Gastrula stage. Embryo development at the gastrula stage
The content of the article
EMBRYOLOGY, science that studies the development of an organism early stages preceding metamorphosis, hatching, or birth. The fusion of gametes - an egg (ovum) and a spermatozoon - with the formation of a zygote gives rise to a new individual, but before becoming the same creature as its parents, it has to go through certain stages of development: cell division, the formation of primary germ layers and cavities, the emergence of embryonic axes and axes of symmetry, the development of coelomic cavities and their derivatives, the formation of extraembryonic membranes and, finally, the appearance of organ systems that are functionally integrated and form one or another recognizable organism. All this is the subject of the study of embryology.
Development is preceded by gametogenesis, i.e. formation and maturation of sperm and egg. The process of development of all eggs of a given species proceeds in general in the same way.
Gametogenesis.
Mature spermatozoa and eggs differ in their structure, only their nuclei are similar; however, both gametes are formed from identical-looking primordial germ cells. In all sexually reproducing organisms, these primary germ cells separate from other cells in the early stages of development and develop in a special way, preparing to perform their function - the production of sex, or germ, cells. Therefore, they are called germplasm - in contrast to all other cells that make up the somatoplasm. It is quite obvious, however, that both germplasm and somatoplasm originate from a fertilized egg - a zygote that gave rise to a new organism. So basically they are the same. The factors that determine which cells will become sexual and which will become somatic have not yet been established. However, in the end, germ cells acquire fairly clear differences. These differences arise in the process of gametogenesis.
In all vertebrates and some invertebrates, primary germ cells arise far from the gonads and migrate to the gonads of the embryo - the ovary or testis - with the blood flow, with layers of developing tissues, or through amoeboid movements. In the gonads, mature germ cells are formed from them. By the time of development of the gonads, the soma and the germ plasm are already functionally isolated from each other, and, starting from this time, throughout the life of the organism, the germ cells are completely independent of any influences of the soma. That is why the signs acquired by an individual throughout his life do not affect his germ cells.
Primary germ cells, being in the gonads, divide with the formation of small cells - spermatogonia in the testes and oogonia in the ovaries. Spermatogonia and oogonia continue to divide many times, forming cells of the same size, which indicates the compensatory growth of both the cytoplasm and the nucleus. Spermatogonia and oogonia divide mitotically and therefore retain their original diploid number of chromosomes.
After some time, these cells stop dividing and enter a period of growth, during which very important changes occur in their nuclei. Chromosomes originally received from two parents are paired (conjugated), entering into very close contact. This makes possible subsequent crossing over (crossover), during which homologous chromosomes are broken and connected in a new order, exchanging equivalent sections; as a result of crossing over, new combinations of genes appear in the chromosomes of oogonia and spermatogonia. It is assumed that the sterility of mules is due to the incompatibility of the chromosomes received from the parents - a horse and a donkey, because of which the chromosomes are not able to survive when closely connected to each other. As a result, the maturation of germ cells in the ovaries or testes of the mule stops at the stage of conjugation.
When the nucleus has been rebuilt and a sufficient amount of cytoplasm has accumulated in the cell, the process of division resumes; the whole cell and the nucleus undergo two different types of divisions, which determine the actual process of maturation of germ cells. One of them - mitosis - leads to the formation of cells similar to the original; as a result of the other - meiosis, or reduction division, during which cells divide twice, cells are formed, each of which contains only half (haploid) the number of chromosomes compared to the original, namely one from each pair. In some species, these cell divisions occur in reverse order. After the growth and reorganization of the nuclei in oogonia and spermatogonia and immediately before the first division of meiosis, these cells are called oocytes and spermatocytes of the first order, and after the first division of meiosis, oocytes and spermatocytes of the second order. Finally, after the second division of meiosis, the cells in the ovary are called eggs (eggs), and those in the testis are called spermatids. Now the egg has finally matured, and the spermatid has yet to go through metamorphosis and turn into a spermatozoon.
One important difference between oogenesis and spermatogenesis needs to be emphasized here. From one oocyte of the first order, as a result of maturation, only one mature egg is obtained; the other three cores and not a large number of cytoplasms turn into polar bodies that do not function as germ cells and subsequently degenerate. All the cytoplasm and yolk, which could be distributed over four cells, are concentrated in one - in a mature egg. In contrast, one first-order spermatocyte gives rise to four spermatids and the same number of mature spermatozoa, without losing a single nucleus. During fertilization, the diploid, or normal, number of chromosomes is restored.
Egg.
The ovum is inert and usually larger than somatic cells given organism. The mouse egg is about 0.06 mm in diameter, while the diameter of the ostrich egg is more than 15 cm. The eggs are usually spherical or oval in shape, but can also be oblong, like those of insects, hagfish or mudfish. The size and other features of the egg depend on the amount and distribution of the nutritious yolk in it, which accumulates in the form of granules or, more rarely, in the form of a continuous mass. Therefore, eggs are divided into different types depending on the content of yolk in them.
Homolecithal eggs
(from Greek homós - equal, homogeneous, lékithos - yolk) . In homolecithal eggs, also called isolecithal or oligolecithal eggs, there is very little yolk and it is evenly distributed in the cytoplasm. Such eggs are typical of sponges, coelenterates, echinoderms, scallops, nematodes, tunicates, and most mammals.
Telolecithal eggs
(from Greek télos - end) contain a significant amount of yolk, and their cytoplasm is concentrated at one end, usually referred to as the animal pole. The opposite pole, on which the yolk is concentrated, is called vegetative. Such eggs are typical for annelids, cephalopods, non-cranial (lancelet), fish, amphibians, reptiles, birds, and monotreme mammals. They have a well-defined animal-vegetative axis, determined by the gradient of the distribution of the yolk; the core is usually located eccentrically; in eggs containing pigment, it is also distributed along a gradient, but, unlike the yolk, it is more abundant at the animal pole.
Centrolecithal eggs.
In them, the yolk is located in the center, so that the cytoplasm is shifted to the periphery and fragmentation is superficial. Such eggs are typical for some coelenterates and arthropods.
Sperm.
Unlike a large and inert egg, spermatozoa are small, from 0.02 to 2.0 mm in length, they are active and able to swim a long distance to reach the egg. There is little cytoplasm in them, and there is no yolk at all.
The shape of spermatozoa is diverse, but among them two main types can be distinguished - flagellated and non-flagellated. Flagellated forms are comparatively rare. In most animals, an active role in fertilization belongs to the spermatozoon.
Fertilization.
Fertilization - difficult process during which the sperm enters the egg and their nuclei fuse. As a result of the fusion of gametes, a zygote is formed - in essence, a new individual capable of developing in the presence of the necessary conditions for this. Fertilization causes the activation of the egg, stimulating it to successive changes leading to the development of a formed organism. During fertilization, amphimixis also occurs, i.e. mixing of hereditary factors as a result of the fusion of the nuclei of the egg and sperm. The egg provides half of the necessary chromosomes and usually all the nutrients needed for the early stages of development.
