1) limits the contents of the cell from the external environment
2) provides the movement of substances in the cell
3) provides communication between organelles
4) carries out the synthesis of protein molecules

The membranes of the smooth endoplasmic reticulum perform the function
1) synthesis of lipids and carbohydrates
2) protein synthesis
3) protein breakdown
4) breakdown of carbohydrates and lipids

One of the functions of the golgi complex
1) the formation of lysosomes
2) the formation of ribosomes
3) ATP synthesis
4) oxidation of organic substances

Lipid molecules are part of
1) plasma membrane
2) ribosomes
3) fungal cell membranes
4) centrioles
Thanks in advance for anyone who can help

1. Cytoplasm performs the function of the cell skeleton due to the presence in it

A) microtubules
B) many chloroplasts
C) many mitochondria
D) systems of branched tubules
2. The composition of the cytoplasm of the cell includes
A) protein filaments
B) cilia and flagella
B) mitochondria
D) cell center and lysosomes
3. What is the role of the cytoplasm in plant cell
A) protects the contents of the cell from adverse conditions
B) provides selective permeability of substances
C) communicates between the nucleus and organelles
D) ensures the entry of substances from the environment into the cell
4. In what part of the cell are the organelles and the nucleus
A) in vacuoles
B) in the cytoplasm
B) in the endoplasmic reticulum
D) in the Golgi complex
5. All organelles and the cell nucleus are interconnected using
A) shells
B) plasma membrane
B) cytoplasm
D) vacuoles
6. The cytoplasm in the cell does NOT take part in
A) the transport of substances
B) the placement of organelles
B) DNA biosynthesis
D) communication between organelles
7. All cell organelles are located in
A) cytoplasm
B) Golgi complex
B) core
D) endoplasmic reticulum
8. The internal semi-liquid environment of the cell, penetrated by the smallest filaments and tubes, in which the organelles and the nucleus are located, is
A) vacuole
B) cytoplasm
B) golgi apparatus
D) mitochondria
9. With the help of what part of the cell are connections between organelles established?
A) core
B) cytoplasm
B) vacuole
D) shell

10. The structure and functions of the plasma membrane are determined by its constituent molecules
A) glycogen and starch
B) DNA and ATP
B) proteins and lipids
D) fiber and glucose
11. Animal cells are less stable in shape than plant cells because they don't have
A) chloroplasts
B) vacuoles
B) cell wall
D) lysosomes
12. Exchange of substances between the cell and environment regulated
A) the plasma membrane
B) endoplasmic reticulum
B) nuclear envelope
D) cytoplasm
13. Glycocalyx in the cell is formed
A) lipids and nucleotides
B) fats and ATP
B) carbohydrates and proteins
D) nucleic acids
14) The plasma membrane of an animal cell, in contrast to the cell wall of plants
A) is made up of fiber
B) is made up of proteins and lipids
B) strong, inelastic
D) permeable to all substances
15. The supply of nutrients by phagocytosis occurs in cells
A) prokaryotes
B) animals
B) mushrooms
D) plants
16. Selective entry of substances into the cell through the plasma membrane is associated with
A) the presence of a cellulose shell
B) the constancy of the concentration of substances in the cytoplasm
C) structural features of the bilipid layer
D) the presence of glycocalyx
17. The main properties of the plasma membrane include
A) impenetrability
B) contractility
B) selective permeability
D) excitability and conductivity
18. The shell of a fungal cell, unlike a plant cell, consists of
A) fiber
B) chitin-like substance
B) contractile proteins
D) lipids
19. A derivative of the plasma membrane - glycocalyx is present on the surface of cells
A) animals
B) viruses
B) mushrooms
D) bacteriophages

1) what cell structures are called inclusions, give examples 2) what underlies the structural organization of the cell 3) how they are arranged

cell membrane

4) what functions does the outer cytoplasmic membrane perform

5) how is the exchange of matter between the cell and the environment carried out. What is pinocytosis. What is phagasytosis

6) list the organelles of the cell and indicate their functions

7) what is the difference between smooth and rough membrane of the endoplasmic reticulum

8) what cell organelles contain DNA and are capable of self-reproduction

establish the correct sequence of phases of mitosis: a) chromatids diverge to opposite poles of the cell. b) spindle formation

division, attachment of centromeres to spindle threads. Chromosomes line up in the equatorial plane of the cell. c) chromosomes spiralize, thicken, the nuclear envelope is destroyed. d) chromatids separate from each other e) despiralization of chromosomes. division of the cytoplasm of the mother cell. formation of two daughter cells.

The blastula, also called the germinal sac, is the final result of the cleavage of the egg. The next stage, which occupies an intermediate position between crushing and organogenesis, in embryogenesis is gastrulation. Its main meaning is the formation of three germ layers: endoderm, ectoderm and mesoderm. In other words, it is with gastrulation that the embryonic differentiation and morphogenesis of the organism begins.

Definition of the term "gastrulation"

Back in 1901, gastrulation was described as the pathway through which mesodermal, endodermal, and ectodermal cells enter the embryo. This definition implies the presence of special organ-forming spaces in the blastula. Having understood this rather simple description, it is easy to move on to a more complex one, modern meaning term. Gastrulation is a sequence of morphogenetic movements, the result of which is the movement of tissue rudiments to the places intended for them in accordance with the "plan" of the organization of the organism. The process is complex, changes are accompanied by growth and reproduction, directed movement and differentiation of cells.

