What types of fertilization do animals have? Fertilization. In iso-, hetero- and many oogamous lower plants, gametes emerge from the gametangia into the water, where fertilization occurs

In higher animals, male and female reproductive cells, or gametes, are used to reproduce offspring. They are very different from each other and from the rest of the body's cells.

Women's sex cell called an egg. It is large, oval or spherical shape, contains a large amount of deutoplasm (yolk) – construction material, due to which in mammals (except for oviparous animals: the echidna and the platypus), the egg develops at the first moment after fertilization, and in other animals until the embryo fully matures. That is why the cells of the latter reach a truly gigantic size.

The largest eggs of all animals currently living on Earth, if we take them with all the membranes, which in birds include the shell and protein, are ostrich eggs. The weight of this cage is 2–3 kilograms. Extinct prehistoric reptiles and the passenger pigeon, which lived on Earth relatively recently, had an egg volume the size of a bucket. The human egg is one of the smallest, its diameter is 0.2–0.5 millimeters. In invertebrates, even smaller eggs are known, the diameter of which barely reaches 0.04 millimeters.

The eggs mature in the female genital organs - the ovaries. In humans they are formed in early childhood, by about two and a half years, and later do not change. It was possible to calculate that each ovary at this time contained 30,000 eggs. In more early period there are several more of them, but starting from the sixth month of intrauterine life in the ovary of the embryo, some of them begin to mature, however full development don't reach.

A maturing egg divides twice and loses half of its chromosomes. Only after the girl reaches puberty can the egg reach full development and ovulation occurs, the release of the egg from the ovary. During a woman's life, only a little more than 400 eggs mature, 13 in one year.

Male reproductive cells - spermatozoa - are very unique and unlike other cells in the body. The sperm of different animals differ from each other in appearance. What they have in common is that they are always smaller than an egg and all of them are characterized by mobility.

A mammalian sperm consists of a small head and a long tail, with which it moves. The length of a human sperm is 50–70 microns, and its heads are only 4–5 microns. The spermatozoa of lower animals are especially complex. They are often equipped with a perforator in the form of an awl, drill, chisel or corkscrew for opening the egg shell, as well as blades, fins and other devices.

Spermatozoa develop in the male gonads - testes, which in vertebrates are paired and located in the body cavity. Only in humans and some mammals they are located in special sacs directly under the skin. The exit of the testes occurs during the intrauterine life of the fetus. If for some reason this does not happen, then sperm will not develop in such testes. It is believed that this happens due to high temperature inside the abdominal cavity. In any case, elephants, whose testes lie in the body cavity and the temperature is quite high, during the mating season they rise high into the mountains, where it is cold. Without this, conception is impossible. When these giants are brought to our cold north, they often give birth to offspring in the very first years. But we must remember that even in their homeland, the breeding of elephants in captivity is far from common.

The male testes have about a thousand convoluted tubules each. The walls of the tubules contain large oval cells, the division of which produces sperm. During division, just like female gametes, they lose half of their chromosomes, and then undergo a complex process of morphological reconstruction, turning from an ordinary oval cell into a mature sperm.

Sperm that have not yet acquired the ability to move are pushed through the tubules into the epididymis, which is a highly convoluted tube, where the sperm are stored in the seminal fluid, which contains the necessary nutrients: glucose and fructose.

Regardless of where the sperm meets the egg, whether in the female's reproductive tract or outside the body, each individual sperm has very little chance of reaching the egg. Indeed, in the female genital tract, the human sperm needs to travel a long distance, and the speed of movement does not exceed 1.5–3 millimeters per minute.

To ensure the reliability of the meeting of two sex cells, nature had to go along the line of using huge armies of sperm, even in cases where only one egg needs to be fertilized. So, during sexual intercourse in reproductive system women where in best case scenario There may be only one mature egg, but 200 or more million sperm are introduced.

The matter is further complicated by the fact that reproductive cells (both female and male) are very delicate and short-lived. The human egg dies a day after ovulation; sperm in the woman’s genital tract live somewhat longer, 24–48 hours.