When a spermatozoon comes into contact with the surface of the egg, the yolk membrane of the egg changes, turning into a fertilization membrane. This change is considered proof that egg activation has occurred. At the same time, on the surface of eggs that contain little or no yolk at all, a so-called. a cortical reaction that prevents other sperm from entering the egg. Eggs that contain a lot of yolk have a cortical reaction later, so they usually get a few spermatozoa. But even in such cases, only one spermatozoon, the first to reach the nucleus of the egg, performs fertilization.
In some eggs, at the point of contact of the sperm with the plasma membrane of the egg, a protrusion of the membrane is formed - the so-called. tubercle of fertilization; it facilitates the penetration of the spermatozoon. Usually, the head of the spermatozoon and the centrioles located in its middle part penetrate the egg, while the tail remains outside. Centrioles contribute to the formation of the spindle during the first division of a fertilized egg. The fertilization process can be considered complete when the two haploid nuclei - the egg and the sperm - merge and their chromosomes are conjugated, preparing for the first crushing of the fertilized egg.
Splitting up.
If the appearance of the fertilization membrane is considered an indicator of the activation of the egg, then division (crushing) is the first sign of the actual activity of the fertilized egg. The nature of crushing depends on the amount and distribution of the yolk in the egg, as well as on the hereditary properties of the zygote nucleus and the characteristics of the egg cytoplasm (the latter are entirely determined by the genotype of the mother organism). There are three types of crushing of a fertilized egg.
Holoblastic fragmentation
characteristic of homolecithal eggs. Crushing planes separate the egg completely. They can divide it into equal parts, like a starfish or sea urchin, or into unequal parts, as in gastropod Crepidula. Cleavage of the moderately telolecithal egg of the lancelet occurs according to the holoblastic type, however, uneven division appears only after the stage of four blastomeres. In some cells, after this stage, fragmentation becomes extremely uneven; the resulting small cells are called micromeres, and the large cells containing the yolk are called macromeres. In molluscs, the cleavage planes pass in such a way that, starting from the stage of eight cells, the blastomeres are arranged in a spiral; this process is regulated by the kernel.
meroblastic fragmentation
typical of telolecithal eggs rich in yolk; it is limited to a relatively small area near the animal pole. Cleavage planes do not pass through the entire egg and do not capture the yolk, so that as a result of division at the animal pole, a small disk of cells (blastodisk) is formed. Such crushing, also called discoidal, is characteristic of reptiles and birds.
Surface crushing
typical of centrolecithal eggs. The nucleus of the zygote divides in the central island of the cytoplasm, and the resulting cells move to the surface of the egg, forming a surface layer of cells around the yolk lying in the center. This type of cleavage is seen in arthropods.
crushing rules.
It has been established that fragmentation obeys certain rules, named after the researchers who first formulated them. Pfluger's Rule: The spindle always pulls in the direction of least resistance. Balfour's rule: the rate of holoblastic cleavage is inversely proportional to the amount of yolk (yolk makes it difficult to divide both the nucleus and the cytoplasm). Sacks' rule: cells are usually divided into equal parts, and the plane of each new division intersects the plane of the previous division at a right angle. Hertwig's rule: the nucleus and spindle are usually located in the center of the active protoplasm. The axis of each spindle of division is located along the long axis of the mass of protoplasm. The division planes usually intersect the mass of protoplasm at right angles to its axes.
As a result of crushing of fertilized eggs of any type, cells called blastomeres are formed. When there are a lot of blastomeres (in amphibians, for example, from 16 to 64 cells), they form a structure that resembles a raspberry and is called a morula.
Blastula.
As the crushing continues, the blastomeres become smaller and tighter to each other, acquiring a hexagonal shape. This form increases the structural rigidity of the cells and the density of the layer. Continuing to divide, the cells push each other apart and, as a result, when their number reaches several hundred or thousands, they form a closed cavity - the blastocoel, into which fluid from the surrounding cells enters. In general, this formation is called the blastula. Its formation (in which cell movements do not participate) ends the period of egg crushing.
In homolecithal eggs, the blastocoel may be centrally located, but in telolecithal eggs, it is usually displaced by the yolk and is located eccentrically, closer to the animal pole and directly below the blastodisc. So, the blastula is usually a hollow ball, the cavity of which (blastocoel) is filled with liquid, but in telolecithal eggs with discoidal crushing, the blastula is represented by a flattened structure.
In holoblastic cleavage, the blastula stage is considered complete when, as a result of cell division, the ratio between the volumes of their cytoplasm and nucleus becomes the same as in somatic cells. In a fertilized egg, the volumes of the yolk and cytoplasm do not correspond at all to the size of the nucleus. However, in the process of crushing, the amount of nuclear material increases somewhat, while the cytoplasm and yolk only divide. In some eggs, the ratio of the volume of the nucleus to the volume of the cytoplasm at the time of fertilization is approximately 1:400, and by the end of the blastula stage it is approximately 1:7. The latter is close to the ratio characteristic of both the primary reproductive and somatic cells.
Late blastula surfaces in tunicates and amphibians can be mapped; for this, lifetime (not harmful to cells) dyes are applied to its different parts - the color marks made are stored during further development and allow you to establish which organs arise from each site. These areas are called presumptive, i.e. those whose fate can be predicted under normal conditions of development. If, however, at the stage of late blastula or early gastrula, these areas are moved or swapped, their fate will change. Such experiments show that, up to a certain stage of development, each blastomere is able to turn into any of the many different cells that make up the body.
Gastrula.
The gastrula is the stage of embryonic development in which the embryo consists of two layers: the outer - ectoderm, and the inner - endoderm. This bilayer stage is achieved in different ways in different animals, since the eggs different types contain varying amounts of yolk. However, in any case, the main role in this is played by cell movements, and not cell divisions.
Intussusception.
In homolecithal eggs, for which holoblastic cleavage is typical, gastrulation usually occurs by invagination (invagination) of the cells of the vegetative pole, which leads to the formation of a two-layer, bowl-shaped embryo. The original blastocoel contracts, but a new cavity, the gastrocoel, is formed. The opening leading into this new gastrocoel is called the blastopore (an unfortunate name because it opens not into the blastocoel, but into the gastrocoel). The blastopore is located in the region of the future anus, at the posterior end of the embryo, and in this region develops most of mesoderm - the third, or middle, germ layer. The gastrocoel is also called the archenteron, or primary gut, and it serves as the rudiment of the digestive system.
Involution.
