Stages of gastrulation. Early stages of embryo development
Gastrula stage.
In many multicellular animals, the inner layer of cells is formed by invading the cells of its wall into the blastula cavity. This two-layer developmental stage is called gastrula... The outer layer of gastrula cells is called ectoderm, internal - endoderm... The cavity formed by invagination and limited by the endoderm is the cavity of the primary intestine, which opens outward with an opening - the primary mouth. 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, formation of the intestine and the development of the central nervous system.
The initial stages of oocyte cleavage Development of the newt embryo.
frogs (above) and birds (below).
Sequential stages of cleavage of 2, 4, and 8 blastomeres are visible.
The frog's egg cell splits into blastomeres of different sizes.
In the ovum 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 external integument of the body, the nervous system and the sense organs associated with it.
From the endoderm, the oral and anal openings, intestines, lungs, liver, and pancreas develop.
The mesoderm gives rise to the notochord, muscles, excretory system, cartilaginous and bone skeleton, blood vessels, sex glands.
Early stages of lancelet development
The embryo of animals develops as a single organism in which all cells, tissues and organs are in close interaction. All the organs of the fetus are fully formed by three months. The initial stages of development of animals 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 embryonic organs.
Amnion- an aqueous shell that surrounds the embryo, protecting it from drying out and mechanical damage. In humans, this is a fetal bladder.
Chorion- adjoins the shell or wall of the uterus, penetrated with 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 food in birds, a source of germ cells and blood cells in humans.
The influence of the environment on the development of the body.
There are, however, factors whose impact 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. The number of harmful external factors should primarily include alcoholic beverages and smoking.
The use of alcoholic beverages causes great harm at any stage of a person's individual development and is especially dangerous during adolescence. Alcohol has a detrimental effect on all systems of human organs, 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 human mental activity, the rhythm of movements, breathing and the activity of the heart, leads to numerous errors in work, to the occurrence of diseases. For example, alcohol destroys the liver, causing it to degenerate (cirrhosis). The systematic use of alcohol leads to the emergence of a serious illness - alcoholism, which requires long-term special treatment. Alcoholic parents can give birth to mentally retarded and physically disabled children.
Frontal poll:
Give a definition to the concept of ontogeny and characterize it.
Describe the stage of blastula.
Describe the stage of gastrula.
Describe the stage of neurula.
Describe the temporary embryonic organs.
How does the influence of the external environment affect the external and internal development 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 ontogenesis.
Regeneration and transplantation.
Postembryonic development.
Indirect postembryonic development.
Complete indirect development: egg → larva, which differs in structure from adult→ pupa → adult (housefly, butterfly, frog).
Incomplete indirect development: egg → a larva, which is similar in structure to an adult → an 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 develop indirectly contain a small supply of yolk.
Usually the larva represents a developmental stage 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 sedentary, sedentary worms and mollusks, the larvae swim freely and take up new habitats.
Direct postembryonic development.
At the time of birth, the body resembles the adult stage. Therefore, the postembryonic period is characterized by the growth and acquisition of a state of functional maturity of organs and systems.
Growth, aging and death are the stages of ontogenesis.
Aging- a natural, growing process over time, leading to a decrease in the adaptive capabilities of the organism and an increase in the likelihood of death.
Death- irreversible cessation of all manifestations of the body's vital activity.
Regeneration and transplantation.
1. Physiological regeneration- this is the renewal of cells and organs that are lost in the course of normal life, i.e. occurring as a normal physiological process (regular change of cell generations in the epithelium of the skin, intestines, regrowth of nails, hair, shedding and regrowth of antlers in deer). The circadian rhythm of cellular renewal is noted. The mitotic index (the number of dividing cells per thousand) allows you to compare the mitotic activity of tissues.
2. Reparative regeneration- regenerative processes in cells, organs and tissues in response to damaging influences (mechanical injury, surgical interventions, burns, frostbite, chemical influences, diseases). Living organisms of any kind are capable of reparative regeneration.
The classic example of reparative regeneration is hydra regeneration. The hydra can be decapitated by amputating the tentacled mouth cone and then re-forming. By cutting a hydra into pieces, you can increase the number of hydras, because each part transforms into a whole hydra. Significant regenerative capacity was found in representatives of the types of flat and annelids, at the starfish.
Regeneration in some species of invertebrates.
A - hydra; B - annelid worm; B is a starfish.
In vertebrates, newts and frog tadpoles, newly amputated legs and tails develop. This is an example of the regeneration of an external organ, as a result of which its form and function are restored, but the regenerated organ is distinguished by its reduced size.
Regeneration of the newt limb.
1-7 - successive stages of regeneration, respectively
10, 12, 14, 18, 28, 42, 56 days after amputation.
The regeneration of internal organs takes place in a slightly different way. When one or two liver lobes are removed from a rat, the remaining lobes increase in size and provide function in a 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 regeirrationalhypertrophy.
Regeneration in mammals. A - regenerative hypertrophy of rat liver: 1 - before surgery, 2 - after removal of two lobes, 3 - regenerated liver; B - rat muscle regeneration: 1 - removed muscle stump, 2 - restored muscle; B - healing of a skin incision in humans: 1 - fibrin clot, 2 - movement of cells of the growth layer, 3 - formation of an epithelial layer.
If you remove one of the paired organs, for example a kidney or an ovary, then the remaining one increases in size and performs a function in the volume of two normal organs. After removal of the lymph node or spleen, the remaining lymph nodes increase in size. Such an increase in the mass and function of the remaining organ in response to the removal of a similar one is called compensationtorny substitutionary hypertrophy and also belongs to the category of recovery processes. The term "hypertrophy" in biology and medicine means an increase in the size of organs and parts of the body.