Considering gastrulation in a more general sense, it can be defined as an intermediate stage belonging to a single dynamic process, during which the blastula sections are rearranged, which greatly facilitates the transition to the process of organogenesis.

Cell movement

If giving general characteristics considered process, then we can say that gastrulation is an embolism and an epiboly. Both terms reflect the morphogenetic movement of cells, which occurs at absolutely all stages of the ontogenetic development of an organism. However, they are most pronounced during gastrulation. Epibolism is the process of moving cells along the surface of the embryo, and embolism is their movement into it.

In embryology, the following main types of gastrulation or cell movement are distinguished: invagination, immigration, involution, delamination and epiboly. More details about them - later in the article.

Movement of cell sheets

Not only individual (freely migrating) cells, but also entire cell layers can take part in the process of gastrulation. The direction is determined by constant and distant interactions. The first forces were discovered by P. Weiss in the 1920s and apparently occur in embryogenesis, the second - rare and special, occur with a small degree of probability during normal morphogenesis.

During gastrulation, cell fragmentation does not occur. As mentioned above, the movement of cell masses begins and, as a result, the formation of a two-layer embryo, called the gastrula. Endoderm and ectoderm become clearly visible. In all multicellular organisms (the only exceptions are coelenterates), in parallel with gastrulation or immediately after it, a third germ layer is formed, called the mesoderm. It is a collection of cells located between the ectoderm and endoderm. As a result, the embryo becomes three-layered.

The methods of gastrulation directly depend on the type of blastula.

Invaginated gastrula

The name of the method speaks for itself. Invagination is the invagination of a single-layer wall of the blastula (balstoderm) into the blastocoel. The most primitive and most illustrative example will be with a rubber ball. When you press it, part of the material is pressed inward. The invagination can be brought to the farthest wall or made insignificant. As a result, the blastula is transformed, and the gastrula is obtained in the form of a two-layer bag with an archenteron. Its inner wall is the primary endoderm, and its outer wall is the primary ectoderm. The resulting archenteron (primary intestine) communicates with the external environment through a hole called the blastopore. Its second name is the primary mouth. Its further development depends on the type of organism. In many animals, the blastopore eventually develops into the definitive mouth. In this regard, they are called protostomes (mollusks, worms, arthropods). In deuterostomes, the blastopore turns into a neuro-intestinal canal located in the back of the embryo (in chordates), or into the anus.

immigration gastrula

Immigration gastrulation is a method of formation of a two-layer embryo, the most characteristic of the coelenterates. The gastrula is formed by the active eviction of a part of the blastula cells into the blastocoel. Such immigration is unipolar. Cells move only from the vegetative pole. Later, they form the endoderm, i.e., the inner layer. It is in this way that gastrulation is carried out in a hydroid polyp, a jellyfish.

Blastodermal cells can penetrate into the blastocoel not in any one area, but over the entire surface of the embryo. Such immigration is called multipolar, but it is quite rare.

In many coelenterates, which are characterized by an immigration method of gastrulation, there is a very active "eviction" of blastula cells, and the resulting gastrula completely loses the blastocoel. In this case, the blastopore characteristic of the previous invagination method is absent.

Delamination gastrula

This rare type of gastrula was first described by Mechnikov I.I., and it is typical for intestinal. The processes accompanying gastrulation are very peculiar, but when considering a typical case, they are perceived more simply. For example, the eggs of some scyphomedusa have concentrically located and well-distinguished sections of the cytoplasm: dense and granular (ectoplasm) and cellular (endoplasm). They are characterized by relatively synchronous and uniform division: 2, 4, 8, 16. Ultimately, the embryo contains 32 blastomeres. Further division is carried out parallel to the surface of the embryo. An outer layer of blastomeres is formed, consisting of ectoplasm, and an inner layer, partly of ectoplasm and endoplasm. In other words, the process of formation of a multilayer embryo proceeds by splitting one layer of cells into two. Then only the internal blastomeres are crushed and again parallel to the surface of the embryo, which, as a result of such a peculiar gastrulation, takes on the shape of a ball. It consists of 64 flat cells that form the ectoderm and 32 more convex cells that are the basis of the endoderm.

epibolic gastrula

In animals with a pronounced telolecithal structure of eggs (displacement of the yolk to the vegetative pole), gastrulation occurs according to the epibolic method. Macromeres are large blastomeres that divide very slowly and contain a large amount of yolk. They do not have the ability to move, in connection with this, more active micromeres located on the cell surface literally "creep" on them. With such gastrulation, the blastopore is absent, and the archenteron is not formed. Only in the future, when the macromeres nevertheless decrease in size, does a cavity begin to form, the rudiment of the primary intestine.

involution

Involutionary gastrulation is a process that consists in "tucking" the outer layer of cells into the inside of the embryo. It, increasing in size, spreads along the inner surface. This method of gastrulation is characteristic of animals with mesolecithal eggs - amphibians (amphibians). The movement of the leading deep cells of the marginal region inhibits the development of the archenteron. It is in them that driving force involutions.

Mixed way of gastrulation

As you know, embryogenesis is the most early period development of each individual organism: from conception to birth. Gastrulation is one of its stages, the second in chronology after crushing. Her methods are so different that they can be compared with a high degree of conventionality. Each of them requires detailed study and analysis. However, there are still certain lines of intersection between them. So, as a kind of variant of invagination, the process of epiboly can be considered, and delamination has similarities with immigration.