In reptiles and birds, whose telolecithal eggs contain a large amount of yolk and are meroblastically divided, blastula cells rise above the yolk in a very small area and then begin to screw inward, under the cells of the upper layer, forming the second (lower) layer. This process of screwing in the cell sheet is called involution. The top layer of cells becomes the outer germ layer, or ectoderm, and the bottom layer becomes the inner, or endoderm. These layers merge into one another, and the place where the transition occurs is known as the blastopore lip. The roof of the primary intestine in the embryos of these animals consists of fully formed endodermal cells, and the bottom of the yolk; the bottom of the cells is formed later.
Delamination.
In higher mammals, including humans, gastrulation occurs somewhat differently, namely by delamination, but leads to the same result - the formation of a two-layer embryo. Delamination is a stratification of the original outer layer of cells, leading to the emergence of an inner layer of cells, i.e. endoderm.
Auxiliary processes.
There are also additional processes that accompany gastrulation. The simple process described above is the exception, not the rule. Auxiliary processes include epiboly (fouling), i.e. movement of cell layers over the surface of the vegetative hemisphere of the egg, and concretion - the association of cells in large areas. One of these processes or both of them can accompany both invagination and involution.
results of gastrulation.
The end result of gastrulation is the formation of a bilayer embryo. The outer layer of the embryo (ectoderm) is formed by small, often pigmented cells that do not contain yolk; from the ectoderm, such tissues as, for example, nervous, and the upper layers of the skin further develop. The inner layer (endoderm) consists of almost unpigmented cells that retain some yolk; they give rise mainly to the tissues lining the digestive tract and its derivatives. However, it should be emphasized that there are no profound differences between these two germ layers. The ectoderm gives rise to the endoderm, and if in some forms the boundary between them in the region of the blastopore lip can be determined, then in others it is practically indistinguishable. Transplantation experiments have shown that the difference between these tissues is determined only by their location. If areas that would normally remain ectodermal and give rise to derivatives of the skin are transplanted onto the lip of the blastopore, they screw inward and become the endoderm, which can turn into a lining digestive tract, lungs or thyroid gland.
Often, with the appearance of the primary intestine, the center of gravity of the embryo shifts, it begins to turn in its membranes, and for the first time the antero-posterior (head-tail) and dorso-ventral (back-belly) axes of symmetry of the future organism are established in it.
Germinal leaves.
Ectoderm, endoderm and mesoderm are distinguished based on two criteria. First, by their location in the embryo at the early stages of its development: during this period, the ectoderm is always located outside, the endoderm is inside, and the mesoderm, which appears last, is between them. Secondly, according to their future role: each of these leaves gives rise to certain organs and tissues, and they are often identified by their further fate in the development process. However, we recall that during the period when these leaflets appeared, there were no fundamental differences between them. In experiments on the transplantation of germ layers, it was shown that initially each of them has the potency of either of the other two. Thus, their distinction is artificial, but it is very convenient to use it in the study of embryonic development.
Mesoderm, i.e. the middle germ layer is formed in several ways. It may arise directly from the endoderm by the formation of coelomic sacs, as in the lancelet; simultaneously with the endoderm, like in a frog; or by delamination, from the ectoderm, as in some mammals. In any case, at first the mesoderm is a layer of cells lying in the space that was originally occupied by the blastocoel, i.e. between the ectoderm on the outside and the endoderm on the inside.
The mesoderm soon splits into two cell layers, between which a cavity is formed, called the coelom. From this cavity subsequently formed the pericardial cavity surrounding the heart, pleural cavity surrounding the lungs, and abdomen that contains the digestive organs. The outer layer of the mesoderm - the somatic mesoderm - forms, together with the ectoderm, the so-called. somatopleura. From the outer mesoderm develop striated muscles of the trunk and limbs, connective tissue and vascular elements of the skin. The inner layer of mesodermal cells is called the splanchnic mesoderm and, together with the endoderm, forms the splanchnopleura. From this layer the mesoderm develops smooth muscles and vascular elements of the digestive tract and its derivatives. In the developing embryo, there is a lot of loose mesenchyme (embryonic mesoderm) that fills the space between the ectoderm and endoderm.
In chordates, in the process of development, a longitudinal column of flat cells is formed - a chord, the main hallmark of this type. Notochord cells originate from the ectoderm in some animals, from the endoderm in others, and from the mesoderm in still others. In any case, these cells can be distinguished from the rest at a very early stage of development, and they are located in the form of a longitudinal column above the primary intestine. In vertebrate embryos, the notochord serves as the central axis around which the axial skeleton develops, and above it the central nervous system. In most chordates, this is a purely embryonic structure, and only in the lancelet, cyclostomes, and elasmobranchs does it persist throughout life. In almost all other vertebrates, notochord cells are replaced by bone cells that form the body of the developing vertebrae; it follows that the presence of the chord facilitates the formation of the spinal column.
Derivatives of the germ layers.
The further fate of the three germ layers is different.
From the ectoderm develop: all nervous tissue; outer layers of the skin and its derivatives (hair, nails, tooth enamel) and partly the mucous membrane of the oral cavity, nasal cavities and anus.
Endoderm gives rise to the lining of the entire digestive tract - from the oral cavity to the anus - and all its derivatives, i.e. thymus thyroid gland, parathyroid glands, trachea, lungs, liver and pancreas.
From the mesoderm are formed: all types connective tissue, bone and cartilage tissues, blood and vascular system; all types of muscle tissue; excretory and reproductive systems, dermal layer of the skin.
In an adult animal, there are very few organs of endodermal origin that do not contain nerve cells derived from the ectoderm. Each important organ also contains derivatives of the mesoderm - blood vessels, blood, and often muscles, so that the structural isolation of the germ layers is preserved only at the stage of their formation. Already at the very beginning of their development, all organs acquire a complex structure, and they include derivatives of all germ layers.
GENERAL BODY PLAN
Symmetry.
In the early stages of development, the organism acquires a certain type of symmetry characteristic of a given species. One of the representatives of the colonial protists, Volvox, has central symmetry: any plane passing through the center of the Volvox divides it into two equal halves. Among multicellular organisms, there is not a single animal that has this type of symmetry. For coelenterates and echinoderms, radial symmetry is characteristic, i.e. parts of their body are located around the main axis, forming, as it were, a cylinder. Some, but not all, planes passing through this axis divide such an animal into two equal halves. All echinoderms at the larval stage have bilateral symmetry, but in the process of development they acquire the radial symmetry characteristic of the adult stage.
For all highly organized animals, bilateral symmetry is typical, i.e. they can be divided into two symmetrical halves in only one plane. Since this arrangement of organs is observed in most animals, it is considered optimal for survival. The plane passing along the longitudinal axis from the ventral (abdominal) to the dorsal (dorsal) surface divides the animal into two halves, right and left, which are mirror images of each other.