Vnutricellular regeneration- an increase in the number of organelles (mitochondria, ribosomes) leading to the intensification of 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. On the wound surface, significant morphogenetic processes do not occur, they unfold inside the preserved part, as a result, the whole organism is formed anew, initially the size of the remaining part, which then grows - morphallaxis... With the regeneration of external organs, a new organ grows from the wound surface - epimorphosis.
Various forms of regeneration after damage have some common features. First, the wound is closed, some of the remaining cells die, then the process of dedifferentiation, i.e. loss of specific structural features by cells, and then reproduction, movement and differentiation of cells again. To start the regeneration process great importance has a violation of the previous spatial connections and contacts between cells. In the regulation of regenerative processes, along with intercellular interactions, hormones and influences from the nervous system play an important role. With age, the regenerative capacity decreases.
Of particular interest for medicine is the question of the regenerative abilities of mammals, to which humans also belong. 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, the periosteum is necessary. Thus, if the necessary conditions are created, then it is possible to achieve the regeneration of many internal organs mammals and humans. The inability in mammals, which are distinguished by an active lifestyle, to regenerate limbs and other external organs, is evolutionarily determined. The quick healing of the wound surface could have a greater adaptive value than the long existence of a gentle regenerate in places that are constantly traumatized 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. Often, it is desirable to transplant a healthy organ of one organism to the place of an affected organ of another organism, in addition to purely technical, surgical tasks, biological tasks arise, depending 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 in case of burns of the latter).
The essence of the stage of gastrulation is that a single-layer embryo - blastula - turns into a multilayer - two- or three-layer, called gastrula (from the Greek gaster - stomach in the diminutive sense).
In primitive chordates, for example, in the lancelet, a homogeneous single-layer blastoderm during gastrulation is transformed into the outer germ layer, the ectoderm, and the inner germ layer, the endoderm. The endoderm forms the primary intestine with a cavity inside the gastrocoel. The opening leading to the gastrocoel is called the blastopore or primary mouth. The two germ layers are defining morphological features gastrulation. Their existence at a certain stage of development in all multicellular animals, from coelenterates to higher vertebrates, makes it possible 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 embryonic layer is formed - the mesoderm, which occupies the place between the ecto- and endoderm. The development of the middle embryonic layer, which is the chordomesoderm, is an evolutionary complication of the gastrulation phase in vertebrates and is associated with the acceleration of their development in 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 development time of some organs relative to others in offspring compared with ancestral groups is a manifestation of heterochrony. Changes in the laying time of the most important organs in the process of evolution are not uncommon.
The gastrulation process is characterized by important cellular transformations, such as directed movement of groups and individual cells, selective reproduction and sorting of cells, the onset of cytodifferentiation and induction interactions.
The methods of gastrulation are different... There are four types of cell movements directed in space, leading to the transformation of the embryo from single-layer to multilayer.
Intussusception- invagination of one of the parts of the blastoderm inside a whole layer. In the lancelet, cells of the vegetative pole invaginate; in amphibians, invagination occurs at the border between the animal and vegetative poles in the area of the gray sickle. The process of intussusception is possible only in eggs with a small or medium amount of yolk.
Epibolia- overgrowth of 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 pronounced 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 cleavage, such as reptiles, birds, and oviparous mammals. Delamination manifests itself in the embryoblast of placental mammals, leading to the formation of a hypoblast and an epiblast.
Immigration- movement of groups or individual cells that are not combined into a single layer. Immigration occurs in all embryos, but it 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 cell multiplication continues, and it has a different intensity in different parts the embryo. However, the most feature gastrulation consists in the movement of cell masses. This leads to a change in the structure of the embryo and its transformation from blastula to gastrula. The cells are sorted according to their belonging to different germ layers, inside which they "recognize" each other. Cytodifferentiation begins at the gastrulation phase, which means a transition to the active use of the biological information of its own genome. One of the regulators of genetic activity is various chemical composition the cytoplasm of embryonic cells, established as a result of ovoplasmic segregation. Thus, 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 cells of the endoderm are light, since they originate from the vegetative pole of the egg. During gastrulation, the role of embryonic induction is very important. 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 is characterized by polarity. A change in the position of the hypoblast in relation to the epiblast causes a change in the orientation of the primary stripe. Such manifestations of the integrity of the embryo such as determination, embryonic regulation and integration are inherent in it during gastrulation to the same extent as during cleavage.
30. Primary organogenesis (neurulation) as a process of formation of a complex of axial organs of chordates. Differentiation of germ layers. The formation of organs and tissues.
Organogenesis, consisting in the formation of individual organs, constitute the main content embryonic period... They continue in the larval period 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 of the gastrula stage by the embryo, namely the formation of germ layers. Occupying a certain position in relation to each other, the germ layers, 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 reproduction, migration and sorting of cells, is their selective death.
The very beginning of organogenesis is called neurulation... Neurulation covers processes from the appearance of the first signs of neural plate formation to its closure into a neural tube. In parallel, the notochord and the secondary intestine are formed, and the mesoderm lying on the sides of the notochord is split in the craniocaudal direction into segmented paired structures - somites.
The nervous system of vertebrates, including humans, is distinguished 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 non-specialized 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 nerve ridges that lie on both sides of the shallow longitudinal neural groove. The edges of the nerve rollers are then closed, forming a closed neural tube with a channel inside - neurocelem. First of all, the closure of the nerve folds occurs at the level of the beginning of the spinal cord, and then spreads in the head and tail directions. It has been shown that microtubules and microfilaments of neuroepithelial cells play an important role in the morphogenesis of the neural tube. The destruction of these cellular structures by colchicine and cytochalasin B leaves the neural plate open. Non-closure of the nerve folds leads to congenital malformations of the neural tube.