Note that in many animals gastrulation occurs in a combined way. In such cases, epiboly and invagination, as well as other morphogenetic processes, take place simultaneously. In particular, this is how gastrulation occurs in amphibians. In this regard, many authors distinguish a mixed method.

gastrula

Literally from Latin, the term "gastrula" is translated as "womb, stomach." It denotes a specific germ of multicellular organisms. Distinctive feature gastrula is the presence of two or three germ layers. The process of its formation is the phase of gastrulation.

The simplest device is observed in animals. They are characterized by an ellipsoidal gastrula with a unicellular outer layer (ectoderm) and an internal accumulation of cells (endoderm), as well as a "primary gut". Gastrula is considered typical sea ​​urchin, which is formed by invagination. In humans, gastrulation takes place on the 8-9th day of development. The gastrula is a disk-shaped flattened formation formed from the inner cell mass.

As a rule, in most animals at the gastrula stage, the embryo cannot live freely and is located in the uterus or egg membranes. However, there are exceptions. So, the larvae of the intestinal cavities, planula, are a free-floating gastrula.

gastrulation(from Latin gaster - stomach) - a complex process of chemical and morphological changes, which is accompanied by reproduction, growth, directed movement and differentiation of cells, resulting in the formation of germ layers - sources of rudiments of tissues and organs, and complexes of axial organs.

At this stage of development of organisms, a two-layer embryo is formed - gastrula. In this case, two germ layers are formed - ectoderm (outer) and endoderm (interior). The gastrula corresponds in structure to modern intestinal animals. At the late stage of gastrulation, a third germ layer is formed - mesoderm (average).

These sheets subsequently give rise to embryonic rudiments, from which tissues and organs are formed. There are four types of gastrulation (Fig. 8).

Immigration(invasion) is the most primitive, initial form of gastrulation. All other types of gastrulation are derived from it. In this case, the cells of the blastoderm move to the blastocoel, where they settle on the inner surface and form endoderm , and the outer cells form ectoderm . This forms the gastric cavity gastrocoel - the cavity of the primary intestine (coelenterates).

Intussusception(invagination) - the blastoderm at the vegetative pole bends inside the blastocoel and reaches the cells of the animal pole. In this case, a gastrocoel is formed, which communicates with the external environment through a hole - blastopore - Primary mouth.

With the development of the blastopore, animals are divided into two groups:

protostomes- the blastopore turns into a real mouth (worms, molluscs, arthropods);

deuterostomes- the primary mouth turns into an anus at the posterior end of the body, and at the anterior, the mouth opening reappears (brachiopods, echinoderms, chordates).

epiboly(fouling) - at the animal pole of the blastula, cells divide faster and crawl onto large cells of the vegetative pole. From the cells of the animal pole is formed ectoderm , and from the cells of the vegetative pole - endoderm. This type of gastrulation is typical for animals in which the egg contains an increased amount of yolk (cyclostomes, amphibians).

Delamination(stratification) - blastoderm cells divide, daughter cells move into the blastocoel, forming endoderm , and the outer cells form ectoderm . In this case, the blastopore is not formed, so the gastrocoel does not communicate with the external environment. This type of gastrulation is characteristic of animals that have lost large reserves of yolk in the eggs (coelenterates, higher placental).

Fig.7. Genetic control of the development of mammals (according to B.V. Konyukhov, 1976)

In ontogenetic development, there are periods of greatest sensitivity to the damaging action of various factors. These periods have received - critical periods of development. P.G. Svetlov postulated two general critical periods in the development of placental mammals. The first of them coincides with the process of implantation of the embryo (in humans, the end of the 1st - the beginning of the 2nd week of pregnancy), the second - with the formation of the placenta (in humans, from the 3rd to the 6th week). According to other sources, the second critical period also includes the 7th and 8th weeks, when neurulation occurs and local organogenesis begins.

The damaging effect during implantation usually leads to its violation, early death of the fetus and its abortion. According to some reports, 50-70% of fertilized eggs (embryos) do not survive the implantation period.

Malformations - abnormalities in the structure of an organ or the whole organism, leading to functional disorders: malformations are persistent morphological changes in an organ or organism as a whole, which go beyond normal variations and occur in utero as a result of a developmental disorder of the embryo or fetus, sometimes after the birth of a child due to a violation of the further formation of organs. These changes cause violations of the corresponding functions. Under developmental anomalies, they understand only such defects in which anatomical changes do not lead to a significant dysfunction, for example, deformations of the auricles that do not disfigure the patient's face and do not significantly affect the perception of sounds. Gross malformations, in which the appearance of the child is disfigured, are often called deformities.

Or gastrula(gaster - stomach). The process that leads to the formation of a gastrula is called gastrulation. characteristic feature 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 fabrics and organs.

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. Whereas multipolar immigration is rarer and has been observed in some hydrojellyfish. 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 historical development some cells have 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.

The content of the article

EMBRYOLOGY, science that studies the development of an organism early stages preceding metamorphosis, hatching, or birth. The fusion of gametes - an egg (ovum) and a spermatozoon - with the formation of a zygote gives rise to a new individual, but before becoming the same creature as its parents, it has to go through certain stages of development: cell division, the formation of primary germ layers and cavities, the emergence of embryonic axes and axes of symmetry, the development of coelomic cavities and their derivatives, the formation of extraembryonic membranes, and, finally, the emergence of organ systems that are functionally integrated and form one or another recognizable organism. All this is the subject of the study of embryology.

