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The number and size of germ cells vary in different animals. The following pattern is observed: the less likely the egg and sperm meet, the greater the number of germ cells formed in the body. For example, fish lay eggs (eggs) and sperm directly into the water (external insemination occurs), and the number of eggs in some of them reaches an enormous amount (cod lays about 10 million eggs). During internal fertilization, thanks to the coordinated behavior of the male and female, male reproductive cells enter directly into the female body. In this case, the probability of fertilization is very high and, as a result, the number of germ cells decreases sharply. The number of germ cells produced is greatly reduced in those parents who care for their offspring. Thus, the number of eggs in viviparous fish does not exceed several hundred, and solitary wasps, which provide future larvae with food - paralyzed insects, lay only about ten eggs. Many other factors influence the number of eggs produced. In particular, there is a relationship between the size of eggs and their number - the larger the eggs, the fewer there are (birds). The fertilization process consists of several stages: penetration of the sperm into the egg, fusion of the haploid nuclei of both gametes to form a diploid cell - the zygote and its activation for fragmentation and further development. Most animals' eggs must be fertilized almost immediately after ovulation. In most mammals, the egg usually retains the ability to fertilize for 24 hours, and in humans - 12-24 hours after ovulation. Sperm that are outside the male reproductive system usually live for a very short time. Thus, trout sperm die in water within 30 seconds, in the genital tract of chickens sperm live for 30-40 days, in the uterus and oviducts of a woman - 5-8 days, and in the spermatozoa of female bees, sperm retain the ability to fertilize for a year or more. The location of the egg by the sperm and their interaction is ensured by special substances - gamons produced by germ cells. It is believed that there are at least two types gynogamonov- substances secreted by eggs (one activates the movement of sperm, the other agglutinates them), and two types androgamon, secreted by male reproductive cells (one paralyzes sperm motility, the other dissolves the egg shell). Fertilization occurs only at a certain concentration of sperm ( rice. 76). It has been shown in rabbits that fertilization does not occur both when less than 1000 sperm are involved in the insemination of a female, and when there are more than 100 million of them. This is explained by an insufficient or excessive amount of secreted hyaluronidase- enzyme.

Using a frog as an example, let's look at how fertilization occurs in animals. The unfertilized egg is covered with several protective shells that protect it from external influences. Spermatozoa actively move in water and, when meeting an egg, with the help of hyaluronidase secreted by the acrosome, they dissolve its membranes and penetrate into the cell. As soon as one sperm penetrates the egg, its membranes acquire properties that prevent the penetration of other sperm, and the egg begins to prepare for division.

Experiments show that to induce the egg to fragment, it is not at all necessary for the sperm to penetrate the egg; their surface interaction is sufficient. If you use a micropipette to pull back the sperm that has begun to penetrate the egg, fragmentation may begin. Conversely, if a sperm is introduced directly into the egg with a micropipette, then activation will not occur. In some species, in particular in the silkworm, several sperm can penetrate the egg, but normally only one of them merges with the nucleus of the egg, the rest die.

§22. Fertilization and embryonic development in animals

Remember from the textbook “Animals” how this happens. reproduction in animals. How are eggs fertilized and where do the embryos of insects, fish, amphibians, birds and mammals develop?

The process of formation of male and female germ cells precedes sexual reproduction, that is, reproduction with the participation of sperm and eggs. Sexual reproduction can occur with or without fertilization.

Fertilization. The process of fusion of the nuclei of male and female germ cells is called fertilization. As a result of fertilization, a zygote is formed (from the Greek zygote - united together) - a fertilized egg. Its nucleus always has a double set of chromosomes. From the zygote an embryo develops, which gives rise to a new organism.

The fertilization process begins with the penetration of the sperm into the egg. Under the action of enzymes in the sperm vesicle, the shell of the egg at the point of contact dissolves. The sperm nucleus enters the egg (Fig. 87). In this case, the shell of the egg becomes impermeable to other sperm. After this, the gamete nuclei fuse and the zygote nucleus is formed.