After the closure of the nerve folds, the cells, originally located between the neural plate and the future cutaneous ectoderm, form neural crest... The cells of the neural crest are distinguished by the ability to migrate extensively, but strictly regulated throughout the body, and form two main streams. The cells of one of them- superficial-included in the epidermis or dermis of the skin, where they differentiate into pigment cells. Another stream migrates abdominal, 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 the gill cartilage, some of the covering bones of the skull.
Mesoderm occupying a place on the sides of the notochord and extending further between the cutaneous ectoderm and the endoderm of the secondary intestine, it is subdivided into the dorsal and ventral regions. The dorsal part is segmented and represented by paired somites. The somites are laid 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... Somites are connected to the lateral lamina by an intermediate mesoderm in the form of segmented somite stalks.
All areas of the mesoderm are gradually differentiating. At the beginning of formation, the somites have a configuration characteristic of an epithelium with a cavity inside. Under induction from the notochord and neural tube, the ventromedial parts of the somites - sclerotomes-transformed into a secondary mesenchyme, evicted from the somite and surrounds the notochord and ventral part of the neural tube. In the end, vertebrae, ribs and scapula are formed from them.
The dorsolateral part of the somites from the inner side 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 area of somite legs with primordia nephrotome and gonot excretory organs and sex glands are formed.
The right and left unsegmented lateral plates are split into two sheets that limit the secondary body cavity - the whole. Inner leaf adjacent to the endoderm is called visceral. It surrounds the intestine on all sides and forms the mesentery, covers the pulmonary parenchyma and heart muscle. The outer leaf 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 ultimately forms the epithelium of the secondary intestine and many of its derivatives. The secondary intestine itself is always located under the notochord.
Thus, in the process of neurulation, a complex of axial organs arises, the neural tube - chord - intestine, which is a characteristic feature of the organization of the body of all chordates. The same origin, development and mutual arrangement of axial organs reveal their complete homology and evolutionary succession.
With an in-depth examination and comparison of neurulation processes in specific representatives of the chordate type, some differences are revealed, which are mainly associated with features depending on the structure of the oocytes, the method of cleavage and gastrulation. Noteworthy is the different shape of the embryos and the shift in the time of the initiation of the axial organs relative to each other, i.e. e. the heterochrony described above.
Ectoderm, mesoderm and endoderm during further development interacting with each other, they participate in the formation of certain organs. The emergence of an organ rudiment 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 epithelium, tooth enamel. The most important ectodermal derivatives are the neural tube, the neural crest and all the nerve cells formed from them.
Endoderm derivatives are the epithelium of the stomach and intestines, liver cells, secreting cells of the pancreas, intestinal and gastric glands. The anterior part of the embryonic intestine forms the epithelium of the lungs and airways, as well as secreting cells of the anterior and middle lobes of the pituitary gland, thyroid and parathyroid glands.
The mesoderm, in addition to the skeletal structures already described above, skeletal muscles, dermis of the skin, organs of the excretory and reproductive systems, forms the cardiovascular system, lymphatic system, pleura, peritoneum and pericardium. From the mesenchyme, which is of mixed origin due to the cells of the three germ layers, all types of connective tissue, smooth muscles, blood and lymph develop.
The embryo of a specific organ is formed initially from a specific germ layer, but then the organ becomes more complicated and, as a result, two or three germ layers take part in its formation.
31. Provisional organs of chordates. Anamnia and Amniote group. Formation, structure, features of functioning and evolution of provisional organs and embryonic membranes. Amnion, chorion or serosa, allantois, yolk sac, placenta. Placenta types, its meaning.
In animals different types during the period of embryonic development provisional embryonic organs that provide vital functions: respiration, nutrition, excretion, movement, etc. The underdeveloped organs of the embryo itself are not yet able to function as intended, although they necessarily play some role in the system of a developing integral organism. As soon as the embryo reaches the required degree of maturity, when most of the organs are capable of performing vital functions, the temporary organs are absorbed or discarded.
The time of formation of provisional organs depends on what reserves of nutrients have been 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 ovum and the fact that development takes place in water, the embryo exchanges gas and excretes dissimilation products directly through the egg shells and reaches the tadpole stage. At this stage, provisional respiratory organs (gills), digestion and movement, adapted to the aquatic lifestyle, are formed. The listed larval organs enable the tadpole to continue development. Upon reaching the state of morphological and functional maturity of the organs of the adult type, the temporary organs disappear in the process of metamorphosis.
Reptiles and birds have more yolk reserves in the egg, but development takes place not in water, but on land. In this regard, the need arises very early to ensure respiration and excretion, as well as to protect against 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. Allantois is formed a little later. In placental mammals, the same provisional organs are formed even earlier, since there is very little yolk in the egg cell. 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 more ancient vertebrates, developing exclusively in the aquatic environment and represented by such classes as Cyclones, Pisces and Amphibians, do not need additional aquatic and other membranes of the embryo and constitute the anamnia group. The amniotic group includes primary terrestrial vertebrates, i.e. those in whom embryonic development takes place in terrestrial conditions.
There are 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 the conditions of the land. These classes include a large number of species that have passed into the aquatic environment for the second time. Thus, higher vertebrates were able to master all habitats. Such perfection would be impossible, including without internal insemination and special provisional embryonic organs.
The structure and functions of the provisional organs of various amniotes have much in common. Characterizing in the very general view provisional organs of the embryos of higher vertebrates, also called embryonic 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 that encloses the embryo and is filled with amniotic fluid. The amniotic membrane is specialized for the secretion and absorption of the amniotic fluid that surrounds the fetus. Amnion plays a primary role in protecting the embryo from drying out and from mechanical damage, creating the most favorable and natural aquatic environment for it. 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 vibrational movements imparted to the embryo, apparently, contribute to the fact that its growing parts do not interfere with each other.