Development is preceded by gametogenesis, i.e. formation and maturation of sperm and egg. The process of development of all eggs of a given species proceeds in general in the same way.

Gametogenesis.

Mature spermatozoa and eggs differ in their structure, only their nuclei are similar; however, both gametes are formed from identical-looking primordial germ cells. In all sexually reproducing organisms, these primary germ cells separate from other cells in the early stages of development and develop in a special way, preparing to perform their function - the production of sex, or germ, cells. Therefore, they are called germplasm - in contrast to all other cells that make up the somatoplasm. It is quite obvious, however, that both germplasm and somatoplasm originate from a fertilized egg - a zygote that gave rise to a new organism. So basically they are the same. The factors that determine which cells will become sexual and which will become somatic have not yet been established. However, in the end, germ cells acquire fairly clear differences. These differences arise in the process of gametogenesis.

In all vertebrates and some invertebrates, primary germ cells arise far from the gonads and migrate to the gonads of the embryo - the ovary or testis - with the blood flow, with layers of developing tissues, or through amoeboid movements. In the gonads, mature germ cells are formed from them. By the time of development of the gonads, the soma and the germ plasm are already functionally isolated from each other, and from that time on, throughout the life of the organism, the germ cells are completely independent of any influences of the soma. That is why the signs acquired by an individual throughout his life do not affect his germ cells.

Primary germ cells, being in the gonads, divide with the formation of small cells - spermatogonia in the testes and oogonia in the ovaries. Spermatogonia and oogonia continue to divide many times, forming cells of the same size, which indicates the compensatory growth of both the cytoplasm and the nucleus. Spermatogonia and oogonia divide mitotically and therefore retain their original diploid number of chromosomes.

After some time, these cells stop dividing and enter a period of growth, during which very important changes occur in their nuclei. Chromosomes originally received from two parents are paired (conjugated), entering into very close contact. This makes possible subsequent crossing over (crossover), during which homologous chromosomes are broken and connected in a new order, exchanging equivalent sections; as a result of crossing over, new combinations of genes appear in the chromosomes of oogonia and spermatogonia. It is assumed that the sterility of mules is due to the incompatibility of the chromosomes received from the parents - a horse and a donkey, because of which the chromosomes are not able to survive when closely connected to each other. As a result, the maturation of germ cells in the ovaries or testes of the mule stops at the stage of conjugation.

When the nucleus has been rebuilt and a sufficient amount of cytoplasm has accumulated in the cell, the process of division resumes; the whole cell and the nucleus undergo two different types of divisions, which determine the actual process of maturation of germ cells. One of them - mitosis - leads to the formation of cells similar to the original; as a result of the other - meiosis, or reduction division, during which cells divide twice, cells are formed, each of which contains only half (haploid) the number of chromosomes compared to the original, namely one from each pair. In some species, these cell divisions occur in reverse order. After the growth and reorganization of the nuclei in oogonia and spermatogonia and immediately before the first division of meiosis, these cells are called oocytes and spermatocytes of the first order, and after the first division of meiosis, oocytes and spermatocytes of the second order. Finally, after the second division of meiosis, the cells in the ovary are called eggs (eggs), and those in the testis are called spermatids. Now the egg has finally matured, and the spermatid has yet to go through metamorphosis and turn into a spermatozoon.

One important difference between oogenesis and spermatogenesis needs to be emphasized here. From one oocyte of the first order, as a result of maturation, only one mature egg is obtained; the remaining three nuclei and a small amount of cytoplasm turn into polar bodies that do not function as germ cells and subsequently degenerate. All the cytoplasm and yolk, which could be distributed over four cells, are concentrated in one - in a mature egg. In contrast, one first-order spermatocyte gives rise to four spermatids and the same number of mature spermatozoa, without losing a single nucleus. During fertilization, the diploid, or normal, number of chromosomes is restored.

Egg.

The ovum is inert and usually larger than the somatic cells of the organism. The mouse egg is about 0.06 mm in diameter, while the diameter of the ostrich egg is more than 15 cm. The eggs are usually spherical or oval in shape, but can also be oblong, like those of insects, hagfish or mudfish. The size and other features of the egg depend on the amount and distribution of the nutritious yolk in it, which accumulates in the form of granules or, more rarely, in the form of a continuous mass. Therefore, eggs are divided into different types depending on the content of yolk in them.

Homolecithal eggs

(from Greek homós - equal, homogeneous, lékithos - yolk) . In homolecithal eggs, also called isolecithal or oligolecithal eggs, there is very little yolk and it is evenly distributed in the cytoplasm. Such eggs are typical of sponges, coelenterates, echinoderms, scallops, nematodes, tunicates, and most mammals.

Telolecithal eggs

(from Greek télos - end) contain a significant amount of yolk, and their cytoplasm is concentrated at one end, usually referred to as the animal pole. The opposite pole, on which the yolk is concentrated, is called vegetative. These eggs are typical of annelids, cephalopods, non-cranial (lancelet), fish, amphibians, reptiles, birds and monotreme mammals. They have a well-defined animal-vegetative axis, determined by the gradient of the distribution of the yolk; the core is usually located eccentrically; in eggs containing pigment, it is also distributed along a gradient, but, unlike the yolk, it is more abundant at the animal pole.

