The inner layer of the gastrula. Gastrulation (formation of gastrula) and its methods. The process and methods of gastrulation
Or gastrula(gaster - stomach). The process that leads to the formation of a gastrula is called gastrulation. A characteristic feature of gastrulation and embryonic development is the intensive movement of cells, as a result of which future tissue rudiments move to the places intended for them in accordance with the plan. structural organization organism. In there are cell layers, which are called. Initially, two germ layers are formed. The outer one is called ectoderm (ectos - outside, derma - skin), and the inner one is called endoderm (entos - inside). In vertebrates, in the process of gastrulation, a third, middle germ layer is also formed - the mesoderm (mesos - middle). The mesoderm is always formed later than the ecto- and endoderm, therefore it is called the secondary germ layer, and the ecto- and endoderm are called the primary germ layers. These germ layers, due to further development, give rise to embryonic rudiments, from which various tissues and organs will form.
Types of gastrulation
During gastrulation, the changes that began at the blastula stage continue, and therefore different types of blastula correspond to different types of gastrulation. The transition from the blastula to the blastula can be carried out in 4 main ways: invagination, immigration, delamination and epiboly.Intussusception or invagination is observed in the case of coeloblastula. This is the simplest way of gastrulation, in which the vegetative part invaginates into the blastocoel. Initially, a small depression appears in the vegetative pole of the blastula. Then the cells of the vegetative pole invaginate more and more into the cavity of the blastocoel. Subsequently, these cells reach the inner side of the animal pole. The primary cavity, the blastocoel, is displaced and is visible only on both sides of the gastrula at the sites of cell bending. The embryo takes a domed shape and becomes two-layered. Its wall consists of an outer leaf - the ectoderm and an inner one - the endoderm. As a result of gastrulation, a new cavity is formed - the gastrocoel or the cavity of the primary intestine. It communicates with the external environment through an annular opening - the blastopore or primary mouth. The edges of the blastopore are called lips. There are dorsal, abdominal and two lateral lips of the blastopore.
According to the subsequent fate of the blastopore, all animals are divided into two large groups: primary and deuterostomes. Protostomes include animals in which the blastopore remains a permanent or definitive mouth in an adult (worms, molluscs, arthropods). In other animals (echinoderms, chordates), the blastopore either turns into an anus or grows over, and the oral opening reappears at the anterior end of the body of the embryo. Such animals are called deuterostomes.
Immigration or penetration is the most primitive form of gastrulation. With this method, individual cells or a group of cells move from the blastoderm to the blastocoel with the formation of the endoderm. If cells enter the blastocoel only from one blastula pole, then such immigration is called unipolar, and from different parts of the blastula it is called multipolar. Unipolar immigration is characteristic of some hydroid polyps, jellyfish and hydromedusae. While multipolar immigration is more a rare occurrence and is observed in some hydromedusas. During immigration, the inner germ layer, the endoderm, can be formed immediately in the process of cell penetration into the blastocoel cavity. In other cases, cells may fill the cavity with a solid mass and then line up in an orderly manner near the ectoderm and form the endoderm. In the latter case, the gastrocoel appears later.
Delamination or delamination is reduced to splitting of the blastula wall. The cells that separate inward form the endoderm, and the outer cells form the ectoderm. This method of gastrulation is observed in many invertebrates and higher vertebrates.
In some animals, due to an increase in the amount of yolk in the egg and a decrease in the cavity of the blastocoel, gastrulation only by intussusception becomes impossible. Then gastrulation occurs in the way of epiboly or fouling. This method consists in the fact that small animal cells intensively divide and grow around larger vegetative ones. Small cells form the ectoderm, and cells of the vegetative pole form the endoderm. This method is observed in cyclostomes and.
The process and methods of gastrulation
However, all described ways of gastrulation rarely found separately, usually they are combined. For example, along with fouling, invagination (amphibians) can occur. Delamination can be observed along with invagination and immigration (reptiles, birds, etc.).Therefore, in the process of gastrulation part of the cells from the outer layer of the blastula moves inward. This is due to the fact that in the process of historical development, some cells adapted to development in direct connection with the external environment, while others - inside the body.