· Chorion(serosa) - the outermost embryonic membrane adjacent to the shell or maternal tissues, arising, like the amnion, from the ectoderm and somatopleura. Chorion serves for exchange between the embryo and environment... In oviparous species, its main function is respiratory gas exchange; in mammals, it performs much more extensive functions, participating in nutrition, secretion, filtration, and synthesis of substances, such as hormones, in addition to respiration.
· Yolk sac has an endodermal origin, covered with visceral mezderm 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. It is first 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 is carried with the 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 a place for the formation of primary germ cells, the mesoderm gives the formed elements of the blood of the embryo. In addition, the mammalian yolk sac is filled with a liquid 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 is somewhat different in different animals. 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 by the end of 6 days after hatching it is completely absorbed. 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 shrivels and disappears without a trace before delivery.
· Allantois develops somewhat later than other extraembryonic organs. It is a saccular outgrowth of the ventral wall of the hindgut. Consequently, it is formed by the endoderm from the inside and the splanchnopleura outside. In reptiles and birds, allantois quickly grows to a chorion and performs several functions. First of all, it is a receptacle for urea and uric acid, which are the end products of the exchange of nitrogen-containing organic substances. The vascular network is well developed in allantois, due to which, together with the chorion, it participates in gas exchange. When hatching, the outer part of the allantois is discarded, while the inner part is retained in the form of the bladder.
In many mammals, allantois is also well developed and, together with the chorion, forms the chorioallantoic placenta. Term placenta means the close overlap or fusion of the germ 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 mesoderm of allantois 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 are distinguished types of placentas... Diffuse - the entire surface of the fetal bladder is evenly covered with villi. This placenta is typical for a pig. In ruminants, a cotyledon placenta is observed, where the villi are collected in groups - cotyledons. The cingulate placenta is characteristic of carnivorous 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 the fetal bladder in the form of a disc.
The placenta is of great importance to 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 - metabolic products are released into the blood of the maternal organism;
4) protective - protects the embryo from the penetration of various bacteria;
The most simply arranged gastrula is found in the intestinal cavity - it is an ellipsoid embryo, in which the ectoderm is represented by an outer unicellular layer, and the endoderm is an internal accumulation of cells. In the inner layer of the embryo (endoderm), a cavity is formed - the so-called. "Primary gut", or gastrocoel. Later, at the front end of the embryo, the so-called. The "primary mouth" or blastopore is the opening through which the primary intestine communicates with the external environment.
The gastrula of sea urchins is considered to be a typical gastrula. It is formed by "invagination" inside (invagination) of a part of the surface of the spherical blastula. As a result of invagination, part of the blastoderm (blastula skin) is pushed inward and forms the gastrocoel (primary intestine). The cells of the gastrocoel belong to the endoderm. Part of the blastoderm remains on the surface of the embryo and forms the ectoderm. Some of the cells are "evicted" into the space between the outer layer of the embryo and the primary intestine, these cells form the mesoderm. Also from the primary intestine inside the embryo are separated from the so-called. coelomic sacs, which are also part of the mesoderm. The opening through which intussusception occurs is the primary mouth (blastopore).
The human embryo goes through the gastrula stage on the 8-9th day of development. The human gastrula is a flattened discoidal formation (the so-called "embryonic disc"), which is formed from the "inner cell mass" of the blastocyst. The upper (that is, facing the animal pole) layer of the embryonic disc is referred to as ecdotherm, the middle layer - to the mesoderm, the lower (that is, facing the vegetative pole, to the future yolk sac) layer of the disc is referred to as endoderm. The homologue of the primary intestine in humans is the so-called. "Primary yolk sac" - a space limited from the animal pole by the ectoderm of the embryonic disc, and from other sides by the so-called. hypoblastoma - extraembryonic endoderm.
Gastrula can be formed by invagination (invagination or embolic) or by epiboly (for example, in some invertebrates). With epiboly, small ectodermal cells gradually overgrow large endodermal cells, while the cavity does not form immediately, but appears later.
In most animals, the embryo at the gastrula stage is not free-living, but is located in the egg membranes or in the uterus. But there are animals with free-swimming gastrula (for example, the free-swimming larva of the intestinal cavity - the planula (parenchymula) - is a gastrula).
Evolutionary origin of gastrula
The presence of the gastrula stage in all multicellular animals serves as one of the proofs of the unity of their origin. According to the Haeckel-Müller biogenetic law, this circumstance points to a common ancestor that existed in all multicellular animals, which in structure resembled the gastrula of modern animals. There are several hypotheses regarding the evolutionary origin of this hypothetical gastrula-like ancestor of multicellular organisms.
Ernst Haeckel in 1872 put forward the so-called. "gastric theory". According to this hypothesis, the ancestors of all multicellular organisms were spherical multicellular colonies of flagellates (similar to blastula, Haeckel called this ancestral organism "blastea"), which swam in the sea as part of plankton and fed on small organic particles suspended in water (for example, bacteria). In the course of evolution, the blastea underwent invagination (invagination) and formed an organism consisting of two layers of cells (outer and inner), the inner layer of cells formed a "gut" that opened outward with a "mouth" "). The biological meaning of the transformation of blastea into gastrea according to E. Haeckel consisted in the specialization of cells. All cells of the blastea were the same, with the help of the beating of the flagella, the cells supported the blastea in the water column, and also scooped up food particles for swallowing. Gastrea underwent specialization: the cells of the outer layer, with the help of the beating of the flagella, supported the gastria in the water column, the cells of the inner layer, using the beating of the flagella, created a flow of fluid that draws the particles into the primary intestine. The presence of a cavity in gastria gave it an evolutionary advantage - gastria, in contrast to blastea, had the ability to eat food, the size of which is larger than the cells of the gastria itself, since now the cells of the inner layer could secrete digestive enzymes into the gastric cavity. According to the theory of gastrulation, the most ancient type of gastrulation is intussusception; other types of gastrulation are secondary and appeared later in evolution. Thus, the most primitive form of gastrula, the planula, is a secondarily simplified embryonic form of animals.