Centrolecithal eggs.

In them, the yolk is located in the center, so that the cytoplasm is shifted to the periphery and fragmentation is superficial. Such eggs are typical for some coelenterates and arthropods.

Sperm.

Unlike a large and inert egg, spermatozoa are small, from 0.02 to 2.0 mm in length, they are active and able to swim a long distance to reach the egg. There is little cytoplasm in them, and there is no yolk at all.

The shape of spermatozoa is diverse, but among them two main types can be distinguished - flagellated and non-flagellated. Flagellated forms are comparatively rare. In most animals, an active role in fertilization belongs to the spermatozoon.

Fertilization.

Fertilization is a complex process during which a sperm enters the egg and their nuclei fuse. As a result of the fusion of gametes, a zygote is formed - in essence, a new individual capable of developing in the presence of the necessary conditions for this. Fertilization causes the activation of the egg, stimulating it to successive changes leading to the development of a formed organism. During fertilization, amphimixis also occurs, i.e. mixing of hereditary factors as a result of the fusion of the nuclei of the egg and sperm. The egg provides half of the necessary chromosomes and usually all the nutrients needed for the early stages of development.

When a spermatozoon comes into contact with the surface of the egg, the yolk membrane of the egg changes, turning into a fertilization membrane. This change is considered proof that egg activation has occurred. At the same time, on the surface of eggs that contain little or no yolk at all, a so-called. a cortical reaction that prevents other sperm from entering the egg. Eggs that contain a lot of yolk have a cortical reaction later, so they usually get a few spermatozoa. But even in such cases, only one spermatozoon, the first to reach the nucleus of the egg, performs fertilization.

In some eggs, at the point of contact of the sperm with the plasma membrane of the egg, a protrusion of the membrane is formed - the so-called. tubercle of fertilization; it facilitates the penetration of the spermatozoon. Usually, the head of the spermatozoon and the centrioles located in its middle part penetrate the egg, while the tail remains outside. Centrioles contribute to the formation of the spindle during the first division of a fertilized egg. The fertilization process can be considered complete when the two haploid nuclei - the egg and sperm - merge and their chromosomes conjugate, preparing for the first crushing of the fertilized egg.

Splitting up.

If the appearance of the fertilization membrane is considered an indicator of the activation of the egg, then division (crushing) is the first sign of the actual activity of the fertilized egg. The nature of crushing depends on the amount and distribution of the yolk in the egg, as well as on the hereditary properties of the zygote nucleus and the characteristics of the egg cytoplasm (the latter are entirely determined by the genotype of the mother organism). There are three types of crushing of a fertilized egg.

Holoblastic fragmentation

characteristic of homolecithal eggs. Crushing planes separate the egg completely. They can divide it into equal parts, like a starfish or sea urchin, or into unequal parts, like a gastropod. Crepidula. Cleavage of the moderately telolecithal egg of the lancelet occurs according to the holoblastic type, however, uneven division appears only after the stage of four blastomeres. In some cells, after this stage, fragmentation becomes extremely uneven; the resulting small cells are called micromeres, and the large cells containing the yolk are called macromeres. In molluscs, the cleavage planes pass in such a way that, starting from the stage of eight cells, the blastomeres are arranged in a spiral; this process is regulated by the kernel.

meroblastic fragmentation

typical of telolecithal eggs rich in yolk; it is limited to a relatively small area near the animal pole. Cleavage planes do not pass through the entire egg and do not capture the yolk, so that as a result of division at the animal pole, a small disk of cells (blastodisk) is formed. Such crushing, also called discoidal, is characteristic of reptiles and birds.

Surface crushing

typical of centrolecithal eggs. The nucleus of the zygote divides in the central island of the cytoplasm, and the resulting cells move to the surface of the egg, forming a superficial layer of cells around the yolk lying in the center. This type of cleavage is seen in arthropods.

crushing rules.

It has been established that fragmentation obeys certain rules, named after the researchers who first formulated them. Pfluger's Rule: The spindle always pulls in the direction of least resistance. Balfour's rule: the rate of holoblastic cleavage is inversely proportional to the amount of yolk (the yolk makes it difficult to divide both the nucleus and the cytoplasm). Sacks' rule: cells are usually divided into equal parts, and the plane of each new division intersects the plane of the previous division at a right angle. Hertwig's rule: the nucleus and spindle are usually located in the center of the active protoplasm. The axis of each spindle of division is located along the long axis of the mass of protoplasm. The division planes usually intersect the mass of protoplasm at right angles to its axes.

As a result of crushing of fertilized eggs of any type, cells called blastomeres are formed. When there are a lot of blastomeres (in amphibians, for example, from 16 to 64 cells), they form a structure that resembles a raspberry and is called a morula.

Blastula.

As the crushing continues, the blastomeres become smaller and tighter to each other, acquiring a hexagonal shape. This form increases the structural rigidity of the cells and the density of the layer. Continuing to divide, the cells push each other apart and, as a result, when their number reaches several hundred or thousands, they form a closed cavity - the blastocoel, into which fluid from the surrounding cells enters. In general, this formation is called the blastula. Its formation (in which cell movements do not participate) ends the period of egg crushing.

In homolecithal eggs, the blastocoel may be centrally located, but in telolecithal eggs, it is usually displaced by the yolk and is located eccentrically, closer to the animal pole and directly below the blastodisc. So, the blastula is usually a hollow ball, the cavity of which (blastocoel) is filled with liquid, but in telolecithal eggs with discoidal crushing, the blastula is represented by a flattened structure.