There is no single view on the causes of gastrulation. According to some views, gastrulation occurs due to the uneven growth of cells in different parts of the embryo. Rumbler (1902) explained the process of gastrulation by changing the shape of cells inside and outside the blastula. He believed that the cells were wedge-shaped, the blastula was wider inside and narrower outside. There are views that gastrulation can be caused by a sharp intensity of water uptake by individual cells. But observations show that these differences are very small.
Goltfreter (1943) believed that the animal pole of the blastula is covered with a very thin film (coat) and therefore the cells are connected into a single mass. The cells of the vegetative pole are not interconnected, have a bottle-shaped shape, elongate and retract inward. In the movement of cells, the degree of adhesion and the nature of the intercellular spaces can play a role. There is also an opinion that cells can move due to their ability to amoeboid movement and phagocytosis. The formation of the third germ layer in the process of embryonic development of animals is carried out in four ways: teloblastic, enterocoelous, ectodermal and mixed.
In many invertebrates (protostomes), the mesoderm is formed from two cells - teloblasts. These cells separate early, even at the stage. In the process of gastrulation, teloblasts are located on the border between the ectoderm and endoderm, begin to actively divide, and the cells formed in this process grow in strands between the outer and inner sheets, forming the mesoderm. This method of mesoderm formation is called teloblastic.
In the enterocoelous method, the mesoderm is formed as pocket-like outgrowths on the sides of the endoderm after gastrulation. These protrusions are located between the ecto- and endoderm, forming the third germ layer. This method of mesoderm formation is characteristic of echinoderms,.
Phases of gastrulation in humans and birds
In reptiles, birds, mammals and human mesoderm is formed from ectoderm during the second phases of gastrulation. During the first phase, the ectoderm and endoderm are formed by delamination. During the second phase, ectoderm cells migrate into the space between the ectoderm and endoderm. They form the third germ layer - the mesoderm. This method of mesoderm formation is called ectodermal.In amphibians, a mixed or transitional mode of mesoderm formation is observed. In them, the mesoderm is formed during gastrulation simultaneously with the ectoderm and endoderm, and both germ layers take part in its formation.
SPLITTING UP
As a result of fertilization, a zygote is formed, which begins to split. Cleavage is accompanied by mitotic division. There is no cell growth and the volume of the embryo does not change. This is because there is no post-mitotic period between divisions in the short interphase, and DNA synthesis begins in the telophase of the preceding mitotic division. The cells formed during the crushing process are called blastomeres, and the embryo is called a blastula.
The types of crushing depend on the amount and distribution of the yolk in the eggs. Crushing can be:
complete uniform;
complete uneven;
incomplete discoidal;
incomplete superficial.
Complete uniform crushing is characteristic of isolecithal eggs
The cleavage furrow runs along the meridian, forming two blastomeres. Then the nucleus divides again, and a second cleavage furrow appears on the surface of the embryo, running along the meridian perpendicular to the first. Four blastomeres are formed. The third furrow runs along the equator and divides it into eight parts. Then there is an alternation of meridional and equatorial fragmentation. The number of blastomeres increases. An embryo at the stage of 32 blastomeres is called a morula. Cleavage continues until the formation of a vesicle-like embryo, the walls of which are formed by a single layer of cells called the blastoderm. Blastomeres diverge from the center of the embryo, forming a cavity called the primary or blastocoel. Blastomeres are the same size. As a result of this crushing, a coeloblastula is formed.
GASTRULATION At the end of the cleavage period, multicellular animals begin the period of formation of germ layers - gastrulation. Gastrulation is associated with the movement of embryonic material. First, an early gastrula is formed, which has two germ layers (ectoderm and endoderm), then a late gastrula, when a third germ layer, the mesoderm, is formed. The resulting embryo is called a gastrula.
The formation of an early gastrula occurs as follows:
immigration (cell eviction), as in coelenterates;
invagination (invagination), like in the lancelet;
epiboly (fouling), like a frog;
delamination (splitting), as in some coelenterates.
