderm, nervous tissue and the epidermis of the skin with skin glands are formed, from which the nervous system, sensory organs and epidermis subsequently develop. From the endoderm, a notochord and epithelial tissue are formed, from which mucous membranes, lungs, capillaries and glands (except for the genital and skin ones) are subsequently formed. The mesoderm produces muscle and connective tissue. ODS, blood, heart, kidneys and gonads are formed from muscle tissue.

Neurulation- the formation of the neural plate and its closure into the neural tube in the process of embryonic development of chordates.

Neurulation is one of the key stages of ontogeny. An embryo at the stage of neurulation is called a neurula.

The development of the neural tube in the anterior-posterior direction is controlled by special substances - morphogens (they determine which of the ends will become the brain), and the genetic information about this is embedded in the so-called homeotic or homeotic genes.

For example, the morphogen retinoic acid, with an increase in its concentration, is able to turn rhombomeres (segments of the neural tube of the posterior part of the brain) of one type into another.

Neurulation in lancelets is the growth of ridges from the ectoderm over a layer of cells that becomes the neural plate.

Neurulation in the stratified epithelium - the cells of both layers descend under the ectoderm mixed, and diverge centrifugally, forming a neural tube.

Neurulation in a single-layered epithelium:

Schizocoelous type (in teleosts) - similar to stratified epithelial neurulation, except that the cells of one layer descend.

In birds and mammals, the neural plate invaginates inward and closes into the neural tube.

In birds and mammals, during neurulation, protruding parts of the neural plate called neural folds, are closed along the entire length of the neural tube unevenly.

Usually, the middle of the neural tube closes first, and then the closure goes to both ends, leaving as a result two open sections - the anterior and posterior neuropores.

In humans, closure of the neural tube is more complex. The spinal section closes first, from the thoracic to the lumbar, the second - the area from the forehead to the crown, the third - the front, goes in one direction, to the neurocranium, the fourth - the area from the back of the head to the end of the cervical, the last, fifth - the sacral section, also goes to one direction, away from the coccyx.

When the second section is not closed, a fatal congenital defect is found - anencephaly. The fetus does not develop a brain.

When the fifth section is not closed, a congenital defect that can be corrected is found - spina bifida, or Spinabifida. Depending on the severity, spina bifida is divided into several subtypes.

During neurulation, the neural tube is formed.

In cross section, immediately after formation, three layers can be distinguished in it, from the inside to the outside:

Ependymal - pseudo-stratified layer containing rudimentary cells.

The mantle zone contains migrating, proliferating cells that emerge from the ependymal layer.

The outer marginal zone is the layer where nerve fibers are formed.

There are 4 axial body: notochord, neural tube, intestinal tube and mesoderm.

Regardless of the animal species, those cells that migrate through the region of the dorsal lip of the blastopore are subsequently transformed into a notochord, and through the region of the lateral (lateral) lips of the blastopore into the third germ layer - the mesoderm. In higher chordates (birds and mammals), due to the immigration of germinal shield cells, the blastopore is not formed during gastrulation. Cells that migrated through the dorsal lip of the blastopore form a notochord, a dense cell strand located along the midline of the embryo between the ectoderm and endoderm. Under its influence, the neural tube begins to form in the outer germinal layer, and only lastly does the endoderm form the intestinal tube.

Differentiation (lat. differens. difference) of the mesoderm begins at the end of the 3rd week of development. The mesenchyme arises from the mesoderm.

The dorsal part of the mesoderm, which is located on the sides of the chord, is divided into body segments - somites, from which bones and cartilage, striated skeletal muscles and skin develop (Fig. 134).

From the ventral non-segmented part of the mesoderm - with the planchnotome, two plates are formed: the splanchnopleura and the somatopleura, from which the mesothelium of the serous membranes develops, and the space between them turns into body cavities, the digestive tube, blood cells, smooth muscle tissue, blood and lymphatic vessels, connective tissue, cardiac striated muscle tissue, adrenal cortex and epithelium sex glands.

Derivatives of the germ layers. The ectoderm gives rise to the outer integument, the central nervous system, and the final section of the alimentary canal. From the endoderm, the notochord, the middle section of the digestive tube and the respiratory system, are formed. From the mesoderm, the musculoskeletal, cardiovascular and genitourinary systems are formed.