In holoblastic cleavage, the blastula stage is considered complete when, as a result of cell division, the ratio between the volumes of their cytoplasm and nucleus becomes the same as in somatic cells. In a fertilized egg, the volumes of the yolk and cytoplasm do not correspond at all to the size of the nucleus. However, in the process of crushing, the amount of nuclear material increases somewhat, while the cytoplasm and yolk only divide. In some eggs, the ratio of the volume of the nucleus to the volume of the cytoplasm at the time of fertilization is approximately 1:400, and by the end of the blastula stage it is approximately 1:7. The latter is close to the ratio characteristic of both the primary reproductive and somatic cells.

Late blastula surfaces in tunicates and amphibians can be mapped; To do this, intravital (not harmful to cells) dyes are applied to its different parts - the color marks made are stored in the course of further development and allow you to determine which organs arise from each area. These areas are called presumptive, i.e. those whose fate can be predicted under normal conditions of development. If, however, at the stage of late blastula or early gastrula, these areas are moved or swapped, their fate will change. Such experiments show that, up to a certain stage of development, each blastomere is able to turn into any of the many different cells that make up the body.

Gastrula.

The gastrula is the stage of embryonic development in which the embryo consists of two layers: the outer - ectoderm, and the inner - endoderm. This bilayer stage is achieved in different ways in different animals, since the eggs different types contain varying amounts of yolk. However, in any case leading role this is played by cell movements, not cell divisions.

Intussusception.

In homolecithal eggs, for which holoblastic cleavage is typical, gastrulation usually occurs by invagination (invagination) of the cells of the vegetative pole, which leads to the formation of a two-layer, bowl-shaped embryo. The original blastocoel contracts, but a new cavity, the gastrocoel, is formed. The opening leading into this new gastrocoel is called the blastopore (an unfortunate name because it opens into the gastrocoel rather than the blastocoel). The blastopore is located in the region of the future anus, at the posterior end of the embryo, and in this region most of the mesoderm develops - the third, or middle, germ layer. The gastrocoel is also called the archenteron, or primary gut, and it serves as the rudiment of the digestive system.

Involution.

In reptiles and birds, whose telolecithal eggs contain a large amount of yolk and are meroblastically divided, blastula cells rise above the yolk in a very small area and then begin to screw inward, under the cells of the upper layer, forming the second (lower) layer. This process of screwing in the cell sheet is called involution. The top layer of cells becomes the outer germ layer, or ectoderm, and the bottom layer becomes the inner, or endoderm. These layers merge into one another, and the place where the transition occurs is known as the blastopore lip. The roof of the primary intestine in the embryos of these animals consists of fully formed endodermal cells, and the bottom of the yolk; the bottom of the cells is formed later.

Delamination.

In higher mammals, including humans, gastrulation occurs somewhat differently, namely by delamination, but leads to the same result - the formation of a two-layer embryo. Delamination is a stratification of the original outer layer of cells, leading to the emergence of an inner layer of cells, i.e. endoderm.

Auxiliary processes.

There are also additional processes that accompany gastrulation. The simple process described above is the exception, not the rule. Auxiliary processes include epiboly (fouling), i.e. movement of cell layers over the surface of the vegetative hemisphere of the egg, and concretion - the association of cells in large areas. One of these processes or both of them can accompany both invagination and involution.

results of gastrulation.

The end result of gastrulation is the formation of a bilayer embryo. The outer layer of the embryo (ectoderm) is formed by small, often pigmented cells that do not contain yolk; from the ectoderm, such tissues as, for example, nervous, and the upper layers of the skin further develop. The inner layer (endoderm) consists of almost unpigmented cells that retain some yolk; they give rise mainly to the tissues lining the digestive tract and its derivatives. However, it should be emphasized that there are no profound differences between these two germ layers. The ectoderm gives rise to the endoderm, and if in some forms the boundary between them in the region of the blastopore lip can be determined, then in others it is practically indistinguishable. Transplantation experiments have shown that the difference between these tissues is determined only by their location. If areas that would normally remain ectodermal and give rise to derivatives of the skin are transplanted onto the lip of the blastopore, they screw inward and become the endoderm, which can turn into the lining of the digestive tract, lungs or thyroid gland.

Often, with the appearance of the primary intestine, the center of gravity of the embryo shifts, it begins to turn in its membranes, and for the first time the antero-posterior (head-tail) and dorso-ventral (back-belly) axes of symmetry of the future organism are established in it.

Germinal leaves.

Ectoderm, endoderm and mesoderm are distinguished based on two criteria. Firstly, by their location in the embryo at the early stages of its development: during this period, the ectoderm is always located outside, the endoderm is inside, and the mesoderm, which appears last, is between them. Secondly, according to their future role: each of these sheets gives rise to certain organs and tissues, and they are often identified by their further fate in the development process. However, we recall that during the period when these leaflets appeared, there were no fundamental differences between them. In experiments on the transplantation of germ layers, it was shown that initially each of them has the potency of either of the other two. Thus, their distinction is artificial, but it is very convenient to use it in the study of embryonic development.