During immigration (eviction), part of the blastoderm cells from the surface of the embryo goes into the blastocoel. The outer layer is the ectoderm and the inner layer is the endoderm. The blastocoel is filled with cells. This method of formation of the gastrula is characteristic of the coelenterates. The lancelet is characterized by the formation of gastrula by intussusception (invagination). During invagination, a certain section of the blastoderm (the vegetative pole) bends inward and reaches the animal pole. A two-layer embryo is formed - gastrula. The outer layer of cells is called the ectoderm, the inner layer is called the endoderm. The endoderm lines the cavity of the primary intestine (gastrocoel). The opening through which the cavity communicates with the external environment is called the primary mouth - the blastopore. In protostomes (worms, mollusks, arthropods), it turns into a mouth opening. In deuterostomes - into the anus, and the mouth is formed at the opposite end of the body (chordates).
Epiboly (fouling) is characteristic of animals developing from telolecithal eggs. The formation of the gastrula is due to the rapid division of micromeres, which overgrow the vegetative pole. Macromeres are inside the embryo. Blastopore formation does not occur and there is no gastrocoel.
Epiboly is characteristic of amphibians.
Delamination (stratification) occurs in coelenterates, the blastula of which is similar to the morula. The cells of the blastoderm are divided into outer and inner layers. The outer layer forms the ectoderm, the inner layer forms the endoderm.
In all multicellular organisms, except sponges and coelenterates, a third germ layer, the mesoderm, is formed. The formation of the mesoderm occurs in two ways: Teloblastic; Enterocoel.
The teloblastic method is characteristic of protostomes. On the border between the ectoderm and endoderm on the sides of the blastopore, cells - teloblasts - begin to divide and give rise to the mesoderm.
The enterocele method is characteristic of deuterostomes. The cells that form the mesoderm are isolated in the form of pockets of the primary intestine. The cavities of the pockets are transformed into a whole. The mesoderm is divided into separate sections - somites, from which certain tissues and organs are formed.
At the end of the cleavage period, multicellular animals begin the period of formation of germ layers - gastrulation. gastrulation is the process of formation of a two-layer embryo. The essence lies in the formation of a single-layer embryo - a two-layer one, consisting from ectoderm and endoderm The embryo formed as a result of gastrulation is called gastrula.
Blastopore- an opening through which the cavity of the primary intestine of the animal embryo at the gastrula stage communicates with environment.
The formation of an early gastrula occurs in the following ways:
Immigration (cell eviction), in coelenterates;
Invagination (invagination), in the lancelet;
Epiboly (fouling), in a frog;
Delamination (splitting), in coelenterates.
When immigration part of the blastoderm cells from the surface of the embryo goes into the blastocoel. The outer layer is the ectoderm and the inner layer is the endoderm. The blastocoel is filled with cells.
With intussusception a certain section of the blastoderm (the vegetative pole) bends inward and reaches the animal pole. A two-layered embryo-gastrula is formed. The outer layer of cells is the ectoderm, the inner layer is the endoderm that lines the cavity of the primary intestine (gastrocoel). The opening through which the cavity communicates with the external environment is called the primary mouth - the blastopore. In protostomes (worms, mollusks, arthropods), it turns into a mouth opening, in deuterostomes, into an anus, and a mouth forms at the opposite end (chordates).
epiboly characteristic of animals that develop from telolecithal eggs. The formation of the gastrula is due to the rapid multiplication of micromeres that overgrow the vegetative pole. Macromeres are inside the embryo. Blastopore formation does not occur and there is no gastrocoel.
Delamination occurs in coelenterates whose blastula is similar to the morula. The cells of the blastoderm are divided into outer and inner layers. The outer layer forms the ectoderm, the inner layer forms the endoderm
All multicellular organisms, except sponges and coelenterates, have a 3rd germ layer - mesoderm - neurulation. The formation of the mesoderm occurs in two ways: teloblastic or enterocelous.
Teloblastic the method is characteristic of primary mouths. At the border between the ectoderm and endoderm on the sides of the blastoporcell, teloblasts begin to divide and give rise to the mesoderm.