Ilya Ilyich Mechnikov in 1876-1886 formulated the so-called "phagocytella theory". According to this hypothesis, the evolution of the blastea proceeded not through invagination, but through the expulsion of the cells of the outer layer into the spherical blastea. Such eviction ("immigration") was substantiated by Mechnikov as follows: after the capture of food particles (phagocytosis), the cells of the blastea detached from the outer layer and sank into the blastea for digestion. At the end of digestion, the cells were re-inserted into outer layer... This process took place continuously. Mechnikov called this hypothetical ancient organism "phagocytella" or "paranchimella". The phagocytella theory is supported by the fact that the most primitive multicellular animals form a gastrula through the immigration of the cells of the outer layer inward, as well as the fact that the simplest multicellular animals do not have cavity digestion, but only intracellular. According to the phagocytella theory, immigration is the most ancient type of gastrulation. The weak link in the phagocytella theory is that it does not explain the biological meaning of the eviction of phagocyte cells into the colony.
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Synonyms:See what "Gastrula" is in other dictionaries:
One of the embryonic forms of the embryo of animals. Dictionary foreign words included in the Russian language. Chudinov A.N., 1910. GASTRULA special form, the stage of development that the embryo of an animal passes through. Complete Dictionary foreign words included in ... ... Dictionary of foreign words of the Russian language
- (novolat. gastrula) stage of embryonic development of multicellular animals, following the blastula. Gastrula has two, then a three-layer wall and a cavity (gastrocoel), usually communicating with the external environment of the blastopore ... Big Encyclopedic Dictionary
GASTRULA, early stage of development of the EMBRYO of animals. It is preceded by the BLASTULA stage. Gastrula is a cavity with two layers (see GERMAN LAYER) of cells. Inner layer of ENDODERM, outer EKTODERM. The cavity of the embryo is called gastrocoel, and its ... ... Scientific and technical encyclopedic dictionary
Convexity, embryo Dictionary of Russian synonyms. gastrula n., number of synonyms: 2 bulge (41) ... Synonym dictionary
- (from the Greek gaster stomach), the embryo of multicellular animals during gastrulation. G. was first described by A.O. Kovalevsky in 1865 and called "intestinal larva", the term "G." introduced in 1874 by E. Haeckel. Usually stages of early, middle and late G ... Biological encyclopedic dictionary
GASTRULA- (from the Greek. gaster stomach), an embryological term introduced by Haeckel to denote the third stage of development following the stage of blastula (see); G.'s formation process is called gastrulation. The embryo in the G. stage has two ... ... Great medical encyclopedia
gastrula- The embryo of a multicellular animal during gastrulation, which has three main germ layers, ectoderm, endoderm (except for sponges and coelenterates) and mesoderm; G. was first described by A.O. Kovalevsky in 1865, and the term ... ... Technical translator's guide
- (novolat. gastrula), the stage of embryonic development of multicellular animals, following the blastula. Gastrula has two, then a three-layer wall and a cavity (gastrocoel), usually communicating with the external environment of the blastopore. * * * GASTRULA GASTRULA ... ... encyclopedic Dictionary
Gastrula gastrula. The embryo of a multicellular animal during gastrulation
GASTRULA- (gastrula) early stage of embryonic development in many animals. The gastrula is a two-layer wall and a central cavity of the archentheron that opens outward through the blastopore. True gastrulation takes place only ... ... Explanatory Dictionary of Medicine
The essence of the gastrulation stage lies in the fact that a single-layer embryo - blastula - turns into multilayer - two- or three-layer, called gastrula(from the Greek. gaster - stomach in the diminutive sense).
In primitive chordates, for example, in the lancelet, a homogeneous single-layer blastoderm during gastrulation is transformed into the outer germ layer - the ectoderm - and the internal germ layer - endoderm. The endoderm forms the primary intestine with a cavity inside gastrocoel. The hole leading to the gastrocoel is called blastopore or the primary mouth. Two germ layers are the defining morphological signs of gastrulation. Their existence at a certain stage of development in all multicellular animals, from coelenterates to higher vertebrates, makes it possible 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 embryonic layer is formed - mesoderm, occupying a place between the ecto- and endoderm. The development of the middle embryonic layer, which is the chordomesoderm, is an evolutionary complication of the gastrulation phase in vertebrates and is associated with the acceleration of their development in the early stages of embryogenesis. In more primitive chordates, such as the lancelet, chordomesoderm usually forms at the beginning of the phase following gastrulation - organogenesis. A shift in the development time of some organs relative to others in offspring compared to ancestral groups is a manifestation heterochrony. Changes in the laying time of the most important organs in the process of evolution are not uncommon.
The gastrulation process is characterized by important cellular transformations, such as directed movement of groups and individual cells, selective multiplication and sorting of cells, the beginning of cytodifferentiation and induction interactions. The listed cellular mechanisms of ontogenesis are discussed in detail in Ch. 8.2.
Rice. 7.3. Presumptive rudiments, gastrulation and neurulation in the lancelet.