Rice. 87. Fertilization in animals

There are two methods of fertilization - external and internal. During external fertilization, the female releases eggs (spawn), and the male releases sperm into the external environment, where fertilization occurs. This method is typical for aquatic animals, such as fish and amphibians. During internal fertilization, the fusion of gametes occurs in the female genital tract. This is how terrestrial and some aquatic inhabitants, such as insects, reptiles, birds and mammals, reproduce.

A fertilized egg can develop both in the female body, for example in mammals, and in the external environment. In the latter case, the eggs are covered with a special membrane or shell, and the female lays them in a safe place, for example in a nest (in birds).

Parthenogenesis. A type of sexual reproduction, when the development of an adult occurs from an unfertilized egg, is called parthenogenesis (from the Greek parthenos - virginity). Parthenogenesis occurs in crustaceans (daphnia), insects (bees, aphids), and some birds (turkeys) (Fig. 88). Development without fertilization most often alternates with normal sexual reproduction. From unfertilized eggs, cells begin to develop in which, in the first division of mitosis, the chromosomes do not diverge and a double set of chromosomes is restored.

Rice. 88. Animals capable of parthenogenesis: 1 - daphnia; 2 - bees

Ontogenesis of the organism and embryonic development. The individual development of an organism - ontogenesis (from the Greek ontos - existence and genesis - birth) covers the entire period of its life. During this time, the body goes through several successive stages: an embryo is formed, a new organism is born, it grows, develops, reproduces, ages and dies. Ontogenesis is divided into two periods - embryonic and postembryonic.

The embryonic period, or embryogenesis (from the Greek embryo - embryo and genesis) lasts from the moment of formation of the embryo from the zygote until its exit from the egg or birth. It occurs in several stages.

After fertilization, the embryo begins to develop from the zygote. A fertilized egg is divided by mitosis into 2, then 4, 8, 16, etc. cells. This process is called fragmentation, since, unlike ordinary division, the cells do not enlarge, that is, they do not grow (Fig. 89). The crushing cells use the nutrients accumulated in the egg. The process goes very quickly. For example, within 4 hours from the moment of fertilization, 64 cells arise from one cell of the zygote.

Rice. 89. Stages of development of the embryo of a chordate animal: 1 - crushing; 2 - blastula; 3 - invagination and formation of two layers of cells; 4 - gastrula: 5 - organogenesis (a - neural tube; b - notochord; c - digestive tube)

Fragmentation ends with the formation of a blastula (from the Greek blastos - sprout), an embryonic vesicle with a cavity inside. The walls of the vesicle consist of a single layer of cells (Fig. 89).

After the formation of the blastula, the second stage of development of the embryo begins - gastrula (from the Greek gaster - stomach). It is a two-layer bag. Its formation begins with the invagination of the lower wall of the blastula into the cavity. As a result, two germ layers are formed: the outer one - ectoderm (from the Greek ectos - outside and dermis - skin) and the inner one - endoderm (from the Greek entos - inside).

Rice. 90. Crushing frog eggs

At the gastrula stage, development ends in sponges and coelenterates. In more highly organized multicellular animals, the formation of the third germ layer further occurs. Between the ectoderm and endoderm is laid the mesoderm (from the Greek mesos - middle, intermediate and dermis). It is formed due to the movement of some cells from the outer and inner layers. As a result, a three-layer embryo is formed. At the same time, at this stage, axial organs are formed, for example, in chordates, the neural tube, notochord and digestive tube.

The subsequent development of the embryo in chordates is associated with the interaction of three germ layers, from which all the tissues and organs of the future organism develop. The stage of organ formation in the embryo is called organogenesis.

From the ectoderm, epithelial and nervous tissues, the epidermis of the skin and its derivatives (nails, hair), as well as the nervous system and sensory organs develop. The mucous epithelium and digestive organs are formed from the endoderm. Muscle and all types of connective tissue are formed from the mesoderm. From the notochord, in most chordates, a cartilaginous or bony skeleton is subsequently formed, and from the lateral sections of the mesoderm - muscles, blood vessels, heart, kidneys and organs of the reproductive system.

The influence of various factors on the development of the embryo. All cells of the embryo develop from one initial cell - the zygote (Fig. 90) and have the same set of chromosomes and genetic information. However, in the cells of different germ layers, the hereditary information of different genes is realized, which leads to the biosynthesis of different proteins and, consequently, to the formation of different tissues and organs from them.