Almost all unfertilized eggs have radial symmetry, but some lose it at the time of fertilization. For example, in a frog egg, the site of penetration of the spermatozoon is always shifted to the front, or head, end of the future embryo. This symmetry is determined by only one factor - the gradient of the distribution of the yolk in the cytoplasm.
Bilateral symmetry becomes apparent as soon as organ formation begins during embryonic development. In higher animals, almost all organs are laid in pairs. This applies to the eyes, ears, nostrils, lungs, limbs, most muscles, skeletal parts, blood vessels and nerves. Even the heart is laid down as a paired structure, and then its parts merge, forming one tubular organ, which subsequently twists, turning into the heart of an adult with its complex structure. Incomplete fusion of the right and left halves of the organs is manifested, for example, in cases of cleft palate or cleft lip, which occasionally occur in humans.
Metamerism (dismemberment of the body into similar segments).
The greatest success in the long process of evolution was achieved by animals with a segmented body. The metameric structure of annelids and arthropods is clearly visible throughout their life. In most vertebrates, the initially segmented structure later becomes little distinguishable; however, at the embryonic stages, their metamerism is clearly expressed.
In the lancelet, metamerism is manifested in the structure of the coelom, muscles and gonads. Vertebrates are characterized by a segmental arrangement of some parts of the nervous, excretory, vascular and supporting systems; however, already at the early stages of embryonic development, this metamerism is superimposed by the advanced development of the anterior end of the body - the so-called. cephalization. If we consider a 48-hour chicken embryo grown in an incubator, we can simultaneously reveal both bilateral symmetry and metamerism in it, which is most clearly expressed at the anterior end of the body. For example, groups of muscles, or somites, first appear in the head region and form sequentially, so that the least developed segmented somites are posterior.
Organogenesis.
In most animals, the alimentary canal is one of the first to differentiate. In essence, the embryos of most animals are a tube inserted into another tube; the inner tube is the intestine, from the mouth to the anus. Other organs that are part of the digestive system, and the respiratory organs are laid in the form of outgrowths of this primary intestine. The presence of the roof of the archenteron, or primary gut, under the dorsal ectoderm causes (induces), possibly together with the notochord, the formation on the dorsal side of the embryo of the second most important body system, namely the central nervous system. This happens as follows: first, the dorsal ectoderm thickens and the neural plate forms; then the edges of the neural plate rise, forming neural folds that grow towards each other and eventually close, - as a result, the neural tube, the rudiment of the central nervous system, appears. The brain develops from the front of the neural tube, and the rest of it turns into the spinal cord. The cavity of the neural tube almost disappears as the nervous tissue grows, leaving only a narrow central canal. The brain is formed as a result of protrusions, protrusions, thickenings and thinnings of the anterior part of the neural tube of the embryo. Paired nerves originate from the formed brain and spinal cord - cranial, spinal and sympathetic.
The mesoderm also undergoes changes immediately after its appearance. It forms paired and metameric somites (muscle blocks), vertebrae, nephrotomes (rudiments of excretory organs) and parts of the reproductive system.
Thus, the development of organ systems begins immediately after the formation of the germ layers. All development processes (under normal conditions) occur with the accuracy of the most advanced technical devices.
METABOLISM OF GERMS
Embryos developing in an aquatic environment do not require any other integument, except for the gelatinous shells that cover the egg. These eggs contain enough yolk to provide nourishment for the embryo; shells protect it to some extent and help retain metabolic heat and, at the same time, are sufficiently permeable so as not to interfere with free gas exchange (i.e., the supply of oxygen and the release of carbon dioxide) between the embryo and the environment.
Extra-embryonic membranes.
In animals that lay eggs on land or are viviparous, the embryo needs additional shells that protect it from dehydration (if eggs are laid on land) and provide nutrition, removal of end products of metabolism and gas exchange.
These functions are performed by extraembryonic membranes - amnion, chorion, yolk sac and allantois, which are formed during development in all reptiles, birds and mammals. Chorion and amnion are closely related in origin; they develop from the somatic mesoderm and ectoderm. Chorion - the outermost shell surrounding the embryo and three other shells; this shell is permeable to gases and gas exchange occurs through it. Amnion protects the cells of the embryo from drying out due to amniotic fluid secreted by its cells. The yolk sac filled with yolk, together with the yolk stalk, supplies the embryo with digested nutrients; this shell contains a dense network of blood vessels and cells that produce digestive enzymes. The yolk sac, like the allantois, is formed from the splanchnic mesoderm and endoderm: the endoderm and mesoderm spread over the entire surface of the yolk, overgrowing it, so that in the end the entire yolk is in the yolk sac. In reptiles and birds, allantois serves as a reservoir for the end products of metabolism coming from the kidneys of the embryo, and also provides gas exchange. In mammals, these important functions are performed by the placenta, a complex organ formed by chorionic villi, which, growing, enter the recesses (crypts) of the uterine mucosa, where they come into close contact with its blood vessels and glands.
In humans, the placenta fully provides the respiration of the embryo, nutrition and the release of metabolic products into the mother's bloodstream.
Extraembryonic membranes are not preserved in the postembryonic period. In reptiles and birds, when they hatch, the dried shells remain in the egg shell. In mammals, the placenta and other extraembryonic membranes are shed from the uterus (rejected) after the birth of the fetus. These shells provided the higher vertebrates with independence from the aquatic environment and undoubtedly played an important role in the evolution of vertebrates, especially in the emergence of mammals.
BIOGENETIC LAW
In 1828, K. von Baer formulated the following provisions: 1) the most common signs of any large group of animals appear in the embryo earlier than the less common signs; 2) after the formation of the most common features less common ones appear, and so on until the appearance of special features characteristic of this group; 3) the embryo of any animal species, as it develops, becomes less and less similar to the embryos of other species and does not pass through later stages their development; 4) the embryo of a highly organized species may resemble the embryo of a more primitive species, but never resembles adult form of this kind.
The biogenetic law formulated in these four propositions is often misunderstood. This law simply states that certain stages of development of highly organized forms have a clear resemblance to certain stages of development of forms lower on the evolutionary ladder. It is assumed that this similarity can be explained by descent from a common ancestor. Nothing is said about the adult stages of the lower forms. In this article, similarities between germline stages are implied; otherwise, the development of each species would have to be described separately.
Apparently, in the long history of life on Earth, the environment played a major role in the selection of embryos and adult organisms most adapted for survival. The narrow limits created by the environment in relation to possible fluctuations in temperature, humidity and oxygen supply reduced the variety of forms, leading them to relatively general type. As a result, that similarity of structure arose, which underlies the biogenetic law, if we are talking about the embryonic stages. Of course, in the process of embryonic development, in the currently existing forms, features appear that correspond to the time, place and methods of reproduction of this species.
Literature:
Carlson b. Fundamentals of Embryology according to Patten, vol. 1. M., 1983
Gilbert S. developmental biology, vol. 1. M., 1993
Which is called gastrula, and the process of its formation gastrulation.