A - presumptive rudiments at the stage of blastula (outside view) and early gastrula (cut view); B - late gastrula and neurulation on the sagittal (left row) and transverse (right row) sections; V - plastic model of the embryo at the end of the neurulation period:
1- animal pole, 2- vegetative pole, 3- blastocel, 4- gastrocoel, 5-dorsal and abdominal lips of the blastopore, 6 - the head end of the embryo, 7- modular plate, 8 - tail end of the embryo, 9-dorsal part of the mesoderm, 10- secondary intestinal cavity. 11 - segmented somites, 12- the abdominal part of the mesoderm; a, b, c, d, e - designation of presumptive and developing organs: a- cutaneous ectoderm, b - neural tube v - chord, G - endotherm, intestinal epithelium, d - mesoderm
Gastrulation methods are different. There are four types of cell movements directed in space, leading to the transformation of the embryo from single-layer to multilayer.
Intussusception - invagination of one of the sections of the blastoderm inside a whole layer. In the lancelet, cells of the vegetative pole invaginate; in amphibians, invagination occurs at the border between the animal and vegetative poles in the area of the gray sickle. The process of intussusception is possible only in eggs with a small or medium amount of yolk.
Epibolia - overgrowth of 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 pronounced in amphibians.
Denomination - 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 cleavage, such as reptiles, birds, and oviparous mammals. Delamination manifests itself in the embryoblast of placental mammals, leading to the formation of a hypoblast and an epiblast.
Immigration - movement of groups or individual cells that are not combined into a single layer. Immigration occurs in all embryos, but it 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.
Morphology of gastrulation. A more detailed examination of gastrulation in the lancelet, frog, chick, and mammals, to which we turn, will help to better understand evolutionary relationships and understand the patterns of individual development.
Gastrulation lancelet shown in fig. 7.3. Different markers at the blastula stage (Fig. 7.3, A) marked presumptive(putative) rudiments. These are areas of blastula, from the cellular material of which, during gastrulation and early organogenesis (neurulation), completely definite germ layers and organs are usually formed (Fig. 7.3, B and V).
Intussusception begins at the vegetative pole. Due to the faster division, the cells of the animal pole grow and push the cells of the vegetative pole into the blastula. This is facilitated by a change in the state of the cytoplasm in the cells that form the lips of the blastopore and adjacent to them. Due to intussusception, the blastocoel decreases and the gastrocoel increases. Simultaneously with the disappearance of the blastocoel, the ectoderm and endoderm come into close contact. In the lancelet, as in all deuterostomes (they include the Echinoderm type, the Chordate type, and some other small animal types), the blastopore area turns into the tail part of the body, in contrast to the protostomes, in which the blastopore corresponds to the head part. The mouth opening in deuterostomes is formed at the end of the embryo opposite the blastopore.
Rice. 7.4. Flask-shaped cells in the blastopore region of early amphibian gastrula: 1 - flask-shaped glues, 2 - dorsal lip of blasgopor
Gastrulation in amphibians has much in common with gastrulation of the lancelet, but since they have a much larger yolk in their oocytes and it is located mainly at the vegetative pole, large amphiblastula blastomeres are not able to invade inward. Intussusception goes a little differently. On the border between the animal and vegetative poles in the area of the gray sickle, the cells first stretch inward strongly, taking the form of "flask-shaped" (Fig. 7.4), and then pull the cells of the surface layer of the blastula with them. A sickle-shaped groove and a dorsal lip of the blastopore appear.
Simultaneously more small cells the animal poles, dividing faster, begin to move towards the vegetative pole. In the area of the dorsal lip, they twist and protrude, and from the sides and from the side opposite to the crescent groove, larger cells grow. Then the process epiboli leads to the formation of the lateral and abdominal lips of the blastopore. The blastopore closes into a ring, inside which for some time large light cells of the vegetative pole are visible in the form of the so-called yolk plug. Later, they completely submerge inward, and the blastopore narrows.
Using the method of labeling with vital (vital) dyes in amphibians, the movements of blastula cells during gastrulation were studied in detail. presumptive(from Lat. praesumptio - an assumption), with normal development, they are first found in the composition of certain primordia of organs, and then in the composition of the organs themselves (Fig. 7.5). It is known that in tailless amphibians the material of the presumptive chord and mesoderm at the blastula stage does not lie on its surface, but in the inner layers of the amphiblastula wall, however, at approximately the same levels as shown in the figure. Analysis of the early stages of development of amphibians allows us to conclude that ovoplasmic segregation, which is clearly manifested in the ovum and zygote (Fig. 7.6), is of great importance in determining the fate of cells that have inherited one or another part of the cytoplasm. A certain similarity between the processes of gastrulation and the area of presumptive organs in amphibians and lancelet, i.e. the homology of the main organs, such as the neural tube, notochord, and secondary intestine, indicates their phylogenetic relationship.
Rice. 7.5. Map of areas of presumptive organ rudiments in the early stages of amphibian embryonic development. A - blastula stage (flaccid on the left); B-D - successive stages of gastrulation (sagittal slices); E - onset of neurulation (cross section):
1 - cutaneous ectoderm, 2- neural tube 3- notochord, 4-mesoderm of somites, 5- mesoderm of splanchnotomes, 6 - endoderm, 7 - blastocoel, 8 -suliform groove, 9-gastrocoel, 10- dorsal lip of the blastopore, 11 -yellow plug, 12- secondary intestinal cavity, 13- nerve rollers
Gastrulation in embryos with a meroblastic type of cleavage and development has its own characteristics. Have birds it begins following the cleavage and formation of blastula during the passage of the embryo through the oviduct. By the time the egg is laid, the embryo already consists of several layers: the top layer is called epiblastoma, bottom - primary hypoblastoma(fig. 7.2, V). There is a narrow gap between them - the blastocoel. Then formed secondary hypoblast, the way of formation of which is not entirely clear. There is evidence that primary germ cells originate in the primary hypoblast of birds, and the secondary one forms an extraembryonic endoderm. The formation of primary and secondary hypoblasts is considered as a phenomenon preceding gastrulation.