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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 - 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 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). The ectoderm forms the skin (epidermis) and nervous system. 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, Live nature 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 gill apparatus of vertebrates do not develop from the mesoderm (as one would expect from classical theory germ layers), 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 got busy social activities, became a prominent politician in the state of California, did a lot for nature 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 significance for our worldview can 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 jaws 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 solid 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 further from spinal cord but, after all, it is 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 neural crest derivatives 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 an eminent 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 a 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 body in ancient animals was associated with the emergence of new regulatory molecules, "Elements", 04.10.2010). But the neural crest example is even brighter.

Germ layers is a basic term in embryology. They designate the layers of the fetal body at an early stage. In most cases, these layers are epithelial in nature.

The germ layers are usually classified into three types:

Ectoderm - the outer sheet, which is also called the epiblast or skin-sensitive layer;

Endoderm is the inner layer of cells. It may also be called a hypoblast or entero-glandular sheet;

Middle layer (mesoderm or mesoblast).

Germinal sheets (depending on their location, they are characterized by certain cell features. Thus, the outer layer of the embryo consists of light and tall cells, which are similar in structure to cylindrical epithelium. The inner leaf consists in most cases of large cells that are filled with specific yolk plates. They have a flattened appearance, which makes them look like

The mesoderm at the first stage consists of spindle-shaped and stellate cells. They later form the epithelial layer. I must say that many researchers believe that the mesoderm is the middle germ layers, which are not an independent layer of cells.

The germ layers at first have the form of a hollow formation, which is called the blastodermal vesicle. At one of its poles, a group of cells gathers, which is called a cell mass. It gives rise to the primary intestine (endoderm).

It should be said that different organs are formed from embryonic sheets. Thus, the nervous system arises from the ectoderm, the digestive tube originates from the endoderm, and the skeleton and muscles originate from the mesoderm.

It should also be noted that special embryonic membranes are formed during embryogenesis. They are temporary, do not participate in the formation of organs and exist only during embryonic development. Each class has certain features in the formation and structure of these shells.

With the development of embryology, they began to determine the similarity of embryos, which was first described by K.M. Baer in 1828. A little later, Charles Darwin identified the main reason for the similarity of embryos of all organisms - their common origin. Severov, on the other hand, argued that the common signs of embryos are associated with evolution, which proceeds in most cases through anabolism.

When comparing the main stages of development of embryos of different classes and species of animals, certain features were found, which made it possible to formulate the law of embryonic similarity. The main provisions of this law was that the embryos of organisms of the same type in the early stages of their development are very similar. Subsequently, the embryo is characterized more and more individual characteristics, which indicate its belonging to the corresponding genus and species. At the same time, the embryos of representatives of the same type are increasingly separated from each other, and their primary similarity is no longer traced.

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 modern theory 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 evidence 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 correct form 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 the capillaries surrounding them) of the brain and spinal cord, the tail muscles of amphibian embryos, as well as 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, 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.

Provisory 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 ensure 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 is used for exchange between the embryo and 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 big amount yolk, he 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 nitrogen-containing metabolism. organic matter. 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.

"MORDOVA STATE UNIVERSITY named after A.I. N. P. OGAREVA»

Department of Biology

Department of Genetics

on the topic: germ layers

Completed by: 3rd year student

specialty "Biology"

Introduction

1. The structure of the germ layers

2. History of the development of the theory of germ layers

3. Formation of germ layers

4. Origin and evolutionary significance of the germ layers

5. Provisions of the theory of germ layers and objections to this theory

Conclusion

Literature

Introduction

Along with the possibility of interpreting the germ layers in terms of their phylogenetic significance, it is important 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 common origin and similar heredity in the various members of this vast group of animals.

It can be thought that in these germ layers, for the first time, differences of different classes begin to be created above the general plan of the body structure, characteristic of all vertebrates.

The formation of germ layers ends the period 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.


Various structures arise from the germ layer. 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.

1. The structure of the germ layers

The germ layers consist of cellular materials that are used for the development of various organs and tissues. In their structure, the cells of various germ layers differ from each other; endoderm cells are always larger and less regular than ectodermal cells. The endoderm is distinguished by the properties of the future bookmark, which has trophic significance. The ectoderm remains on the surface and initially has a protective value. Unlike the endoderm, it consists of regularly arranged cells of a more uniform shape. Gastrulation leads to a noticeable difference between the outer and inner layers and the germinal material becomes heterogeneous. The process that leads to the appearance of differences in an initially homogeneous material is called differentiation.