Blastula, as a single-layer embryo, has not yet differentiated into germ layers, or cell layers. The embryo acquires the signs of a multicellular animal only when its body is divided into outer and inner germ layers - ecto- and endoderm. The ectoderm forms the primary covering of the body. The endoderm gives rise to the primary gut.
The concept of the germ layer was introduced by the famous natural scientist Karl Baer, who discovered germ layers in the chicken embryo. He showed that in all vertebrates the formation of certain organs can be associated with three germ layers. The ectoderm forms the epidermis and its derivatives, such as hair, feathers, as well as the nervous system and sensitive epithelium. From the endoderm arise the intestines and associated organs, such as the liver and lungs. Third germ layer mesoderm, forms the muscles, skeleton, excretory system and part of the sex glands. Subsequently, it was proved that the theory of germ layers is quite applicable to the development of invertebrates, being, thus, universal. Of course, the germ layers in reality are not strictly specialized, since the boundaries between them can be violated due to the wide potential capabilities of cells in the course of individual development. At the same time, the main position of the theory of germ layers, according to which the basic plan of the structure of multicellular animals is consistent with two or three poorly differentiated primordia, indicating the phylogenetic commonality of these animals, is not justified.
So, the embryo acquires a metacellular level of development when its body is divided into ecto- and endoderm. This separation is achieved in the process of gastrulation.
The two-layer embryo at the vegetative pole forms the primary mouth, or blastopore leading to the cavity of the primary intestine. Depending on the position of the primary mouth, two main groups are distinguished among bilaterally symmetrical animals: primary- and secondarily. In protostomes, the blastopore turns into the oral opening of the animal, while the anus arises from the secondarily bent ectoderm, which is connected to the posterior region of the endodermal gut (Fig. 30a). In deuterostomes, the primary mouth is transformed into an anus, and in the head region, in the form of an ectodermal protrusion, the mouth opening is re-formed (Fig. 30, b).
Thus, the main processes occurring at this stage of embryogenesis are significant movements of cells relative to each other ( morphogenetic movements). The result is an embryo with a complex anatomical structure.
Gastrulation is followed by a period when both cell division and morphogenetic movements continue. At this time, the processes of cell differentiation and organogenesis are important. In representatives of different types of animals, they differ greatly.
Methods of gastrulation (formation of a two-layer embryo - gastrula)
There are several ways to form a two-layer nucleus − gastrulae.
Immigration
The simplest way is immigration (crawling) of some cells from the surface layer into the cavity of the blastula, their reproduction there and filling the entire blastocoel with a randomly located mass. The outer layer of cells is the ectoderm, and the inner layer is the endoderm (Fig. 29). In many lower multicellular organisms, two main structures are formed due to the inner layer: the midgut epithelium (endoderm proper) and the tissues surrounding it, which make up the third germ layer, or mesoderm. These two layers (endo- and mesoderm), at the suggestion of I. I. Mechnikov, are called phagocytoblastoma, while the ectoderm kinoblastoma. The functions of these layers are different.
Intussusception
In less primitive animals, the gastrula is formed not by the cells crawling into the blastocoel, but by screwing in the outer epithelium, after which the screwed-in part becomes the endoderm. This process is called invagination.
Delamination
If, after crushing the egg, not a hollow ball is obtained, but a morula, then two-layer is achieved by delamination (splitting). The essence of delamination lies in the fact that the outer cells turn into the epithelium, while the inner cells remain the endoderm.
Epiboly (fouling)
Another way of forming a two-layer embryo is called epiboly or fouling. Epiboly is observed in the case of eggs rich in yolk, when future endoderm cells are inside due to fouling with cells of the animal pole. material from the site
Evolution of gastrula
Embryologists and evolutionists attach great importance to the processes that transform a unicellular fertilized egg (zygote) into a multicellular two-layer embryo. But I. I. Mechnikov's multicellular theories arose from spherical colonies of protozoa. Separate individuals of such a colony, having captured food, left to digest it in the cavity of the colony, then returning back. Over time, there was a division of cells into nourishing and motor cells, equipped with flagella. The colony ceased to be a hollow ball, since there were always nutrient cells inside, forming a phagocytoblast. This structure of multicellular Mechnikov called parenchymula. The mule parenchyma is a hypothetical primordial multicellular animal.
On the other hand, the no less famous zoologist E. Haeckel, again based on the observed processes occurring in the developing egg, suggested that the primary two-layer animal occurred by invagination in a certain place of the protozoan colony-ball. Haeckel called this hypothetical animal gastrea.
What comes first - immigration or invagination - is difficult to decide. But you should keep in mind general rule: if in one organism some process occurs by the movement of individual cells, and in another - by bending of the epithelial layers, the first organism is more primitive in this respect than the second. The fact is that intussusception requires that the body already has regulatory mechanisms that ensure friendly, coordinated behavior of the invaginating cells.
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In many multicellular animals, the inner layer of cells is formed by invagination of the cells of its wall into the cavity of the blastula. This two-layer stage of development is called gastrula. The outer layer of cells of the gastrula is called ectoderm, internal - endoderm. Formed by invagination and limited by the endoderm, the cavity is the cavity of the primary intestineopening outwards with a hole - the primary mouth. The ectoderm and endoderm are called germ layers.
Further development of the initially two-layer gastrula is associated with the formation of the third germ layer - mesoderm, isolation of the notochord, the formation of the intestine and the development of the central nervous system.
Initial stages crushing of eggs Development of the newt embryo.
frogs (top) and birds (bottom).
Successive stages of crushing 2, 4 and 8 blastomeres are visible.
The frog egg is divided into blastomeres of different sizes.
In the egg cell of birds, only the surface area is crushed
The active cytoplasm in which the nucleus is located.
Neurula stage.
The ectoderm gives rise to the body's outer integument, the nervous system, and associated sensory organs.
Mouth and anus, intestines, lungs, liver, pancreas develop from the endoderm.
The mesoderm gives rise to the chord, muscles, excretory system, cartilaginous and bone skeleton, blood vessels, sex glands.
Early stages of lancelet development
The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. Completely all the organs of the fetus are formed by three months. The initial stages of animal development have much in common for all organisms, which is one of the proofs of the unity of the origin of all living organisms on Earth.
Temporary germinal organs.
Amnion- an aqueous membrane that surrounds the embryo, protecting it from drying out and mechanical damage. In humans, this is the fetal bladder.
Chorion- adjacent to the shell or wall of the uterus, penetrated by capillaries, providing nutrition and respiration of the embryo.
Allantois- Urinary sac, which serves to excrete metabolic products. Its vessels are the umbilical veins and arteries for nutrition and excretion.
Yolk sac- serves for nutrition in birds, a source of germ cells and blood cells in humans.
The influence of the environment on the development of the organism.