Mesoderm, i.e. the middle germ layer is formed in several ways. It may arise directly from the endoderm by the formation of coelomic sacs, as in the lancelet; simultaneously with the endoderm, like in a frog; or by delamination, from the ectoderm, as in some mammals. In any case, at first the mesoderm is a layer of cells lying in the space that was originally occupied by the blastocoel, i.e. between the ectoderm on the outside and the endoderm on the inside.

The mesoderm soon splits into two cell layers, between which a cavity is formed, called the coelom. From this cavity subsequently formed the pericardial cavity surrounding the heart, the pleural cavity surrounding the lungs, and the abdominal cavity, in which the digestive organs lie. The outer layer of the mesoderm - the somatic mesoderm - forms, together with the ectoderm, the so-called. somatopleura. From the outer mesoderm develop striated muscles of the trunk and limbs, connective tissue and vascular elements of the skin. The inner layer of mesodermal cells is called the splanchnic mesoderm and, together with the endoderm, forms the splanchnopleura. Smooth muscles and vascular elements of the digestive tract and its derivatives develop from this layer of mesoderm. In the developing embryo, there is a lot of loose mesenchyme (embryonic mesoderm) that fills the space between the ectoderm and endoderm.

In chordates, in the process of development, a longitudinal column of flat cells is formed - a chord, the main distinguishing feature of this type. Notochord cells originate from the ectoderm in some animals, from the endoderm in others, and from the mesoderm in still others. In any case, these cells can be distinguished from the rest at a very early stage of development, and they are located in the form of a longitudinal column above the primary intestine. In vertebrate embryos, the notochord serves as the central axis around which the axial skeleton develops, and above it the central nervous system. In most chordates, this is a purely embryonic structure, and only in the lancelet, cyclostomes, and elasmobranchs does it persist throughout life. In almost all other vertebrates, notochord cells are replaced by bone cells that form the body of the developing vertebrae; it follows that the presence of the chord facilitates the formation of the spinal column.

Derivatives of the germ layers.

The further fate of the three germ layers is different.

From the ectoderm develop: all nervous tissue; the outer layers of the skin and its derivatives (hair, nails, tooth enamel) and partially the mucous membrane of the oral cavity, nasal cavities and anus.

Endoderm gives rise to the lining of the entire digestive tract - from the oral cavity to the anus - and all its derivatives, i.e. thymus thyroid gland, parathyroid glands, trachea, lungs, liver and pancreas.

From the mesoderm are formed: all types of connective tissue, bone and cartilage tissue, blood and the vascular system; all types of muscle tissue; excretory and reproductive systems, dermal layer of the skin.

In an adult animal, there are very few organs of endodermal origin that do not contain nerve cells derived from the ectoderm. Each important organ also contains derivatives of the mesoderm - blood vessels, blood, and often muscles, so that the structural isolation of the germ layers is preserved only at the stage of their formation. Already at the very beginning of their development, all organs acquire a complex structure, and they include derivatives of all germ layers.

GENERAL BODY PLAN

Symmetry.

In the early stages of development, the organism acquires a certain type of symmetry characteristic of a given species. One of the representatives of the colonial protists, Volvox, has central symmetry: any plane passing through the center of the Volvox divides it into two equal halves. Among multicellular organisms, there is not a single animal that has this type of symmetry. For coelenterates and echinoderms, radial symmetry is characteristic, i.e. parts of their body are located around the main axis, forming, as it were, a cylinder. Some, but not all, planes passing through this axis divide such an animal into two equal halves. All echinoderms at the larval stage have bilateral symmetry, but in the process of development they acquire the radial symmetry characteristic of the adult stage.

For all highly organized animals, bilateral symmetry is typical, i.e. they can be divided into two symmetrical halves in only one plane. Since this arrangement of organs is observed in most animals, it is considered optimal for survival. The plane passing along the longitudinal axis from the ventral (abdominal) to the dorsal (dorsal) surface divides the animal into two halves, right and left, which are mirror images of each other.

Almost all unfertilized eggs have radial symmetry, but some lose it at the time of fertilization. For example, in a frog egg, the site of penetration of the spermatozoon is always shifted to the front, or head, end of the future embryo. This symmetry is determined by only one factor - the gradient of the distribution of the yolk in the cytoplasm.

Bilateral symmetry becomes apparent as soon as organ formation begins during embryonic development. In higher animals, almost all organs are laid in pairs. This applies to the eyes, ears, nostrils, lungs, limbs, most muscles, skeletal parts, blood vessels and nerves. Even the heart is laid down as a paired structure, and then its parts merge, forming one tubular organ, which subsequently twists, turning into the heart of an adult with its complex structure. Incomplete fusion of the right and left halves of the organs is manifested, for example, in cases of cleft palate or cleft lip, which occasionally occur in humans.

Metamerism (dismemberment of the body into similar segments).

The greatest success in the long process of evolution was achieved by animals with a segmented body. The metameric structure of annelids and arthropods is clearly visible throughout their life. In most vertebrates, the initially segmented structure later becomes hardly distinguishable, however, at the embryonic stages, their metamerism is clearly expressed.

In the lancelet, metamerism is manifested in the structure of the coelom, muscles and gonads. Vertebrates are characterized by a segmental arrangement of some parts of the nervous, excretory, vascular and supporting systems; however, already at the early stages of embryonic development, this metamerism is superimposed by the advanced development of the anterior end of the body - the so-called. cephalization. If we consider a 48-hour chicken embryo grown in an incubator, we can simultaneously reveal both bilateral symmetry and metamerism in it, which is most clearly expressed at the anterior end of the body. For example, groups of muscles, or somites, first appear in the head region and form sequentially, so that the least developed segmented somites are posterior.