Enterocoelic the method is typical for deuterostomes. The cells that form the mesoderm are isolated in the form of pockets of the primary intestine. The cavities of the pockets turn into a whole. The mesoderm is divided into separate sections - somites, from which certain tissues and organs are formed.
Organogenesis. Neirula. Bookmark somites.
After the formation of the mesoderm, the process of histo- and organogenesis begins. First, axial organs are formed - neural tube, chord, then all other organs. A neurula is formed.
Organogenesis- laying and formation of organs. Primary organogenesis- laying of axial organs (chord, neural tube, intestinal tube). secondary organogenesis- the formation of all other organs.
Formation of axial organs in chordate embryos:
1. The ectoderm on the dorsal side of the embryo bends, forming a longitudinal groove, the edges of which close. Formed neural tube plunges into the ectoderm.
2. The dorsal part of the endoderm, located under the nerve bud, gradually separates and forms chord.
3. From the ectoderm and endoderm is formed intestinal tube.
ectoderm- epidermis, skin glands, hair, enamel, conjunctiva, lens, retina, ears, epithelial lining of the nasal cavity and oral cavity, anus and vagina, anterior and posterior pituitary, CNS, adrenal medulla, jaws.
Mesoderm- skeletal muscles, diaphragm, vertebrae, dentin, renal tubules, ureters, oviducts, uterus, part of the ovaries and testicle, adrenal cortex, heart, blood, lymphatic system, lungs, sclera, choroid and cornea of the eye.
Endoderm- chord, most digestive tract, intestinal lining, Bladder, lungs, pancreas, thymus, thyroid, parathyroid gland.
Under the chord is the intestinal tube, on the sides of the chord - somite mesoderm.
1. The outer part of the somite, adjacent to the ectoderm, is called dermotome. From it is formed connective tissue skin.
2. Inner part - sclerotome- gives rise to the skeleton.
3. Between the dermotome and the sclerotome is myotome giving rise to striated muscles.
4. There are legs under the somites ( nephrogonotome), from which the genitourinary system is formed.
Coelomic bags are formed symmetrically on the sides. The walls of the coelomic sacs facing the intestines are called splanchnopleura, towards the ectoderm somatopleura. These sheets are involved in education of cardio-vascular system, pleura, peritoneum, pericardium.
supervisory bodies.
Provisional bodies- these are temporary organs necessary for the life of the embryo. The time of their formation depends on the egg and environmental conditions.
The presence or absence of provisional organs underlies the division of vertebrates into groups: Amniota and Anamnia.
To the group anamniev include evolutionarily more ancient animals that develop in an aquatic environment and do not need additional water and other shells of the embryo. (Cyclostomes, fish, amphibians)
To the group amniote include primary terrestrial vertebrates, the embryonic development of which takes place in terrestrial conditions. (Reptiles, birds, mammals)
There is much in common in the structure and functions of the provisional organs of amniotes. The provisional organs of higher vertebrates are called germinal membranes. They develop from the cellular material of already formed germ layers.
Provisional bodies:
Amnion- bag filled amniotic fluid, which creates an aquatic environment and protects the embryos from drying out and damage.
Chorion- the outer germinal membrane adjacent to the shell or maternal tissues. Serves for exchange with the environment, participates in respiration, nutrition and excretion.
Yolk sac- it is involved in the nutrition of the embryo and is a hematopoietic organ.
Alantois- outgrowth of the hindgut is involved in gas exchange, is a receptacle for urea and uric acid. In mammals it together with the chorion forms the placenta.From the allantois to the chorion, vessels grow through which the placenta performs excretory, respiratory and nutritional functions.
Types of placenta:
1.Epitheliochorional- (semi-placenta) has the simplest structure. When it is formed, villi in the form of small tubercles appear on the surface of the chorion. They sink into the corresponding recesses of the uterine mucosa without disturbing it. (the chorion is in contact with the epithelium of the uterine glands) Horse pigs
2. Desmochorionic- characterized by the establishment of the closest connection between the chorion of the embryo and the wall of the uterus. At the point of contact with the villi of the chorion, the epithelium is destroyed. The branched plates are immersed in the connective tissue. (The chorion is in contact with the connective tissue.)
