Types of gastrulation.

At the end of the period of fragmentation, the embryos of all multicellular animals enter the period of formation of germ layers (leaves). This stage is called gastrulation.

There are two stages in the gastrulation process. First, an early gastrula is formed, which has two germ layers: the outer one is the ectoderm and the inner one is the endoderm. Then comes the late gastrula, when the middle germ layer, the mesoderm, is formed. Gastrula formation occurs in different ways.

There are 4 types of gastrulation:

1) Immigration- gastrulation by the eviction of individual cells from the blastoderm into the interior. It was first described by I.I. Mechnikov in jellyfish embryos. Immigration can be unipolar, bipolar and multipolar, i.e. during immigration, cells are evicted from one, two or several zones at once. Immigration, observed in coelenterates, which are lower in the evolutionary series than all multicellular organisms, is the most ancient type of gastrulation.

2) Intussusception- gastrulation by invagination of the vegetative pole. It is characteristic of lower chordates, echinoderms, and some coelenterates, i.e. it is observed in embryos developing from isolecithal eggs, characterized by complete uniform crushing.

3) Epiboly- fouling.

If the embryo develops from a telolecithal egg, and at the vegetative pole of the blastula there are large, yolk-rich macromeres, then the bending of the vegetal pole is difficult, and gastrulation occurs due to the rapid proliferation of micromeres that overgrow the vegetal pole. In this case, macromeres end up inside the embryo. Epiboly is observed in amphibians; it is combined with the movement of the blastoderm into the embryo (invagination) at the border of the animal and vegetative poles, i.e. epiboly in its pure form practically does not occur.

4) Delamination- delamination. With this type of gastrulation, observed in some coelenterates that have a blastula in the form of a morula (there is no blastocoel in the blastula), blastoderm cells are divided into external and internal. As a result, the gastrula ectoderm is formed due to the external cells, and the endoderm is formed due to the internal cells.

Rice. 4. Types of gastrula: a – intussusception gastrula; b, c – two stages of development of the immigration gastrula; d, e – two stages of development of the delamination gastrula; f, g – two stages of development of the epibolic gastrula; 1 – ectoderm; 2 – endoderm; 3 – blastocoel.

Despite the variety of types of gastrulation, the essence of the process comes down to one thing: a single-layer embryo (blastula) turns into a two-layer embryo (gastrula).

1.5.4. Methods of formation of the third germ layer

In all multicellular animals, except sponges and coelenterates, following the formation of ecto- and endoderm, the third germ layer, the mesoderm, develops. Mesoderm has a dual origin. One part of it looks like a loose mass of cells moving out one by one from other germ layers. This part is called mesenchyme. All types of connective tissue, smooth muscles, circulatory and lymphatic systems are subsequently formed from mesenchyme. In the process of phylogenesis, it arose earlier. The second part of the mesoderm is called the mesoblast. It appears in the form of a compact, bilaterally symmetrical rudiment. The mesoblast formed in phylogeny later than the mesenchyme. During ontogenesis it develops in various ways.

Teloblastic method, is mainly observed in protostomes (typically occurs in mollusks, annelids, crustaceans). It occurs by ingrowth of multicellular primordia on both sides of the blastopore or by the introduction of two large cells - teloblasts - into the same places. As a result of the proliferation of teloblasts, from which small cells are separated, mesoderm is formed.

Enterocelic method observed in deuterostomes (typical course in echinoderms, lancelet). In them, the mesoblast is detached from the wall of the primary intestine in the form of paired mesodermal pockets with the rudiments of the coelomic cavity inside.

Consequently, at the stage of formation of the germ layers, the same process takes place, varying only in details. The essence of the occurring phenomena lies in the differentiation of three germ layers: the outer - ectoderm, the inner - endoderm and the middle layer located between them - the mesoderm. Later, due to these layers, various tissues and organs develop.

Rice. 5. Methods of formation of the third germ layer: A - teloblastic, B - enterocoelic, 1 - ectoderm, 2 - mesenchyme, 3 - endoderm, 4 - teloblast (a) and coelomic mesoderm (b).

From the ectoderm develop: nervous system, epidermis of the skin, epithelium of the skin and mammary glands, horny formations (scales, hair, feathers, nails), epithelium of the salivary glands, lens of the eye, auditory vesicle, peripheral sensory apparatus, tooth enamel.

From endoderm: chord, epithelial lining of the intestinal tract and its derivatives - the liver, pancreas, gastric and intestinal glands; epithelial tissue lining the organs of the respiratory system and partly the genitourinary system, as well as the secreting parts of the anterior and middle lobes of the pituitary gland, thyroid and parathyroid glands.