The main gastrulation events and the final formation of the three germ layers begin after oviposition with the start of incubation. An accumulation of cells occurs in the posterior part of the epiblast as a result of the uneven rate of cell division and their movement from the lateral sections of the epiblast to the center, towards each other. The so-called primary strip, which extends towards the head end. In the center of the primary strip is formed primary groove, and along the edges - primary rollers. A thickening occurs at the head end of the primary strip - Hensen's knot, and in it is the primary fossa (Fig. 7.7).
When epiblast cells enter the primary groove, their shape changes. They resemble in shape the "flask" cells of the gastrula of amphibians. Then these cells take on a stellate shape and submerge under the epiblast, forming the mesoderm (Fig. 7.8). Endoderm is formed on the basis of primary and secondary hypoblasts with the addition of a new generation of endodermal cells migrating from the upper layers of the blastoderm. The presence of several generations of endodermal cells indicates an extension of the gastrulation period over time.
Rice. 7.6. Ovoplasmic segregation in the grass frog egg.
A - immediately after fertilization; B- 2 hours after fertilization (left view): 1 - pigmented animal area, 2- unpigmented negative area, 3 - the head-tail axis of the future organism, 4- gray sickle, 5 - dorsal side, 6 - abdominal side
Rice. 7.7. Chicken embryo in the primordial streak stage
(dorsal view):
1 - dark area, 2 - translucent region of the embryonic disc
Part of the cells migrating from the epiblast through the Hensen's nodule forms the future notochord. Simultaneously with the initiation and lengthening of the notochord, the Hensen knot and the primary stripe gradually disappear in the direction from the cephalic to the caudal end. This corresponds to the narrowing and closing of the blastopore. As it contracts, the primary stripe leaves behind the formed areas of the axial organs of the embryo in the direction from the head to the tail. It seems reasonable to consider cell movements in the chick embryo as homologous epiboli, and the primary stripe and Hensen's nodule as homologous to the blastopore in the dorsal lip of the gastrula of amphibians.
It is interesting to note that the cells of mammalian embryos (Ch. 7.6.1), despite the fact that in these animals the eggs have a small amount of yolk, and the cleavage is complete, in the gastrulation phase they retain the movements characteristic of the embryos of reptiles and birds. This confirms the idea of the origin of mammals from an ancestral group in which eggs were rich in yolk.
Rice. 7.8. Chicken embryo at the primordial streak stage (cross section).
A, B - at low and high magnification: 1 - ectoderm, 2 - endoderm, 3 - mesoderm, 4 - primary roller, 5 - primary groove
Features of the stage of gastrulation. Gastrulation is characterized by a variety of cellular processes. The mitotic continues cell multiplication, moreover, it has different intensity in different parts of the embryo. At the same time, the most characteristic feature of gastrulation is movement of cell masses. This leads to a change in the structure of the embryo and its transformation from blastula to gastrula. Is happening sorting cells by their belonging to different germ layers, inside which they "recognize" each other.
The gastrulation phase begins cytodifferentiation, which means the transition to the active use of biological information of its own genome. One of the regulators of genetic activity is the different chemical composition of the cytoplasm of the 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 cells of the endoderm are light, since they originate from the vegetative pole of the egg.
During gastrulation, the role of embryonic induction. 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 is characterized by polarity. A change in the position of the hypoblast in relation to the epiblast causes a change in the orientation of the primary strip.
All the above processes are described in detail in chapter 8.2. It should be noted that such manifestations integrity embryo like determination, embryonic regulation and integration are inherent in him during gastrulation to the same extent as during cleavage (see Section 8.3).
Blastula
Blastula- a single-layer embryo. It consists of a layer of cells - the blastoderm, which limits the cavity - the blastocoel. Blastula begins to form in the early stages of cleavage due to the divergence of blastomeres. The resulting cavity is filled with liquid. The structure of the blastula largely depends on the type of cleavage.
Celloblastula(typical blastula) is formed by uniform crushing. It looks like a single-layer vesicle with a large blastocele (lancelet).
Amphiblastula formed by crushing telolecital eggs; blastoderm is built of blastomeres of different sizes: micromeres on the animal and macromeres on the vegetative poles. In this case, the blastocoel is shifted towards the animal pole (amphibians).
Blastul types: 1 - celloblastula; 2 - amphiblastula; 3 - discoblastula; 4 - blastocyst; 5 - embryoblast; 6 - trophoblast.
Discoblastula formed during discoidal cleavage. The blastula cavity looks like a narrow slit located under the embryonic disc (bird).
Blastocyst It is a single-layer vesicle filled with liquid, in which an embryoblast is distinguished (from which an embryo develops) and a trophoblast, which provides nutrition to the embryo (mammals).
Gastrula:
1 - ectoderm; 2 - endoderm; 3 - blastopore; 4 - gastrocoel.
After the blastula has formed, the next stage of embryogenesis begins - gastrulation(formation of germ layers). As a result of gastrulation, a two-layer and then a three-layer embryo (in most animals) is formed - the gastrula. Initially, the outer (ectoderm) and inner (endoderm) layers are formed. Later, between the ecto- and endoderm, the third germ layer, the mesoderm, is laid.