3. Endotelochorional- not only the epithelium but also the connective tissue is destroyed. The villi are in contact with the vessels and are separated from the maternal blood only by their thin endothelial wall. (predators)
4. Hemochorionic- there are profound changes in the uterus. The villi are bathed in blood and absorb nutrients from it.
By appearance:
1.Diffuse- The villi are evenly distributed over the entire surface of the chorion.
2. Cotyledonnaya- the villi are collected in groups in the form of bushes
3.Waist- villi form a girdle surrounding the water bladder.
4.Discoid– The villi are located within the discoid region on the surface of the chorion.
Per cleavage and blastulation processes a number of further developmental processes follow, which lead to the formation of primitive, primary anlages of the organs of the embryo, that is, to the emergence of first two and then three germ layers, or layers, from the initially single blastoderm of the blastula (outer germ layer, or ectoderm, middle leaf, or mesoderm, and inner leaf, or endoderm). The stage of development with two rudimentary germ layers is called gastrula.
After the end of the period gastrulation there are already more complex changes that first cause the formation of the dorsal part of the embryo (notogenesis), later - the formation of its body, the laying of the body cavity, the so-called coelom in the mesoderm (coelomatization), then the laying of the dorsal string, that is, the chord (chordulation), and the formation germs nervous system in the form of neural, medullary plate and medullary tube (neurulation). The older the animal is phylogenetically, that is, the higher the stage of its phylogenetic development, the more complex these processes (although their basic scheme remains unchanged and can be derived from the development of the lancelet) and the more these processes overlap chronologically (heterochronically).
In the region of the vegetative pole blastula in blastoderm somewhat larger than blastomeres at animal pole. The region of these larger blastomeres of the vegetative pole begins to gradually indent, invaginate into the blastocoel towards the animal pole.
Because of this, the cavity blastocoel begins to decrease, and the blastoderm of the vegetative pole approaches the cell layer of the blastoderm of the animal pole. Finally, both of these layers adjoin each other, which simultaneously leads to the disappearance of the blastocoel cavity. This process can be compared purely morphologically with the indentation of one wall of a perforated rubber ball inward, towards the opposite wall. We can say that gastrulation in the lancelet occurs according to the method of intussusception (invagination).
There is no active ingrown areas of the vegetative pole towards the animal pole; the process of invagination is actually due to uneven growth of the blastula. Due to the fact that cells in the region of the animal pole multiply faster than cells at the vegetative pole, the blastoderm of the animal pole, which grows in width, begins to close, including the more slowly growing region of vegetative blastomeres.
Along with the, no doubt, changes in the colloidal state of the surface layer of the cytoplasm of cells along the edges of the blastopore are also important.
Thus, as a result invaginations a cup-like formation appears, the wall of which is already two-layered, since the opposite areas of the blastoderm adjoin one another during invagination. At the bottom of the new cavity resulting from invagination, there is an inner layer of cells corresponding to the former vegetative pole of the blastula. Its outer surface, on the contrary, is covered with a layer of blastoderm, which was previously located on the animal pole. This stage of development, which is characterized by a wall consisting of two adjoining cell layers, is called the gastrula. The outer epithelial layer of the gastrula is the outer germ layer, the layer is the ectoderm, the inner layer is the inner germ layer - the endoderm.
EMBRYOLOGY
Lesson number 4
TOPIC: “Embryogenesis. Cleavage, gastrulation
TEST QUESTIONS
Definition of the concept of "crushing".
The nature of the crushing of the zygote in humans.
Dark and light blastomeres, their morphological characteristics, development potential.
Morula and blastocyst, their morphological characteristics.
Implantation of the embryo in the uterine mucosa. stages of implantation.
Histiotrophic and hematotrophic types of nutrition of the embryo.
Methods of gastrulation in humans.
Stages of gastrulation (delamination and immigration), their timing and mechanisms.