Organogenesis.

In most animals, the alimentary canal is one of the first to differentiate. In essence, the embryos of most animals are a tube inserted into another tube; the inner tube is the intestine, from the mouth to the anus. Other organs that are part of the digestive system, and the respiratory organs are laid in the form of outgrowths of this primary intestine. The presence of the roof of the archenteron, or primary gut, under the dorsal ectoderm causes (induces), possibly together with the notochord, the formation on the dorsal side of the embryo of the second most important body system, namely the central nervous system. This happens as follows: first, the dorsal ectoderm thickens and the neural plate forms; then the edges of the neural plate rise, forming neural folds that grow towards each other and eventually close, - as a result, the neural tube, the rudiment of the central nervous system, appears. The brain develops from the front of the neural tube, and the rest of it turns into the spinal cord. Neural tube cavity as it grows nervous tissue almost disappears - only a narrow central channel remains from it. The brain is formed as a result of protrusions, protrusions, thickenings and thinnings of the anterior part of the neural tube of the embryo. Paired nerves originate from the formed brain and spinal cord - cranial, spinal and sympathetic.

The mesoderm also undergoes changes immediately after its appearance. It forms paired and metameric somites (muscle blocks), vertebrae, nephrotomes (rudiments of excretory organs) and parts of the reproductive system.

Thus, the development of organ systems begins immediately after the formation of the germ layers. All development processes (under normal conditions) occur with the accuracy of the most advanced technical devices.

METABOLISM OF GERMS

Embryos developing in an aquatic environment do not require any other integument, except for the gelatinous shells that cover the egg. These eggs contain enough yolk to provide nourishment for the embryo; shells protect it to some extent and help retain metabolic heat and, at the same time, are sufficiently permeable so as not to interfere with free gas exchange (i.e., the supply of oxygen and the release of carbon dioxide) between the embryo and the environment.

Extra-embryonic membranes.

In animals that lay eggs on land or are viviparous, the embryo needs additional shells that protect it from dehydration (if eggs are laid on land) and provide nutrition, removal of end products of metabolism and gas exchange.

These functions are performed by extraembryonic membranes - amnion, chorion, yolk sac and allantois, which are formed during development in all reptiles, birds and mammals. Chorion and amnion are closely related in origin; they develop from the somatic mesoderm and ectoderm. Chorion - the outermost shell surrounding the embryo and three other shells; this shell is permeable to gases and gas exchange occurs through it. The amnion protects the cells of the fetus from drying out thanks to the amniotic fluid secreted by its cells. The yolk sac filled with yolk, together with the yolk stalk, supplies the embryo with digested nutrients; this shell contains a dense network of blood vessels and cells that produce digestive enzymes. The yolk sac, like the allantois, is formed from the splanchnic mesoderm and endoderm: the endoderm and mesoderm spread over the entire surface of the yolk, overgrowing it, so that in the end the entire yolk is in the yolk sac. In reptiles and birds, allantois serves as a reservoir for the end products of metabolism coming from the kidneys of the embryo, and also provides gas exchange. In mammals, these important functions are performed by the placenta, a complex organ formed by chorionic villi, which, growing, enter the recesses (crypts) of the uterine mucosa, where they come into close contact with its blood vessels and glands.

In humans, the placenta fully provides the respiration of the embryo, nutrition and the release of metabolic products into the mother's bloodstream.

Extraembryonic membranes are not preserved in the postembryonic period. In reptiles and birds, when they hatch, the dried shells remain in the egg shell. In mammals, the placenta and other extraembryonic membranes are shed from the uterus (rejected) after the birth of the fetus. These shells provided the higher vertebrates with independence from the aquatic environment and, undoubtedly, played important role in the evolution of vertebrates, especially in the emergence of mammals.

BIOGENETIC LAW

In 1828, K. von Baer formulated the following provisions: 1) the most common signs of any large group of animals appear in the embryo earlier than the less common signs; 2) after the formation of the most common features, less common ones appear, and so on until the appearance of special features characteristic of this group; 3) the embryo of any animal species, as it develops, becomes less and less similar to the embryos of other species and does not go through the later stages of their development; 4) the embryo of a highly organized species may resemble the embryo of a more primitive species, but never resembles the adult form of this species.

The biogenetic law formulated in these four propositions is often misunderstood. This law simply states that certain stages of development of highly organized forms have a clear resemblance to certain stages of development of forms lower on the evolutionary ladder. It is assumed that this similarity can be explained by descent from a common ancestor. Nothing is said about the adult stages of the lower forms. In this article, similarities between germline stages are implied; otherwise, the development of each species would have to be described separately.

Apparently, in the long history of life on Earth, the environment played a major role in the selection of embryos and adult organisms most adapted for survival. The narrow limits created by the environment with regard to possible fluctuations in temperature, humidity and oxygen supply reduced the variety of forms, bringing them to a relatively common type. As a result, that similarity of structure arose, which underlies the biogenetic law, if we are talking about the embryonic stages. Of course, in the process of embryonic development, in the currently existing forms, features appear that correspond to the time, place and methods of reproduction of this species.

Literature:

Carlson b. Fundamentals of Embryology according to Patten, vol. 1. M., 1983
Gilbert S. developmental biology, vol. 1. M., 1993