There are, however, factors whose influence on individual development is not only undesirable, but also harmful. Especially it should be said about such influences on the development and functioning of the human body. Among the harmful external factors, alcoholic beverages and smoking should be primarily attributed.
Use alcoholic beverages brings great harm at any stage of individual development of a person and is especially dangerous in adolescence. Alcohol has a detrimental effect on all human organ systems, primarily on the central nervous system, on the heart and blood vessels, on the lungs, kidneys, and the system of organs of movement (muscles). The use of even small doses of alcohol disrupts the mental activity of a person, the rhythm of movements, breathing and heart activity, leads to numerous errors in work, to the occurrence of diseases. For example, alcohol destroys the liver, causing its degeneration (cirrhosis). The systematic use of alcohol leads to the emergence of a serious disease - alcoholism, which requires a long special treatment. Alcoholic parents can have mentally retarded and physically handicapped children.
Front poll:
Define the concept of ontogeny and describe it.
Describe the stage of the blastula.
Describe the gastrula stage.
Describe the stage of neurula.
Describe the temporary germinal organs.
How does the influence of the external environment affect the external and internal development of the organism?
VI. Postembryonic development of the organism.
Postembryonic development.
Indirect postembryonic development.
The biological significance of the larvae.
Direct postembryonic development.
Growth, aging and death are the stages of ontogeny.
Regeneration and transplantation.
Postembryonic development.
Indirect postembryonic development.
Full indirect development: egg → larva, which differs in structure from the adult → pupa → adult (housefly, butterfly, frog).
Incomplete indirect development: egg → larva, which is similar in structure to an adult → adult (cockroach).
The biological significance of the larvae.
Thanks to self-feeding, the larvae ensure the development of an adult, because. the eggs of animals that are characterized by indirect development contain a small supply of yolk.
Usually the larva represents a stage of development specially adapted for active feeding and growth (insects, amphibians). As a rule, larvae and adults of the same species live in different conditions, i.e. occupy different ecological niches, and due to this they do not compete with each other for space and food.
In some organisms, the larvae contribute to the spread of the species. For example, in many sessile, sedentary worms and mollusks, the larvae swim freely and occupy new habitats.
Direct postembryonic development.
By the time of birth, the body resembles the adult stage. Therefore, the postembryonic period is characterized by growth and the acquisition of a state of functional maturity of organs and systems.
Growth, aging and death are the stages of ontogeny.
Aging- a natural process that grows over time, leading to a decrease in the adaptive capabilities of the body and an increase in the likelihood of death.
Death- the irreversible cessation of all manifestations of the vital activity of the body.
Regeneration and transplantation.
1. Physiological regeneration- this is the renewal of cells and organs lost in the course of normal life, i.e. occurring as a normal physiological process (a regular change in cell generations in the epithelium of the skin, intestines, regrowth of nails, hair, shedding and regrowth of antlers in deer). There is a daily rhythm of cellular renewal. The mitotic index (the number of dividing cells per thousand) makes it possible to compare the mitotic activity of tissues.
2. Reparative regeneration- recovery processes in cells, organs and tissues in response to damaging effects (mechanical trauma, surgical effects, burns, frostbite, chemical exposure, diseases). Living organisms of any kind are inherent in the ability to reparative regeneration.
Hydra regeneration is a classic example of reparative regeneration. The Hydra can be decapitated by amputating the tentacled mouth cone and then re-forming it. By cutting the hydra into pieces, you can increase the number of hydras, because. each part is transformed into a whole hydra. A significant regenerative capacity was found in representatives of the types of flatworms and annelids, in starfish.
Regeneration in some species of invertebrates.
A - hydra; B - ringed worm; B is a starfish.
Vertebrates, newts, and frog tadpoles develop newly amputated legs and tails. This is an example of regeneration external body, as a result of which its form and function are restored, however, the regenerated organ is distinguished by its reduced size.
Triton limb regeneration.
1–7 – successive stages regeneration respectively
10, 12, 14, 18, 28, 42, 56 days after amputation.
The regeneration of internal organs occurs somewhat differently. When one or two lobes of the liver are removed from a rat, the remaining lobes increase in size and provide a function in the volume that was characteristic of a normal organ. However, the shape of the liver is not restored. The process by which the mass and function of an organ is restored is called regenon-rationalhypertrophy.
regeneration in mammals. A - regenerative hypertrophy of the rat liver: 1 - before surgery, 2 - after removal of two lobes, 3 - regenerated liver; B - rat muscle regeneration: 1 - stump of the removed muscle, 2 - restored muscle; C - healing of a skin incision in a person: 1 - fibrin clot, 2 - movement of cells of the growth layer, 3 - formation of an epithelial layer.
If one of the paired organs is removed, for example, the kidney or ovary, then the remaining one increases in size and performs the function in the volume of two normal organs. After removal of a lymph node or spleen, the remaining lymph nodes increase in size. This increase in mass and function of the remaining organ in response to the removal of a similar organ is called compensationthorny substitution hypertrophy and also belongs to the category of recovery processes. The term "hypertrophy" in biology and medicine refers to an increase in the size of organs and parts of the body.
ATnutricellular regeneration- an increase in the number of organelles (mitochondria, ribosomes) leading to the intensification of the energy and plastic metabolism of cells.
In all cases of reparative regeneration, complex regular changes in the structure of organs occur. These changes are most noticeable when the whole organism is restored from a part. Significant shaping processes do not occur on the wound surface, they unfold inside the preserved part, as a result, the whole organism is re-formed, initially the size of the remaining part, which then grows - morphallaxis. During the regeneration of external organs, the regrowth of a new organ from the wound surface is observed - epimorphosis.
Various forms of regeneration after injury share some common features. First, the wound closes, some of the remaining cells die, then the process of dedifferentiation, i.e. loss by cells of specific structural features, and then reproduction, movement and again differentiation of cells. To start the process of regeneration, the disruption of previous spatial connections and contacts between cells is of great importance. In the regulation of regenerative processes, along with intercellular interactions, hormones and influences from the nervous system play an important role. With age, regenerative capacity decreases.
Of particular interest to medicine is the question of the regenerative abilities of mammals, to which man also belongs. Skin, tendons, bones, nerve trunks and muscles regenerate well. For muscle regeneration, it is important to preserve at least a small stump, and for bone regeneration, periosteum is necessary. Thus, if the necessary conditions are created, it is possible to achieve the regeneration of many internal organs of mammals and humans. The impossibility of regeneration of limbs and other external organs in mammals with an active lifestyle is evolutionarily determined. Greater adaptive value could have fast healing wound surface than the long-term existence of a tender regenerate in places that are constantly injured during an active lifestyle.