Primary organizers or inductors play an important role in the differentiation of cellular material. Inductors are chemical substances, which are secreted by groups of cells and affect other groups of cells, changing their development path. As a result of differentiation of the germ layers, various organs and tissues are formed. In the study of these processes in different animals, it was found that the fate of each germ layer in all multicellular organisms is, as a rule, the same.

Thus, the epithelium of the skin, skin glands, many horn derivatives, the nervous system and sensory organs develop from the ectoderm. From the endoderm in all animals, the epithelium of the middle part of the intestinal tract, the liver and the digestive glands are formed. In chordates, the epithelium of the respiratory tract is also formed. Blood and lymph, muscle, connective, cartilaginous and bone tissues, kidney epithelium, the wall of the secondary body cavity, part of the tissues of the reproductive system develop from the mesoderm.

2. History of the development of the theory of germ layers

The germ layer theory is one of the largest generalizations of comparative embryology in the 19th century. The germ layers were first described by X. Pander (1817), who discovered that at some stages of development the chicken embryo consists of three thin films or layers, the cellular nature of which was not yet known. Pander called the outer leaf serous, the deepest - mucous, and the intermediate - blood. These observations were confirmed by K. Baer (1828, 1837), who found germ layers in some other animals (Fish, Frogs, Turtles). Baer distinguished two primary layers - animal and vegetative, which are then again divided into secondary germ layers: the animal layer gives skin and muscular, and the vegetative - vascular and mucous. According to modern terminology, the skin sheet corresponds to the ectoderm, the mucous sheet corresponds to the endoderm, and the muscular and vascular sheet corresponds to the parietal and visceral sheet of the mesoderm. Baer's mistake was only that he described the origin of these two mesodermal layers in Vertebrates from different sources. The terms "ectoderm" and "endoderm" were borrowed by embryologists from zoology (this is how the epithelial layers that make up the body of adult Cnidarians were called even earlier). The cellular structure of the germ layers of the chicken embryo was established by Remak in 1855.


Initially, it was believed that germ layers are formed only during the development of Vertebrates. However, after the work of A. O. Kovalevsky and I. I. Mechnikov, who studied the development of almost all classes of invertebrates, it became clear that germ layers are present in one form or another in all multicellular animals. A. O. Kovalevsky (1871) in the article “Embryological Studies of Worms and Arthropods” wrote in the final part: “If we now compare the development of the worms we have described with the development of other animals, then the analogy of the germ layers with those of vertebrate animals is especially striking to us, down to individual details; the same two primary leaves that play a major role in the development of worms are also present in vertebrates; as in some, so in others, the middle leaf appears only later. The destinies of the leaves and the laying of the organs coincide extremely, right down to individual processes.

I. I. Mechnikov discovered germ layers in some animals with greatly altered development and for the first time raised the question of the evolution of gastrulation processes.

3. Formation of germ layers

Germ layers are formed in animals and humans in a process called gastrulation.

Among animals, two-layer and three-layer taxa are distinguished. Starting with flatworms, animals have 3 germ layers: ectoderm (outer), endoderm (inner) and mesoderm (middle). Mesoderm is present only in three-layered animals, while ectoderm and endoderm are found in two-layered (sponges, bryozoans, coelenterates) and three-layered animals.

The nervous system, skin, skin glands, skin derivatives, such as feathers, hair, nails, claws, scales, as well as the epithelium of the anterior and posterior sections of the digestive tube, and the bones of the visceral skeleton develop from the ectoderm in ontogenesis.

The intestinal lining is formed from the endoderm; the endoderm provides nourishment to the embryo; from this germ layer, the respiratory organs, the mucous membranes of the digestive system, and the digestive glands (liver, etc.) develop.

From the mesoderm, the organs of the circulatory, excretory and reproductive systems, the serous membranes of the coelom and internal organs, as well as the bones of the supporting skeleton and muscles, are formed.

Modern methods of studying the embryonic process have made it possible to establish that the germ layers do not have the significance of a primitive organ and do not repeat any stage of phylogenetic development. They should be considered as the material of a certain complex of future organs that are at the same level of development and are morphologically similar. The process of formation of germ layers signifies a certain stage in the development of organs, which the vast majority of animals go through.

Usually, each organ includes tissues originating from different germ layers, but we classify an organ as a derivative of one or another leaf, depending on what its main primordium develops from. Thus, the wall of the midgut in Vertebrates consists of endodermal epithelium and mesodermal smooth muscles in origin and a layer of connective tissue. But since the first rudiment of the midgut is formed from the endoderm, and the mesodermal elements join it later, and the digestive function is performed by the endodermal epithelium, the midgut is considered an endodermal organ.