But the point is not only in the duration of the existence of the sexual elements, it is also important how long they are capable of fertilization. The shell of salmon eggs, once in the water, hardens so much that the sperm can no longer penetrate through it. And the sperm themselves retain the ability to move in water for a very short time: in salmon - 45 seconds, and in brook trout only 23 seconds. For such short term and the meeting of both cells must occur. Therefore, when artificial breeding Salmon eggs at fish factories are pre-mixed with sperm and only a little later transferred to the water.

The lifespan and ability of sperm to move can be significantly increased if they are kept without water. In a “dry” form, the sperm of some fish can be stored for one to two weeks, and sometimes more.

In some animals, sperm is stored in the female genital tract for a very long time. Mating of bats occurs during wintering, but fertilization does not occur at this moment. Sperm introduced into the female's body is stored in the genital tract until spring. Snail sperm can be stored for years. In bees, mating occurs once in a lifetime. The sperm is stored in a special sac connected to the genital tract. When the queen bee lays eggs, she voluntarily opens the sphincter of the pouch and allows sperm to fertilize the laid eggs. If laying is done with the sphincter closed, the eggs are unfertilized.

How does a sperm find an egg? Little is known about this now. Due to the huge numbers of sperm, encounters with eggs can occur as a result of random collisions. Special devices are also known. The eggs of some animals contain special substances that, released into the environment in completely insignificant quantities, either extend the life of sperm or force them to move towards the source of this substance.

The existence of huge armies of sperm is not necessary. Where the structure of the reproductive apparatus greatly facilitates the meeting of the sperm with the egg, animals manage a small amount male reproductive cells. Thus, in some lower crustaceans belonging to the daphnids, two eggs are stored in a close brood chamber, where sperm enter during mating, after which the hole through which they are introduced is closed. The spermatozoa of these daphnids are very large, inactive and, what is especially interesting, there are few of them. During mating, no more than five sperm enter the brood chamber, and in total the male has no more than 20 sperm.

Fertilization begins with the attachment of a sperm to the egg shell. Now he must get inside. This is prevented by the shell of the egg. In some animals, such as echinoderms and amphibians, it is very thick. Often it is completely impassable for sperm, with the exception of a narrow canal called the “micropyle”; Only along it can a sperm penetrate into the egg. The efforts that sperm develop in this process are enormous. You can observe how large eggs of sea animals, in comparison with which sperm are negligible, begin to slowly move or rotate under the friendly pressure of their thousands of armies surrounding the egg in a dense ring.

The human egg, in addition to its own shell, is surrounded by a layer of cells - the corona radiata and is therefore inaccessible to one sperm. Only the combined efforts of several hundred thousand sperm can lead to a breakthrough of this barrier, destroying, with the help of a special enzyme hyaluronidase, contained in minute quantities in their heads, the substance that solders the cells of the corona radiata together. Only then will one of the sperm be able to penetrate the egg.

Penetration into the egg immediately causes a number of changes in it, first of all the shell: in a short period of time it becomes so dense that no sperm can penetrate through it anymore. The appearance of this membrane ensures that the egg is fertilized by only one sperm.

In this case, the egg of the egg fuses with the nucleus of the sperm that penetrates inside. Thus, the nucleus of a new cell, resulting from the fusion of male and female gametes, already contains the full number of chromosomes. Then the egg begins to divide.

In some cases, the emergence of the fertilization membrane is delayed, and due to this, several sperm penetrate the egg. When their nuclei merge with the egg core total there are more chromosomes than normal. However, for some organisms, the penetration of several sperm into the egg is normal occurrence. However, in this case, only one of them merges with the nucleus of the female cell. The rest die near the surface of the nucleus, and their substance is used only to nourish the egg.

Only exclusively in rare cases Several sperm may take part in the fusion. Such an egg usually develops incorrectly and soon dies. However, in insects, birds and some other animals, under artificial conditions, it is possible to grow individual specimens obtained from the fusion of an egg with several sperm to adulthood.