The specificity of cell functioning does not arise immediately, but at a certain stage of embryogenesis. It has been established that at the stage of 2-16 cells (depending on the type of animal), each cell can develop into a normal organism. If these cells are separated, then an independent organism is formed from each - identical twins arise. They look alike and are always the same sex (Fig. 91).

Rice. 91. Formation of identical twins

Research has shown that there are critical periods in the development of animal embryos when normal development may be disrupted. For example, such periods are the middle of cleavage, the beginning of gastrulation and the stage of formation of axial organs. At this time, the embryo is especially sensitive to lack of oxygen, temperature changes, and mechanical stress. The better the egg is protected, the less susceptible it is to external influences. Some viral diseases, such as measles in humans, have a negative effect on the development of the embryo. A number of medications have the same effect, for example, antibiotics, hormonal drugs, drugs and alcohol. Powerful factors causing disturbances in the embryonic development of animals and humans are X-rays and radioactive radiation. Their impact can lead to the death of the embryo or the birth of organisms with deformities (Fig. 92).

Rice. 92. Violation of embryonic development can lead to the development of deformities: 1 - two-headed snake; 2 - Siamese twins Chang and Eng

Exercises based on the material covered

How does the process of fertilization occur in animals?
What are the advantages of internal fertilization compared to external fertilization?
Explain why some animals reproduce by parthenogenesis. Give examples.
What is the difference between cleavage and normal cell division?
At what stage of embryonic development does cell specialization in structure and function occur?
What environmental factors can have a negative impact on the development of the embryo in animals?

First stage -

For fertilization, complete release of the egg from the cells of the corona radiata is not necessary. There is only enough space for sperm to penetrate through the transparent membrane of the egg into the vitelline space.

Second stage -

Third stage

The head, neck carrying the centrosome, and the initial part of the tail penetrate into the oocyte.

Fertilization in animals and plants

This stage is strictly specific and highly selective: only sperm of their own species can penetrate into the egg.

fertilization membrane,

Fourth stage

In the zygote

blastomeres,

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Fertilization in animals goes through four stages.

First stage - liberation of the egg from the corona radiata and loosening of the transparent membrane under the influence of the enzyme hyaluronidase secreted by sperm.

This requires the participation of a large number (thousands) of sperm. Dispersal of corona radiata cells This is not a specific feature; it can occur under the influence of sperm from animals and other species.

For fertilization, complete release of the egg from the cells of the corona radiata is not necessary.

Fertilization in animals goes through four stages.

There is only enough space for sperm to penetrate through the transparent membrane of the egg into the vitelline space.

Second stage - penetration of sperm into the transparent membrane of the egg, where they accumulate up to several dozen: in cows and sheep - up to 100, in pigs - from 200 to 1000, in mares - up to 10.

Sperm from other (distant) animal species do not pass through the transparent membrane of the egg.

Third stage- penetration of sperm through the vitelline membrane into the protoplasm of the egg cell.

From the moment the sperm comes into contact with the plasmalemma of the egg, a protrusion of cytoplasm is formed on its surface - the receptive tubercle, or fertilization tubercle.

The head, neck carrying the centrosome, and the initial part of the tail penetrate into the oocyte. This stage is strictly specific and highly selective: only sperm of their own species can penetrate into the egg.

As a rule, one sperm penetrates through the vitelline membrane of the egg. Having penetrated the cytoplasm of the egg, the sperm undergoes great changes.

The head of the sperm is separated from the tail, swells, becomes rounded and takes the shape of a rounded nucleus - the male pronucleus.

After the sperm penetrates the egg, a fertilization membrane, preventing other sperm from entering the oocyte. Eggs secrete special substances - agglutinins. They glue other sperm together, and the follicular cells of the corona radiata absorb them.

Fourth stage- fusion of pronuclei (heminuclei) of the egg cell and sperm. This stage lasts 4-7 hours and ends with the formation of a qualitatively new cell (zygote) with a diploid, i.e. normal, or complete, as in somatic cells of the body, set of chromosomes for a given animal species.