Factors affecting the mechanisms of gastrulation.
PURPOSE OF THE LESSON:
Learn the features and nature of the crushing of the zygote in humans. Learn to distinguish between dark and light blastomeres. Understand the difference between morula and blastocyst. Know the stages of implantation and types of nutrition of the embryo depending on the stage of implantation. To study the methods of gastrulation in humans, to know the stages and mechanisms of gastrulation.
The student must know:
the nature of the crushing of the zygote in humans;
morphological characteristics of dark and light blastomeres, development potency;
morphological characteristics of the morula and blastocyst;
stages of implantation of the embryo in the uterine mucosa;
methods of gastrulation in humans, their timing and mechanisms.
The student must be able to:
on diagrams and tables, be able to distinguish between dark and light blastomeres; differentiate morula and blastocyst;
to distinguish between the stages of implantation of the embryo in the uterine mucosa;
distinguish between the ways of gastrulation in humans;
make sketches and make a written record of them.
SPLITTING UP
Splitting up is a series of mitotic divisions of the zygote with the formation of many daughter cells (blastomeres) smaller size. Mitotic divisions of the zygote and subsequently blastomeres occur with an increase in the number of cells, but without an increase in their mass, therefore they are called crushing.
Crushing types:
complete, or holoblastic, (lancelet, amphibians, mammals): the zygote is completely divided into blastomeres;
partial, or meroblastic, (fish, reptiles, birds): only part of the zygote is crushed;
uniform: the resulting blastomeres are the same or close in size;
uneven: blastomeres differ in size;
synchronous: blastomeres divide simultaneously;
asynchronous.
According to the nature of the spatial arrangement of blastomeres, the following are distinguished types of complete crushing:
radial: the resulting blastomeres are located one above the other, creating a figure with radial symmetry (lancelet, amphibians);
spiral: the overlying blastomeres are displaced with respect to the underlying ones at an angle of 45 °, thus arranged in a spiral (mollusks, nemerteans, annelids and some planarians);
bilateral (bilaterally symmetrical): in the early stages, the arrangement of blastomeres occurs according to the law of bilateral symmetry, so that the blastomeres of the right side of the embryo correspond to exactly the same blastomeres of the left side (squirts, nematodes and some other invertebrates)
chaotic (disordered): already after the third division, there is no strict pattern in the arrangement of blastomeres; in the course of further crushing, the resulting disorder increases (some jellyfish). External randomness, however, is regulated by internal mechanisms that have not yet been elucidated, and leads to the formation of an animal of a particular species.
Partial crushing exists in two forms:
discoidal: crushing, in which only part of the cytoplasm at the animal pole is divided into blastomeres (fish, reptiles, birds);
superficial: the surface layer of the cytoplasm is crushed, which, as it develops, completely separates from the yolk (insects and most other arthropods).
In the process of crushing, the embryo moves along fallopian tube and completes crushing in the uterine cavity on the 6th day of development. Cleavage ends with the formation of the blastula. This is a multicellular embryo, usually with a cavity inside. The wall of the blastula is called blastoderm, and the cavity blastocoel(primary body cavity). The blastula is also usually distinguished roof, bottom and delimiting them edge zone. Blastula formed during complete uniform crushing is called uniform coloblastula (lancelet), with complete uneven - uneven coeloblastula, also called amphiblastula (amphibians) or blastocyst (mammals, humans), with partial discoidal - discoblastula (reptiles, birds), with superficial - sterroblastula (coelenterates).
At the blastula stage, the polarity of the embryo is finally established, the degree of its integration increases, and exactly the order of cell arrangement and the degree of their interaction appear that are necessary for directed cell movements at the next stage of development - gastrulation.
At the stage of late blastula, the so-called presumptive regions are established in the lancelet and lower vertebrates, which contain the material of certain organs and systems. The location and boundaries of the presumptive areas, or presumptive rudiments, were studied using the method of marking areas of the embryo with vital dyes (V. Vogt's method, proposed in 1925). After applying a label to the surface of the embryo, one can trace its movements and find out the fate of this area during gastrulation and further development.