Transplantation, or transplantation of cells, tissues and organs from one place to another in one organism, as well as from one organism to another. It is often desirable to transplant a healthy organ of one organism to the place of the affected organ of another organism, in addition to purely technical, surgical tasks, there are biological tasks that depend on the immunological incompatibility of the donor's tissues with the recipient's body, as well as moral and ethical problems.
Distinguish three types of transplant: auto-, homo- and heterotransplantation. Autotransplantation- transplantation of organs and tissues within the same organism (skin transplantation for burns and cosmetic defects, transplantation of the intestine to the place of the esophagus for burns of the latter).
Which the embryo of a multicellular animal passes during its development. Blastula transforms into gastrula. This is the earliest stage in the development of the embryo. The process of formation and growth of the gastrula is called gastrulation. Then comes the neurula stage.
The structure of the embryo during this period
As you know, the cells of the gastrula form the so-called petals. They correspond to three layers. The outer is called the exoderm, and in the future it turns into the epidermis - nails, hair and the nervous system of an adult organism.
The middle lobe of the gastrula is called the mesoderm. Muscles, skeleton, endocrine and circulatory system. But not all living organisms have a middle layer of cells. Some simple invertebrates develop from a bilayer gastrula.
The endoderm is the innermost layer of the embryo. It forms the lungs, liver and intestines. The human fetus also has a gastrula stage. It is formed in a form resembling a disk, already on the 8th - 9th day of fertilization. But, nevertheless, it is a gastrula, as in amphibians with reptiles.
Gastrulation methods
Modern biology knows several of them:
- Intussusception. Occurs in coelenterates and even higher animals. Scyphoid jellyfish and corals in the embryonic phase develop precisely by invagination. This method leads to the retraction of the wall inward, and the formation of a hole, which in the future often becomes a mouth in protostomes, and an anus or cloaca in deuterostomes. Protostomes are simple animals small size. Some are not even visible to the human eye. These are arthropods, mollusks, nematodes, annelids, tardigrades, etc. Deuterostomes include higher creatures: echinoderms and chordates. Including a person.
- Immigration. Indicates that the cells invade inside the blastula and form from the inside a special important tissue called the parenchyma. It is usually observed in sponges and coelenterates, on the example of which the great Russian scientist I. I. Mechnikov established that the gastrula is not a simple stage of the embryo, but an unusual discovery in world embryology.
- Delamination. Translated from Latin as "dividing into layers." This method of gastrulation is possible due to the splitting of blastula cells into two layers, from which the ectoderm and endoderm are later formed. This simple type of organogenesis is inherent in higher mammals.
- Epiboly. In some fish and amphibians, the gastrula develops in this way. In this case, small, yolk-poor cells grow around one large one, in which there is enough yolk. The result is a gastrula, similar in composition to a bird's egg.
These four modes of gastrulation are rarely found in nature in their pure form. Their combinations are more often observed.
Name history
The Russian biologist G. Kovalevsky in 1865 believed that the gastrula is an "intestinal larva", due to the similarity of the gastrula to the larva and its location in the area close to the intestines. Less than one decade later, in 1874, the German philosopher and naturalist E. Haeckel introduced the term "gastrula" itself, which is translated from ancient Greek as "womb", "stomach", which is also explained by the location of the embryo.
independent organism
As a rule, a gastrula is an embryo that does not exist by itself. It is located in the egg or uterus. But in nature there are also animals that develop from free-swimming gastrulae. Most often - it is intestinal. This group of creatures is interesting for its simple structure, which in an adult is similar to the composition of the gastrula. From this it follows that it is the same independent organism as the animal that eventually grows out of it. It can perform all the functions necessary to maintain vital activity in the embryonic state.
The essence of the gastrulation stage lies in the fact that a single-layer embryo - blastula - turns into a multilayer - two- or three-layer, called gastrula (from the Greek gaster - stomach in a diminutive sense).
In primitive chordates, for example, in the lancelet, a homogeneous single-layer blastoderm during gastrulation is transformed into an outer germ layer, the ectoderm, and an inner germ layer, the endoderm. The endoderm forms the primary gut with a cavity inside, the gastrocoel. The opening leading to the gastrocoel is called the blastopore or primary mouth. Two germ layers are the defining morphological features of gastrulation. Their existence at a certain stage of development in all multicellular animals, from the coelenterates to the higher vertebrates, allows us to think about the homology of the germ layers and the unity of the origin of all these animals.
In vertebrates, in addition to the two mentioned, during gastrulation, a third germ layer is formed - the mesoderm, which occupies a place between the ecto- and endoderm. The development of the middle germ layer, which is a chordomesoderm, is an evolutionary complication of the gastrulation phase in vertebrates and is associated with an acceleration of their development at the early stages of embryogenesis. In more primitive chordates, such as the lancelet, chordomesoderm usually forms at the beginning of the next phase after gastrulation - organogenesis. The shift in the time of development of some organs relative to others in descendants compared with ancestral groups is a manifestation of heterochrony. Change bookmark time the most important organs in the process of evolution is not rare.
The process of gastrulation is characterized by important cellular transformations, such as directed movements of groups and individual cells, selective reproduction and sorting of cells, the beginning of cytodifferentiation and induction interactions.
The ways of gastrulation are different. Four types of spatially directed cell movements are distinguished, leading to the transformation of the embryo from a single layer to a multilayer one.
Intussusception- invagination of one of the sections of the blastoderm inward as a whole layer. In the lancelet, the cells of the vegetative pole invaginate; in amphibians, intussusception occurs on the border between the animal and vegetative poles in the region of the gray crescent. The process of invagination is possible only in eggs with a small or medium amount of yolk.
epiboly- fouling with small cells of the animal pole of larger, lagging in the rate of division and less mobile cells of the vegetative pole. This process is clearly expressed in amphibians.
Delamination- stratification of blastoderm cells into two layers lying one above the other. Delamination can be observed in the discoblastula of embryos with a partial type of crushing, such as reptiles, birds, and oviparous mammals. Delamination manifests itself in the embryoblast of placental mammals, leading to the formation of hypoblast and epiblast.
Immigration- movement of groups or individual cells that are not united into a single layer. Immigration occurs in all embryos, but is most characteristic of the second phase of gastrulation in higher vertebrates.
In each specific case of embryogenesis, as a rule, several methods of gastrulation are combined.