In the zygote metabolic processes are sharply accelerated: oxygen absorption increases 3-4 times compared to an unfertilized egg; in the very first minutes, an increase in carbohydrate metabolism is noted, phosphate metabolism increases sharply (100 times or more), potassium and calcium metabolism increases 10 times or more;

In the zygote, a restructuring of the cytoplasm occurs and an intensive process of fragmentation begins. In this case, the zygote divides into two daughter cells - blastomeres, from two, 4 are formed, from four - 8, and so on, and the formation and development of a new organism begins.

Fertilization occurs in the third of the oviduct closest to the ovary. The zygote formed after fertilization gradually moves in the oviduct towards the uterus.

Polyspermy (refertilization) - penetration into the protoplasm of the egg by two or even several sperm. Normally, after one sperm penetrates the egg, others are not allowed through due to the so-called zonal reaction. In aging eggs, the zonal reaction is disrupted and several sperm can penetrate into the egg, with which the egg merges.

Polyspermy disrupts the course of embryogenesis, leads to the death of the zygote or causes the development of malformed fetuses.

Multiple fertilization (superfecundation) - fertilization of several eggs in one female with sperm from males of different breeds during the hunting period. It is often observed in multiparous animals (pigs, dogs), less often in cows and mares.

As a result of superfecundation, fruits with characteristics of different breeds are born. In livestock breeding practice, insemination of females in one heat by producers of different breeds is used to increase the fertility and viability of the offspring. For this purpose, during natural insemination, a female is mixed with two males with an interval of 10-15 minutes, and during artificial insemination, the sperm obtained from two or more males is mixed and the females are inseminated with the mixture.

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Fertilization is the process of fusion of germ cells. Fertilization is preceded by insemination.

Fertilization in animals (briefly)

Insemination is the process that ensures the meeting of sperm and eggs. There are:

1) external insemination, when sperm and eggs are released into the water, where they merge. Characteristic of aquatic animals (fish);

2) internal insemination, in which males, with the help of copulatory organs, introduce sperm into the reproductive tract of the female (terrestrial animals).

Fertilization leads to: a) the formation of a diploid zygote; b) the zygote is encouraged to develop.

When meeting an egg, the sperm releases the enzymes hyaluronidase and mucinase, which destroy the membranes of the egg, and the sperm penetrates the egg. After this, a fertilization membrane is formed, preventing the penetration of other sperm. The sperm nucleus swells for some time, reaching the size of the egg nucleus, they merge, the diploid set of chromosomes is restored - a zygote is formed.

After the formation of the zygote, ontogenesis begins - the individual development of the organism.

Ontogenesis - the process of development of an organism from the moment of formation of the zygote until death.

Ontogenesis is divided into two periods: embryonic (from the moment of formation of the zygote until exit from the egg membranes or birth) and postembryonic (from exit from the egg membranes or birth to death).

The embryonic period includes three stages: cleavage, gastrulation, and organogenesis.

1) Crushing - the crushing process begins with the formation of a zygote and ends with the formation of a blastula.

2) Gastrulation - from the blastula and ends with the formation of a two- or three-layer embryo - the gastrula.

3) Organogenesis - begins with the formation of a complex of axial organs, ends with the emergence from the egg membranes).

Splitting up– division leading to an increase in the number of cells without their growth. The cleavage process is characterized by rapidly successive mitotic divisions. The resulting cells are called blastomeres. This produces 2 - 4 - 8 - 16 - 32, etc. blastomeres. The interphase between divisions is very short, so the cells do not have time to grow to their previous sizes, resulting in a gradual decrease in the size of the blastomeres. Crushing is complete, when the entire egg is crushed (in eggs containing a small amount of yolk - mammals) and incomplete, when only part of the egg is crushed (in eggs containing a large amount of yolk - birds). In humans, there are two types of blastomeres: small ( trophoblast), which form a shell that provides nutrition to the embryo; and larger ones ( embryoblast), from which the embryo itself develops.

Rice. Crushing and development of the lancelet egg: a – fertilized egg; b-g – crushing; h – blastula; and – gastrulation; l – formation of the neural tube (neurula); m – histo- and organogenesis.