From mesoderm: From the outer (lateral) part of the somites, i.e. the dermatome, the connective tissue of the skin - the dermis - is formed. From the middle (central) part of the somites, i.e. the myotome, striated skeletal muscles are formed. The inner (medial) part of the somites, i.e. the sclerotome, gives rise to supporting tissues, first cartilaginous, and then bone (primarily the vertebral bodies) and connective tissue, forming an axial skeleton around the notochord.

The somite legs (nephrogonatomes) give rise to excretory organs (renal tubules) and gonads.

The cells forming the visceral and parietal layers of the splanchnotome are the source of the epithelial lining of the secondary cavity of the coelom. The splanchnotome also produces connective tissue of internal organs, the circulatory system, smooth muscles of the intestines, respiratory and genitourinary tracts, and skeletal mesenchyme, which gives rise to the rudiments of the skeleton of the limbs.

Chapter 3. Provisional authorities

Provisional organs are temporary special extra-embryonic organs that ensure the connection of the embryo with the environment during embryonic development.

Rice. 6. Provisional organs of vertebrates.

a – anamnesia; b – non-placental amniotes; c – placental amniotes; 1 – embryo; 2 – yolk sac; 3 – amnion; 4 – allantois; 5 - chorion; 6 – chorionic villi; 7 – placenta; 8 – umbilical cord; 9 - reduced yolk sac; 10 – reduced allantois.

Since the embryonic development of organisms with different types of development (larval, non-larval, intrauterine) occurs under different conditions, the degree of development and functions of their provisional organs are different.

3.1. Yolk sac

The yolk sac is characteristic of all animals with a non-larval type of development, the eggs of which are rich in yolk (fish, reptiles, birds). In fish, the yolk sac is formed from the cellular material of the three germ layers, that is, ecto-, ento- and mesoderm. In reptiles and birds, the inner layer of the yolk sac is of endodermal origin, and the outer layer is of mesodermal origin.

In mammals, although there is no yolk reserve in the eggs, there is a yolk sac. This may be due to its important secondary functions. It is formed from splanchnopleura, which arises from formations of mesodermal and endodermal origin. The splanchnopleura is split into intraembryonic and extraembryonic parts. The yolk sac is formed from the extraembryonic part.

Blood vessels grow into the walls of the yolk sac, forming a dense capillary network. The cells of the yolk sac wall secrete enzymes that break down the nutrients in the yolk, which enter the blood capillaries and then into the embryo’s body. Thus, the yolk sac performs trophic function. The yolk sac is also the site of reproduction of blood cells, that is, it performs hematopoietic function.

In mammals, the endoderm of the yolk sac serves as the site of formation of primary germ cells. In addition, the yolk sac of mammals is filled with a fluid characterized by a high concentration of amino acids and glucose, which indicates the possibility protein metabolism in the yolk sac. In different mammals, the yolk sac is developed differently: in predators it is large with a highly developed network of vessels. And in primates it shrinks greatly and disappears without a trace before birth.

The fate of the yolk sac varies from animal to animal. In birds, by the end of incubation, the remains of the yolk sac are inside the embryo, after which it quickly dissolves and disappears. In mammals, the reduced yolk sac is part of the placenta.

What are germ layers or layers? What is the meaning of this term? The article will provide brief information about these isolated groups of cells present in all embryos of faunal representatives at a certain stage of embryonic development.

From the history

Back in the 60s of the 18th century, the German and Russian physiologist Caspar Friedrich Wolf observed and later described the formation and transformation of one of the germ layers into an intestinal tube. For the first time, all three germ layers were discovered and described by Christian Heinrich Pander, academician of the Imperial Academy of Sciences in St. Petersburg (1821), naturalist, embryologist and paleontologist. He studied their structure by also studying the chicken embryo. In addition, academician of the same academy Karl Baer discovered the presence of germ layers in the embryos of other animals - fish, reptiles, amphibians. Thanks to the works of these scientists, an impetus was given to the study of these structures.

Formation of germ layers

The zygote (fertilized animal egg) begins to divide. At the early stage of embryonic development, cells intensively divide by mitosis, forming a spherical structure - a morula, and then a blastula. Its difference from morula is that at this stage the cells (they are called blastomeres) diverge from the center to the periphery, and in the middle a so-called blastoderm vesicle is formed. The blastula is thus a single-layer embryo.