Features of the stage of gastrulation. Gastrulation is characterized by a variety of cellular processes. Mitotic reproduction of cells continues, and it has a different intensity in different parts of the embryo. However, the most characteristic gastrulation is the movement of cell masses. This leads to a change in the structure of the embryo and its transformation from blastula to gastrula. Cells are sorted according to their belonging to different germ layers, inside which they “recognize” each other. The gastrulation phase is the beginning of cytodifferentiation, which means a transition to the active use of the biological information of one’s own genome. One of the regulators of genetic activity is various chemical composition cytoplasm of embryonic cells, established as a result of ovoplasmic segregation. So, the ectodermal cells of amphibians have a dark color due to the pigment that got into them from the animal pole of the egg, and the endoderm cells are light, as they come from the vegetative pole of the egg. During gastrulation, the role of embryonic induction is very large. It has been shown that the appearance of the primary streak in birds is the result of an inductive interaction between the hypoblast and the epiblast. The hypoblast has polarity. A change in the position of the hypoblast in relation to the epiblast causes a change in the orientation of the primary streak. Such manifestations of the integrity of the embryo as determination, embryonic regulation and integration are inherent in it during gastrulation to the same extent as during crushing
30. Primary organogenesis (neurulation) as a process of formation of a complex of chordate axial organs. Germ layer differentiation. Formation of organs and tissues.
Organogenesis, which consists in the formation of individual organs, constitutes the main content of the embryonic period. They continue in the larval and end in the juvenile period. Organogenesis is distinguished by 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 is the most important consequence of the interaction between the germ layers.
In the course of organogenesis, the shape, structure and chemical composition of cells change, cell groups are isolated, which are the rudiments of future organs. A certain form of organs gradually develops, spatial and functional connections between them are established. 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. A prerequisite for organogenesis, along with cell reproduction, migration, and sorting, is their selective death.
The beginning of organogenesis is called neurulation. Neurulation covers the processes from the appearance of the first signs of the formation of the neural plate to its closing into the neural tube. In parallel, the chord and secondary gut are formed, and the mesoderm lying on the sides of the chord is split in the craniocaudal direction into segmented paired structures - somites.
Nervous system vertebrates, including humans, is characterized by the stability of the basic structural plan throughout the evolutionary history of the subtype. In the formation of the neural tube, all chordates have much in common. Initially unspecialized dorsal ectoderm, responding to the induction effect from the chordomesoderm, turns into neural plate, represented by cylindrical neuroepithelial cells.
The neural plate does not remain flattened for long. Soon, its lateral edges rise, forming neural folds that 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 - neurocele. 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 in the morphogenesis of the neural tube big role play microtubules and microfilaments of neuroepithelial cells. Destruction of these cell structures by colchicine and cytochalasin B causes the neural plate to remain open. Non-closure of the neural folds leads to congenital defects neural tube development.
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 ventral direction, forms sensitive spinal ganglia, sympathetic ganglia, 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.
mesoderm, which occupies a place on the sides of the chord 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 represented by paired 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 an 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 chord and neural tube, the ventromedial parts of the somites - sclerotomes-turn into secondary mesenchyme, are evicted from the somite and surround the notochord and 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 gonotome 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. 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.
Endoderm in all embryos it eventually 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 of axial organs arises - the neural tube - the notochord - the gut, which are the most 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.
An in-depth examination and comparison of neurulation processes in specific representatives of the chordate type reveals some differences that are mainly associated with features that depend on the structure of the eggs, the method of crushing and gastrulation. Attention is drawn to the different shape of the embryos and the shift in the time of laying the axial organs relative to each other, i.e. e. heterochrony described above.
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. So, the epidermis of the skin and its derivatives (feather, hair, nails, skin and mammary glands), components of the organs of vision develop from the ectoderm; hearing, smell, oral cavity epithelium, tooth enamel. The most important ectodermal derivatives are the neural tube, the neural crest, and all the nerve cells formed from them.
Derivatives of the endoderm are the epithelium of the stomach and intestines, liver cells, secreting cells of the pancreas, intestinal and gastric glands. Anterior section The embryonic intestine 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.
The mesoderm, in addition to the skeletal structures already described above, the skeletal muscles, the dermis of the skin, the organs of the excretory and reproductive systems, forms 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 specific germ layer, but then the organ becomes more complex and, as a result, two or three germ layers take part in its formation.
31. Provisional organs of chordates. Anamnia and Amniota group. Formation, structure, features of functioning and evolution of provisional organs and germinal membranes. Amnion, chorion or serosa, allantois, yolk sac, placenta. Types of placenta, its significance.
In animals of different types during the period of embryonic development, provisional embryonic organs, providing vital functions: respiration, nutrition, excretion, movement, etc. The underdeveloped organs of the embryo itself are not yet able to function as intended, although they certainly 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 stocks nutrients were accumulated in the egg and under 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.
Reptiles and birds have more yolk reserves in the egg, but development does not take place in water, but on land. In this regard, very early there is a need to ensure respiration and excretion, as well as protection from drying out. In them already in early embryogenesis, almost in parallel with neurulation, the formation of provisional organs begins, such as amnion, chorion and yolk sac. A little later, allantois is formed. In placental mammals, these same provisional organs are formed even earlier, since there is very little yolk in the egg. The development of such animals occurs in utero, the formation of provisional organs in them coincides in time with the period of gastrulation.
The presence or absence of the amnion and other provisional organs underlies the division of vertebrates into two groups: Amniota and Anamnia. Evolutionarily older vertebrates that develop exclusively in the aquatic environment and are represented by such classes as cyclostomes, fishes, and amphibians do not need additional aquatic and other shells of the embryo and constitute the anamnia group. The group of amniotes includes primary terrestrial vertebrates, i.e. those whose embryonic development takes place in terrestrial conditions.
These are the three classes: Reptiles, Birds and Mammals. They are the highest vertebrates, as they have coordinated and highly efficient organ systems that ensure their existence in the most difficult conditions, which are land conditions. These classes include a large number of species that have secondarily passed into the aquatic environment. Thus, higher vertebrates were able to master all habitats. Such perfection would be impossible, including without internal insemination and special provisional embryonic organs.
There is much in common in the structure and functions of the provisional organs of various amniotes. Describing in the general view 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. There are some features in the development of the embryonic membranes of placental mammals, which will be discussed below.
· 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 contribute to the fact 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 has an endodermal origin, is covered with a visceral mesderm and is directly connected with 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 mammalian yolk sac is filled with a fluid with 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 remains 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. At hatching, the outer part of the allantois is discarded, while the inner part is preserved as 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.
Placentas differ in the shape and placement of the villi. On this basis, the following types of placenta. Diffuse - the entire surface of the fetal bladder is evenly covered with villi. Such a placenta is characteristic of a pig. In ruminants, a cotyledon placenta is observed, where the villi are collected in groups - cotyledons. The cingulate placenta is characteristic of predatory mammals. In this case, the villi surround the fetal bladder in the form of a wide belt. The next type of placenta is discoidal. It is observed in monkeys and humans, when the villi are located on amniotic sac in the form of a disk.
The placenta is of great importance for the developing baby.
It performs a number of important functions:
1) trophic - through the placenta, the fetus is nourished;
2) respiratory - carries out the supply of oxygen;
3) excretory - there is a release into the blood of the maternal organism of metabolic products;
4) protective - protects the embryo from the penetration of various bacteria;