Blastula- This is a single-layer multicellular embryo with a cavity inside. The wall of the blastula is called the blastoderm (it is formed by cells - blastomeres), and the cavity inside is the blastocoel (primary body cavity).

The next stage is the formation of a two-layer embryo - gastrulation- in the lancelet it is carried out by invagination of the blastoderm into the cavity of the blastocoel. Gastrula has two layers of cells (germ layers): the outer - ectoderm and the inner - endoderm. The cavity of the gastrula is called the gastrocoel (primary intestine), and the entrance to the intestine is called the blastopore (primary mouth). Animals (worms, mollusks) in which a mouth opening develops in place of a blastopore are called protostomes. Chordates and echinoderms are classified as deuterostomes, since an anus is formed in place of the blastopore, and the mouth develops at the opposite end of the body. At the stage of two germ layers, bilayer animals (coelenterates) complete their development. In the rest, a third germ layer, the mesoderm, is formed between the ectoderm and endoderm. In chordates, this occurs by detaching mesodermal pockets from the endoderm.

Each germ layer, at the stage of histo- and organogenesis, gives rise to only certain organs.

From ectoderm develop: the spinal cord and brain, peripheral nerves; the epidermis of the skin and its derivatives (nails, hair, sebaceous and sweat glands, tooth enamel, sensory cells of the organs of vision, hearing, etc.).

From uh ntoderm epithelium develops: body cavities, digestive and respiratory systems, liver and pancreas, etc., as well as the notochord. The notochord in lower chordates is preserved throughout life, but in higher chordates it is replaced by a spine.

From mesoderm develop: skeletal muscles, all types of connective tissue, circulatory, excretory and reproductive systems.

Organogenesis is completed mainly by the end of the embryonic period of development. However, differentiation and complication of organs continues in the postembryonic period.

First stage - release of the egg from the corona radiata and loosening of the transparent membrane under the influence of the enzyme hyaluronidase secreted by sperm.

This requires the participation of a large number (thousands) of sperm. Dispersal of corona radiata cells - This is not a specific feature; it can occur under the influence of sperm from animals and other species.

It has been established in female rabbits that fertilization does not occur if less than 1000 sperm are introduced during insemination of a female, and also if there are excess sperm - more than 100 million, which is associated in the first case with a lack of hyaluronidase or with its excess in the second case.

Sperm from other (distant) animal species do not pass through the transparent membrane of the egg.

Third stage- penetration of sperm through the vitelline membrane into the protoplasm of the egg. When the germ cells come together, the enzymes in the acrosome of the sperm head destroy the secondary membrane of the egg. From the moment the sperm comes into contact with the plasmalemma of the egg, a protrusion of cytoplasm is formed on its surface - the receptive tubercle, or fertilization tubercle.

As a rule, one sperm penetrates the vitelline membrane of the egg. Having penetrated the cytoplasm of the egg, the sperm undergoes great changes.

The head of the sperm separates from the tail, swells, becomes rounded and takes the form of a rounded nucleus - the male pronucleus.

After the sperm penetrates the egg, a fertilization membrane is formed around it, preventing other sperm from penetrating the oocyte. Eggs secrete special substances - agglutinins. They glue other sperm together, and the follicular cells of the corona radiata absorb them.

Fourth stage- fusion of pronuclei (heminuclei) of the egg and sperm. This stage lasts 4-7 hours and ends with the formation of a qualitatively new cell (zygote) with a diploid, i.e. normal, or complete, as in the somatic cells of the body, for a given animal species, set of chromosomes: in somatic cells of cattle 60 , and in reproductives with a haploid (half) set (eggs and sperm) - 30 each, in horses - 66 and 33, respectively; in pigs - Z8 and 19 each; sheep - 54 and 27 each; in dogs - 22 and 11 each. The number of chromosomes remains constant from generation to generation. The resulting zygote has double inheritance (mother and father).

In the zygote, a restructuring of the cytoplasm occurs and an intensive process of fragmentation begins. In this case, the zygote divides into two daughter cells - blastomeres, from two 4 are formed, from four - 8 and so on, and the formation and development of a new organism begins.

Fertilization occurs in the third of the oviduct closest to the ovary. The zygote formed after fertilization gradually moves in the oviduct towards the uterus.