After the end of this period of embryonic development of representatives of the animal world, called cleavage, the stage of gastrulation begins. The difference between these stages of ontogenesis is cardinal. In the first case, the fertilized egg is divided into many blastomeres (smaller cells), without changing in mass and volume. The main significance of cleavage is the transition of the embryo from one cell to multicellularity. Gastrulation, which occurs after fragmentation, involves cell differentiation. At this stage, the so-called germ layers appear. These are certain groups of cells from which certain tissues and organs are subsequently formed.

Differences between germ layers

The structure of the embryo at the stage of gastrulation and those preceding it is shown in the image below. At the stage following gastrulation, called neurula, the neural plate, notochord rudiment, epithelium, and intestine are formed. The posterior and anterior sections of the body become distinguishable.

During gastrulation, as mentioned above, not only cell multiplication occurs, but also their growth and directed movement, subsequently leading to clearly defined differentiation. Groups of related cells are combined into separate layers of cells, external and internal. They are called ectoderm and endoderm.

In sponges and coelenterates (jellyfish, corals, ctenophores), only these two germ layers develop. In higher animals, three of them are formed: the mentioned ectoderm and endoderm, as well as the middle layer - mesoderm.

Their differences lie primarily in their functions, as well as in what organs and tissues they give rise to. They will be discussed in more detail below.

Ectoderm

The outer layer of germ cells is responsible for motor, sensory and integumentary functions. From it the organs of the nervous system subsequently develop. In addition, the skin and everything that is found on it in animals develops from the ectoderm: protective scales, claws, nails, feathers, scutes, etc., as well as tooth enamel.

This germ layer in vertebrates contains three parts: the outer layer, as well as the neural tube and neural crest. The last two components are also known as neuroectoderm. Since 2000, the neural crest, at the suggestion of Canadian embryologist Brian Hall, has been called the fourth germ layer in many publications.

Endoderm

The germ layer from which internal organs are partially formed. This is the digestive system, including the glands (pancreas, liver). Respiratory organs also develop from the endoderm (in fish - gills and swim bladder).

Mesoderm

The middle layer of germ cells, characteristic only of higher animals. Responsible for the implementation of trophic and support functions. From it develop bones and muscles, cartilage, notochord, excretory organs, as well as organs of the reproductive and circulatory systems.

Finally

The article briefly described the germ layers of animals, their functions, and listed the organs and systems that develop from the mesoderm, ectoderm, and endoderm.

An interesting fact is that all representatives of the animal world have tissues from 2-3 of these structures in most organs.

Types of gastrulation.

At the end of the period of fragmentation, the embryos of all multicellular animals enter the period of formation of germ layers (leaves). This stage is called gastrulation.

There are two stages in the gastrulation process. First, an early gastrula is formed, which has two germ layers: the outer one is the ectoderm and the inner one is the endoderm. Then comes the late gastrula, when the middle germ layer, the mesoderm, is formed. Gastrula formation occurs in different ways.

There are 4 types of gastrulation:

1) Immigration- gastrulation by the eviction of individual cells from the blastoderm into the interior. It was first described by I.I. Mechnikov in jellyfish embryos. Immigration can be unipolar, bipolar and multipolar, i.e. during immigration, cells are evicted from one, two or several zones at once. Immigration, observed in coelenterates, which are lower in the evolutionary series than all multicellular organisms, is the most ancient type of gastrulation.

2) Intussusception- gastrulation by invagination of the vegetative pole. It is characteristic of lower chordates, echinoderms, and some coelenterates, i.e. it is observed in embryos developing from isolecithal eggs, characterized by complete uniform crushing.

3) Epiboly- fouling.

If the embryo develops from a telolecithal egg, and at the vegetative pole of the blastula there are large, yolk-rich macromeres, then the bending of the vegetal pole is difficult, and gastrulation occurs due to the rapid proliferation of micromeres that overgrow the vegetal pole. In this case, macromeres end up inside the embryo. Epiboly is observed in amphibians; it is combined with the movement of the blastoderm into the embryo (invagination) at the border of the animal and vegetative poles, i.e. epiboly in its pure form practically does not occur.

4) Delamination- delamination. With this type of gastrulation, observed in some coelenterates that have a blastula in the form of a morula (there is no blastocoel in the blastula), blastoderm cells are divided into external and internal. As a result, the gastrula ectoderm is formed due to the external cells, and the endoderm is formed due to the internal cells.

Rice. 4. Types of gastrula: a – intussusception gastrula; b, c – two stages of development of the immigration gastrula; d, e – two stages of development of the delamination gastrula; f, g – two stages of development of the epibolic gastrula; 1 – ectoderm; 2 – endoderm; 3 – blastocoel.

Despite the variety of types of gastrulation, the essence of the process comes down to one thing: a single-layer embryo (blastula) turns into a two-layer embryo (gastrula).