The plan of the mutual arrangement of the presumptive rudiments of future organs is called the map of the presumptive rudiments. The map of the rudiments, for example, in amphibians, accurately indicates the position of the areas from which the epidermis of the skin, the organs of smell, hearing, the lens of the eye, and others will develop.
Crushing of the human embryo complete or holoblastic(furrows of crushing pass through the entire embryo) , unevennoe(as a result of crushing, blastomeres of unequal size are formed) and asynchronous(Different blastomeres cleave at different rates, so the embryo at certain stages of cleavage contains an odd number of cells). The first crushing (division) of the zygote is completed after 30 hours, resulting in the formation of two blastomeres covered with a fertilization membrane. The stage of two blastomeres is followed by the stage of three blastomeres.
From the very first crushing of the zygote, two types of blastomeres are formed - “dark” and “light”. "Light", smaller, blastomeres are crushed faster and are arranged in one layer around the large "dark", which are in the middle of the embryo. From the surface "light" blastomeres, a trophoblast subsequently arises, connecting the embryo with the mother's body and providing its nutrition. Internal, "dark", blastomeres form the embryoblast, from which the body of the embryo and extraembryonic organs (amnion, yolk sac, allantois) are formed.
Starting from the 3rd day, cleavage proceeds faster, and on the 4th day the embryo consists of 7-12 blastomeres. After 50-60 hours, a dense accumulation of cells is formed - morula, and on the 3-4th day. formation begins blastocysts- a hollow vial filled with liquid. The blastocyst travels through the fallopian tube to the uterus within 3 days and enters the uterine cavity after 4 days. The blastocyst is in the uterine cavity in a free form (free blastocyst) for 2 days (days 5 and 6). By this time, the blastocyst increases in size due to an increase in the number of blastomeres - embryoblast and trophoblast cells - up to 100 and due to increased absorption of the secretion of the uterine glands by the trophoblast and active production of fluid by trophoblast cells.
The trophoblast during the first 2 weeks of development provides nutrition to the embryo due to the decay products of maternal tissues - a histiotrophic type of nutrition. The embryoblast is located in the form of a bundle of germ cells ("germ bundle"), which is attached internally to the trophoblast at one of the poles of the blastocyst.
IMPLANTATION
Implantation (ingrowing, rooting) - the introduction of the embryo into the uterine mucosa. There are two stages of implantation:
adhesion (sticking): the embryo is attached to the inner surface of the uterus;
invasion (immersion) - the introduction of the embryo into the tissue of the mucous membrane of the uterus.
On the 7th day. in the trophoblast and embryoblast, changes occur associated with the preparation for implantation:
the blastocyst retains the fertilization membrane;
in the trophoblast, the number of lysosomes with enzymes increases, ensuring the destruction (lysis) of the tissues of the uterine wall and thereby contributing to the introduction of the embryo into the thickness of its mucous membrane;
microvilli appearing in the trophoblast gradually destroy the fertilization membrane;
the germinal nodule flattens and turns into the germinal shield, in which preparations for the first stage of gastrulation begin.
Implantation lasts about 40 hours. Simultaneously with implantation gastrulation(formation of germ layers). This is the first critical period of development.
In the first stage The trophoblast is attached to the epithelium of the uterine mucosa, and two layers are formed in it - cytotrophoblast and symplastotrophoblast.
In the second stage symplastotrophoblast, producing proteolytic enzymes, destroys the uterine mucosa. The trophoblast villi that form in this case, penetrating into the uterus, sequentially destroy its epithelium, then the underlying connective tissue and vessel walls, and the trophoblast comes into direct contact with the blood of the maternal vessels. The nutrition of the embryo is carried out directly from the maternal blood (hematotrophic type of nutrition). From the mother's blood, the fetus receives not only all the nutrients, but also the oxygen necessary for breathing.
The hematotrophic type of nutrition, replacing the histiotrophic one, is accompanied by a transition to a qualitatively new stage of embryogenesis - second phasegastrulation and laying of extra-embryonic organs.
gastrulation
The period of gastrulation is characterized by active movements, both of individual cells of the embryo and of cell masses, as a result of which three main layers of the body are formed in vertebrates, they are called germ layers:
ectoderm- outer germ layer;
mesoderm- middle germ layer;
endoderm- inner germ layer.