The presence of germ layers, similarly involved in the construction of the body of all Metazoa, made it possible to compare the development of systematically distant groups of animals. At the present time it is simply impossible to describe the development of any animal without mentioning the germ layers.

4. Origin and evolutionary significance of the germ layers

The question arises of what is the origin and evolutionary significance of the germ layers. According to E. Haeckel (1874), the primary germ layers (ecto- and endoderm) repeat in development (recapitulate) the primary organs (skin and intestines) of the hypothetical common ancestor of the Metazoa - Gastrea. It follows from this that the germ layers in all animals are homologous. I. I. Mechnikov (1886) also attached recapitulation significance to the germ layers, but he represented the common ancestor of the Metazoa in the form of Phagocytella. According to Mechnikov, the kinoblast is represented during development by the ectoderm, and all organs that arose in the process of evolution from the kinoblast have an ectodermal origin during individual development. The evolution of the phagocytoblast occurred in two directions. In coelenterates, it completely epithelialized and turned into a lining of the gastric cavity; in individual development, it is represented by the endoderm. In three-layered animals, only the central part of the phagocytoblast turned into the intestine and is represented in ontogenesis by the endoderm, while the peripheral part gave rise to tissues of the internal environment and is represented in ontogenesis by the mesoderm.

5. Provisions of the theory of germ layers and objections to this theory

Thus, by the end of the XIX century. the classical theory of germ layers has developed, the content of which is the following provisions:

1. In the ontogenesis of all multicellular animals, two or three germ layers are formed, from which all organs develop.

2. The germ layers are characterized by a certain position in the body of the embryo (topography) and are respectively designated as ecto-, ento- and mesoderm.

3. The germ layers are specific, that is, each of them gives rise to strictly defined primordia, which are the same in all animals.

4. The germ layers recapitulate in ontogeny the primary organs of the common ancestor of all Metazoa and are therefore homologous.

5. The ontogenetic development of an organ from one or another germ layer indicates its evolutionary origin from the corresponding primary organ of the ancestor.

To date, many facts have accumulated that, at first glance, do not fit into the framework of the classical theory of germ layers. Therefore, statements began to appear that this theory is outdated, is in crisis, and needs to be revised. All of these criticisms are based on an overly formal anti-evolutionary understanding of the germ layers Let us consider some of the most significant objections to the germ layer theory.

1. The fact that the mesoderm can originate from both the ectoderm and the endoderm has been the subject of many disagreements, and this casts doubt on its unity as a germ layer. Many authors consider it necessary to distinguish between the mesoblast (entomesoderm) and the mesenchyme (ectomesoderm). But the differences between these parts of the mesoderm are not as significant as it seems at first glance. In forms with spiral fragmentation, the mesenchyme originates from micromeres of the 2nd and 3rd quartets, and the mesoblast belongs to the 4th quartet: all these cells are located along the edges of the blastopore, i.e., in the border zone between the ecto- and endoderm. Migration of mesenchymal elements into the blastocoel is part of gastrulation. It can also be assumed that the evolutionary formation of the phagocytoblast, the peripheral part of which is represented by the mesoderm, was a long process, and its replenishment due to the kinoblast continued for a very long time, which is reflected in ontogeny.

2. In some animals, the germ layers are presented in a very complicated form. In Insects and Birds, for example, the so-called biphasic or even multiphase gastrulation is observed, which, as it were, breaks up into a number of independent acts. Often, even before the formation of the germ layers, organogenesis begins, the rudiments of organs are isolated. The germ layers are not clearly expressed. But this situation can easily be explained as the result of a secondary change in the course of development. We must not forget that all ontogenetic processes are subject to evolution to the same extent as the organs of adult animals. Even within the phylum Cnidaria, gastrulation has undergone a considerable evolution, so it is not surprising that in higher animals far removed from the origins of the Metazoa, gastrulation processes have undergone such profound secondary changes. Rather, one should be surprised that we still distinguish germ layers in them, albeit in a modified form.