1.5.4. Methods of formation of the third germ layer

In all multicellular animals, except sponges and coelenterates, following the formation of ecto- and endoderm, the third germ layer, the mesoderm, develops. Mesoderm has a dual origin. One part of it looks like a loose mass of cells moving out one by one from other germ layers. This part is called mesenchyme. All types of connective tissue, smooth muscles, circulatory and lymphatic systems are subsequently formed from mesenchyme. In the process of phylogenesis, it arose earlier. The second part of the mesoderm is called the mesoblast. It appears in the form of a compact, bilaterally symmetrical rudiment. The mesoblast formed in phylogeny later than the mesenchyme. During ontogenesis it develops in various ways.

Teloblastic method, is mainly observed in protostomes (typically occurs in mollusks, annelids, crustaceans). It occurs by ingrowth of multicellular primordia on both sides of the blastopore or by the introduction of two large cells - teloblasts - into the same places. As a result of the proliferation of teloblasts, from which small cells are separated, mesoderm is formed.

Enterocelic method observed in deuterostomes (typical course in echinoderms, lancelet). In them, the mesoblast is detached from the wall of the primary intestine in the form of paired mesodermal pockets with the rudiments of the coelomic cavity inside.

Consequently, at the stage of formation of the germ layers, the same process takes place, varying only in details. The essence of the occurring phenomena lies in the differentiation of three germ layers: the outer - ectoderm, the inner - endoderm and the middle layer located between them - the mesoderm. Later, due to these layers, various tissues and organs develop.

Rice. 5. Methods of formation of the third germ layer: A - teloblastic, B - enterocoelic, 1 - ectoderm, 2 - mesenchyme, 3 - endoderm, 4 - teloblast (a) and coelomic mesoderm (b).

Without having in your disposal of early embryos humans, showing some of the most important stages of the formation of germ layers, we tried to trace their formation in other mammals. The most noticeable feature of early development is the formation of many cells from a single fertilized egg through successive mitoses. Even more important is the fact that even during the early phases of rapid proliferation, the cells thus formed do not remain an unorganized mass.

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

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

Germ layers are of interest to the embryologist from several points of view. The simple structure of the embryo, when it first contains one, then two and finally three primary layers of cells, is a reflection of the phylogenetic changes that took place in lower animals - the ancestors of vertebrates. From the point of view of possible ontogenetic recapitulations, some facts fully allow this.

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

Skeletal, muscular and circulatory systems originate in vertebrates almost exclusively from the mesoderm - a layer that is relatively unnoticeable in small, low-organized creatures, but whose role increases as their size and complexity increase due to their increasing needs for support and circulatory systems.

Along with the possibility interpretation of germ layers from the point of view of their phylogenetic significance, it is also important for us to establish the role they play in individual development. The germ layers are the first organized groups of cells in the embryo, which are clearly distinguished from each other by their features and relationships. The fact that these relationships are essentially the same in all vertebrate embryos strongly suggests a common origin and similar heredity in the various members of this huge group of animals.

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

Formation of embryonic leaflets The period when the main process of development is only an increase in the number of cells ends, and the period of differentiation and specialization of cells begins. Differentiation occurs in the germ layers before we can see its signs using any of our microscopic techniques. In the leaf, which has a completely homogeneous appearance, localized groups of cells with different potencies for further development constantly appear.

We have known about this for a long time, for we can see how from the germ layer various structures arise. At the same time, no visible changes are noticeable in the germ layer due to which they arise. Recent experimental studies indicate how early this invisible differentiation precedes the visible morphological localization of cell groups that we easily recognize as the rudiment of the definitive organ.

So, for example, if you cut from any place of Hensen's node a narrow transverse strip of the ectoderm of a twelve-hour embryo and grown in tissue culture, then at a certain time specialized cellular elements of a type that is found only in the eye will be discovered, although the rudiment of the optic vesicle of a chick embryo does not appear before 30 hours of incubation. A strip taken from another area, although it appears the same, when grown in culture does not form cells characteristic of the eye, but exhibits a different specialization.

Experiments show how early in the germ layers groups of cells with different potencies for development are determined. As development progresses, these cell groups become more and more prominent. In some cases, they are separated from the mother leaf by protrusion, in other cases - by migration of individual cells, which later accumulate somewhere in a new place.

From the primary groups of cells thus formed, gradually definitive organs are formed. Therefore, the origin of various parts of the body in embryogenesis depends on the growth, division and differentiation of the germ layers. This diagram shows us the general path along which the early processes discussed above develop. If we follow the process of development further, we see that each normal division of an object is more or less clearly centered around a certain branch of this family tree of germ layers.