Germ leaves- separate layers of cells that occupy a certain position in the embryo and give rise to the corresponding organs and organ systems. The germ layers arise not only as a result of the movement of cell masses, but also as a result of the differentiation of similar, relatively homogeneous blastula cells. In the process of gastrulation, the germ layers occupy a position corresponding to the plan of the structure of an adult organism. Differentiation- the process of the appearance and growth of morphological and functional differences between separate cells and parts of the embryo. Depending on the type of blastula and on the characteristics of cell movement, the following main methods of gastrulation are distinguished: intussusception, immigration, delamination, epiboly.
Types of gastruli: 1 - invagination; 2 - epibolic; 3 - immigration; 4 - delamination;
a - ectoderm; b - endoderm; c - gastrocoel.
At intussusception one of the sections of the blastoderm begins to protrude into the blastocoel (near the lancelet). In this case, the blastocoel is almost completely displaced. A two-layer sac is formed, the outer wall of which is the primary ectoderm, and the inner wall is the primary endoderm lining the cavity of the primary intestine, or gastrocoel... The hole through which the cavity communicates with the environment is called blastopore, or primary mouth... In representatives of different groups of animals, the fate of the blastopore is different. In primitive animals, it turns into a mouth opening. In deuterostomes, blastopores overgrow, and in its place an anus often arises, and the oral opening breaks out at the opposite pole (anterior end of the body).
Immigration- "eviction" of a part of the cells of the blastoderm into the cavity of the blastocoel (higher vertebrates). Endoderm is formed from these cells.
Delamination occurs in animals with blastula without blastocel (birds). With this method of gastrulation, cellular movements are minimal or completely absent, since stratification occurs - the outer cells of the blastula are transformed into ectoderm, and the inner ones form the endoderm.
Epibolia occurs when the smaller blastomeres of the animal pole split faster and overgrow the larger blastomeres of the vegetative pole, forming an ectoderm (amphibians). The cells of the vegetative pole give rise to the internal germ layer, the endoderm.
The described methods of gastrulation are rarely found in their pure form and their combinations are usually observed (intussusception with epiboly in amphibians or delamination with immigration in echinoderms).
Most often, the cellular material of the mesoderm is part of the endoderm. It invades the blastocoel in the form of pocket-like outgrowths, which are then detached. With the formation of the mesoderm, a secondary body cavity, or coelom, is formed.
The process of organ formation in embryonic development is called organogenesis... Organogenesis can be divided into two phases: neurulation- the formation of a complex of axial organs (neural tube, notochord, intestinal tube and mesoderm of somites), in which almost the entire embryo is involved, and construction of other organs, the acquisition by various parts of the body of their typical shape and features of the internal organization, the establishment of certain proportions (spatially limited processes).
By Karl Baer's germ-leaf theory, the emergence of organs is due to the transformation of one or another germ layer - ecto-, meso- or endoderm. Some organs can be of mixed origin, that is, they are formed with the participation of several germ layers at once. For example, the musculature of the digestive tract is a derivative of the mesoderm, and its inner lining is a derivative of the endoderm. However, simplifying somewhat, the origin of the main organs and their systems can still be associated with certain germ layers. The embryo at the stage of neurulation is called neurula... The material used to build the nervous system in vertebrates - neuroectoderm, is part of the dorsal part of the ectoderm. It is located above the anlage of the notochord.
Neirula:
1 - ectoderm; 2 - chord; 3 - secondary body cavity; 4 - mesoderm; 5 - endoderm; 6 - intestinal cavity; 7 - neural tube.
First, in the area of the neuroectoderm, a flattening of the cell layer occurs, which leads to the formation of a neural plate. Then the edges of the neural plate thicken and rise, forming nerve ridges. In the center of the plate, due to the movement of cells along the midline, a neural groove arises, dividing the embryo into the future right and left halves. The neural plate begins to fold along the midline. Its edges touch and then close. As a result of these processes, a neural tube with a cavity appears - neurocelem.
The closure of the ridges occurs first in the middle and then in the back of the nerve groove. This happens last in the head, which is wider than the others. The anterior expanded section further forms the brain, the rest of the neural tube - the dorsal. As a result, the neural plate turns into a neural tube that lies under the ectoderm.
During neurulation, some of the cells in the neural plate are not part of the neural tube. They form the ganglion plate, or neural crest, a collection of cells along the neural tube. Later, these cells migrate throughout the embryo, forming cells of nerve nodes, adrenal medulla, pigment cells, etc.
From the material of the ectoderm, in addition to the neural tube, the epidermis and its derivatives (feather, hair, nails, claws, skin glands, etc.), components of the organs of vision, hearing, smell, epithelium of the oral cavity, and enamel of teeth develop.
The mesodermal and endodermal organs are not formed after the formation of the neural tube, but simultaneously with it. Pockets or folds are formed along the lateral walls of the primary intestine by protruding the endoderm. The area of endoderm located between these folds thickens, bends, folds and detaches from the bulk of the endoderm. This is how it appears chord... The resulting pocket-like protrusions of the endoderm are detached from the primary intestine and turn into a series of segmental-located closed sacs, also called coelomic sacs. Their walls are formed by the mesoderm, and the cavity inside is a secondary body cavity (or the whole).
All types of connective tissue, the dermis, the skeleton, striated and smooth muscles, the circulatory and lymphatic systems, and the reproductive system develop from the mesoderm.
From the endoderm, the epithelium of the intestine and stomach, liver cells, secreting cells of the pancreas, intestinal and gastric glands develop. The anterior section of the embryonic intestine forms the epithelium of the lungs and airways, secreting sections of the anterior and middle lobe of the pituitary gland, thyroid and parathyroid glands.
Embryonic induction:
1 - primordium of chordomesoderm; 2 - blastula cavity; 3 - induced neural tube; 4 - induced chord; 5 - primary neural tube; 6 - primary chord; 7 - the formation of a secondary embryo connected to the host embryo.
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