3. In the case of strictly determined cleavage (in Nematodes, Annelids, Mollusks, Ascidians), individual blastomeres or groups of blastomeres already represent the rudiments of certain organs. So, in the ringed worm Arenicola, at the stage of 64 blastomeres, the so-called rosette, consisting of 4 cells, is distinguished at the animal pole, which is the rudiment of a sensitive sultan, and in equatorial zone there are 4 groups of cells, 4 in each - trochoblasts, from which prototroch develops. At the vegetative pole there are 7 large cells rich in yolk - the rudiment of the intestine, to which a cell is adjacent from the future dorsal side, giving rise to mesodermal teloblasts. One gets the impression that the germ layers formed later have no independent significance, but are only a temporary association of already existing heterogeneous rudiments.

However, this association of primordia in the germ layers is not accidental, but is historically conditioned. So, the composition of the ectoderm includes the rudiments of only those organs that develop from it and with non-deterministic fragmentation (skin, sensory organs, etc.). In addition, the early determination of blastomeres is also a consequence of secondary changes in the course of development - this is an adaptation that allows the embryo to quickly turn into a larva, consisting of a few more cells, but already capable of independently performing all vital functions (except, of course, sexual).

4. Critics of the theory of germ layers usually point to the existence of various exceptions, which include the perversion of germ layers in Sponges, the absence of clearly expressed leaves in many flatworms, the absence of endoderm in most Bryozoans, etc. We will consider all these specific examples together with a detailed description of the development of these animals. We only note that the occurrence of all particular deviations from the general rule can be fully understood from an evolutionary point of view, and the reasons that caused them are clear in most cases. In addition, these deviations are usually observed in rather low organized animals, while in higher animals (Arthropods, Vertebrates), the specificity of the germ layers is strictly observed. This suggests that the germ layers of the lower Metazoa are highly labile, while their specificity appeared later and progresses in the course of evolution.

5. In asexual reproduction, various restorative processes, and experimental intervention in the course of development, a violation of the principle of specificity of germ layers is often observed. So, during the budding of Bryozoans and some Ascidia, tissues of an endodermal nature are not included in the composition of the kidney, and the intestine develops from the ectoderm. In Nemertine Lineus lacteus, a small pre-oral part of the body can be cut off, which also does not contain endodermal organs, and a whole animal develops from this fragment.

To understand the nature of these phenomena, it is necessary to remember what the specificity of the germ layers is based on. In embryogenesis, from each leaf, those organs develop that historically separated from the composition of the corresponding cell layer, i.e., the specificity of the leaves is based on the phenomenon of recapitulation. The recapitulation itself (as shown by I. I. Shmalgauzen) is largely due to the fact that there are certain historically established morphogenetic correlations between the parts of the embryo. But in the course of recovery processes and asexual reproduction, development proceeds not on the basis of the gastrula, but on the basis of the tissues of an adult animal, between which there are other physiological relationships. The germ layers are exclusively embryonic formations and, as such, are absent in adult animals. Therefore, the specificity of the germ layers loses its significance.

To this we can add that the ability for asexual reproduction and wider morphogenetic abilities of tissues are characteristic only of animals that have not reached a very high evolutionary level, which indicates the progressive specificity of the germ layers and tissues of an adult animal.

The modern point of view on the germ layers is well expressed by the following quote from V. N. Beklemishev’s “Comparative Anatomy of Invertebrates”: “... the kinoblast and phagocytoblast are the main layers of the body and the direct organs of the animal only in the larvae of coelenterates and sponges and in the most simply arranged of hydroids , like Protohydra. In all other Enterozoa, due to the concentration of functions and the integration of organs, the primary layers break up into a number of derivatives, which are intertwined in a complex way. Because of this, in the superior Metazoa, the primary layers are reduced to the level of germ layers; they are no longer, as such, in the adult, but they are preserved in the form of primary layers of the embryo, giving rise to certain cellular systems, tissues and elementary organs of the adult organism. However, these germ layers remain homologous to each other in all Metazoa everywhere except adult sponges, retaining the same basic sets of characteristic features of mutual position and prospective significance.

Conclusion

So, the germ layers are not an imaginary concept, they really exist, they manifest a certain type of primary differentiation of cellular material during the development of the Metazoa from the egg. The constancy with which the germ layers are reproduced in the development of the vast majority of animals can only be explained by the existence of "historical traditions", i.e., recapitulation. But the germ layers should not be regarded as something stable and unchanging; one should not forget about possible evolutionary transformations of any ontogenetic processes, including the development of germ layers.

Literature

1. Ivanova-Kazas O. M., Krichinskaya E. B. A course in comparative embryology of invertebrate animals. L. Publishing house Leningrad. University, 1988.

2. http:///biologia/26-zarodyshevye-listki. html

3. Large Soviet Encyclopedia, TSB

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