Germ layers in different animals are homologous entities and, i.e. during their development give identical structures: the ectoderm always transforms into the outer cover of the body, and the lining of the midgut develops from the endoderm.
Another feature gastrulation period lies in the fact that the resulting during cell division, unlike blastomeres, begin to grow and increase to the size of the mother, in this case, active growth and an increase in the size of the embryo itself occur.
Gastrulation in humans proceeds in two stages:
Stage I (delamination) falls on the 7th day of intrauterine development: two leaves are formed from the material of the germinal nodule (embryoblast): the outer leaf is epiblast(cells look like pseudostratified prismatic epithelium) and internal - hypoblast facing into the cavity of the blastocyst (cells are small cubic, with foamy cytoplasm).
Stage II (immigration) - on the 14-15th day of intrauterine development.
Gastrulation methods, i.e., the mechanisms of formation of germ layers, differ in different animals and are largely determined by the structure of the blastula. There are four main types of gastrulation:
intussusception (invagination);
delamination (stratification, splitting);
immigration (eviction);
epiboly (fouling).
As a result of gastrulation, an embryo is formed - gastrula. The gastrula has a cavity - gastrocoel(cavity of the primary intestine), into which the hole leads - blastopore(primary mouth).
Depending on the further fate of the blastopore in development, all animals are divided into protostomes and deuterostomes. In protostomes, which include most invertebrates, a mouth opening is formed in place of the blastopore, and the opposite end becomes the posterior end of the body. In deuterostomes, which include chordates and some invertebrates, the blastopore is transformed into an anus or neurointestinal canal located at the posterior end of the body, and the oral opening erupts on the ventral side at the opposite end of the body. The blastopore is distinguished edges, or lips:dorsal, lateral and ventral.
An important result of gastrulation in chordates is the formation of the so-called axial complex of primordia in the germ layers. The axial complex of rudiments represents the rudiments of the nervous system (nervous plate) and chord (chordal plate) located along the axis of the body of the embryo, as well as the mesoderm rudiments lying laterally in relation to the chordal plate and associated with it. The closely adjacent rudiments of the notochord and mesoderm are often called chordomesodermis.
There are several factors of gastrulation:
physical: these include the faster division of small cells compared to larger ones, which leads to the fouling of large cells with small ones, for example, in the blastula of amphibians and fish that have telolecithal eggs. In the blastula of these animals, there is a gradient in the distribution of blastomeres in size - their sizes decrease in the direction from the vegetative pole to the animal one. The metabolic gradient has the same direction - with a decrease in cell size, their metabolic activity and the rate of division increase. It is in the areas of active cell division that the processes of their movement begin. One of the reasons for the movement of cells is considered to be a change in surface tension with an increase in the number of cells. Cells at the same time are able to make active amoeboid movements;
chemical: the mechanism of inducing influence is associated with the release of special factors: proteins, nucleoproteins, steroids.
organizing centers: the direction of cell movement, and then their differentiation, is determined by induction - the influence of some areas or the rudiments of the embryo on others. Such areas and rudiments are called organizing centers, or inductors. The theory of organizing centers, put forward by G. Spemann, establishes the presence in the embryo at different stages of its development of special areas that have an inducing effect on neighboring areas. The organizing centers, for example, include the anterior (dorsal) lip of the blastopore, which induces the formation of the chordomesoderm, which, in turn, causes the differentiation of part of the ectoderm cells into the neural plate.
An important role in the regulation of intercellular interactions during development, specialized intercellular connections are performed - slot contacts, which appear already during crushing between blastomeres and, possibly, represent the first system of signal transmission between cells.
Types of crushing (read)
BUT - radial crushing (lancelet);
B - bilateral crushing (ascaris);
AT - disordered fragmentation (flukes);
G - spiral crushing (clam) .
The numbers indicate the sequence of crushing stages.
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