Polyploid animals, that is, having several sets of chromosomes, also arise during normal fertilization of an egg by one sperm, if the process of subsequent fragmentation of the egg is disrupted. Polyploidy is especially widespread in plants. The cells of polyploid plants are larger, resulting in a greatly increased plant size. All cultivated plants are polyploid. Cases of polyploidy in animals are much rarer. This may be due to confusion in determining gender. In this case, normal chromosome divergence does not occur, the division of the egg is disrupted, and it dies. Only in same-sex animals does polyploidy occur easily.

Fertilization is a specific reaction. This means that fusion can occur between germ cells of animals belonging to the same or very similar species. Fertilization of an egg by the sperm of unrelated animals occurs only as an exceptional phenomenon.

Another feature of fertilization is that it is irreversible. If the sperm that has penetrated the egg even dies for some reason, it can continue development and fragmentation as if nothing had happened. The development of the egg will continue if the sperm that has penetrated it is carefully removed from there. Not a single sperm will be able to enter this egg again. The embryos developing from such incompletely fertilized eggs die in the early stages of development and only sometimes reach adulthood. The ability of an egg to develop after death or removal of the sperm that has penetrated inside is very important property. Thanks to this ability, amazing things can happen to the egg.

External fertilization carried out in environment, usually in aquatic conditions where male and female reproductive cells are found. An example is fertilization in most animals that live or reproduce in water: annelids, bivalves, most fish, tailless amphibians. The male and female gametes released by these organisms enter the water, where they meet and merge—the formation of a zygote.

With external fertilization the meeting of the egg and sperm depends on the most various factors external environment, therefore, with this type of fertilization, organisms usually form great amount germ cells. For example, the lake frog lays up to 11 thousand eggs, the Atlantic herring lays about 200 thousand eggs, and the sunfish - almost 30 million.

Internal fertilization

Internal fertilization- the meeting and fusion of gametes occurs in the female genital tract. In this case, the probability of fertilization and survival of the zygote is much higher, so much fewer reproductive cells (especially eggs) are formed. Internal fertilization is common to many aquatic organisms, and on land it becomes the only reliable way to ensure the fusion of gametes. With internal fertilization, the zygote gets the opportunity to develop while remaining in the mother's body

Internal fertilization in many animals (reptiles, birds) is accompanied by the laying of eggs during external environment, where over a certain period of time small cubs develop from eggs: chicks, baby turtles, crocodiles etc. In most mammals, the zygote and the embryo formed from it undergo internal development in the female genitals. In mammals (except oviparous - platypus And echidnas) to grow the embryo (embryo), the so-called children's place or placenta. It is present in the form of rudiments even in marsupials. Through the placenta, a connection is established between the bloodstreams of the embryo and the female. Thanks to this, gas exchange in the body of the embryo is ensured, its nutrition and removal of decay products and, of course, protection of the embryo from unfavorable conditions external environment.

Internal fertilization in animals - a process that arose during evolution later than external fertilization, and a much more progressive morphobiological phenomenon. The same should be noted about the appearance of the placenta in the history of the development of the animal world. They ensure healthy reproduction younger generation with significant protection, preservation (and saving) of germ cells of reproducing organisms and mother’s care for the development of embryos.

With internal fertilization the zygote gets the opportunity to develop while remaining in the mother's body.

The number of germ cells that the body produces also depends on the degree of parental care for the offspring. For example, cod lays 10 million eggs and never returns to the laying site, the African tilapia fish, which carries eggs in its mouth, produces no more than 100 eggs, and mammals with complex parental behavior that provides care for their offspring give birth to only one or several young .

In humans, as in all other mammals, fertilization occurs in the oviducts, through which the egg moves towards the uterus. Sperm overcome huge distance before meeting the egg, and only one of them penetrates the egg. After penetration of the sperm, the egg forms a thick shell on the surface, impenetrable to other sperm.

If fertilization has occurred, the egg completes its meiotic division (§ 3.6) and the two haploid nuclei fuse in the zygote, combining the genetic material of the paternal and maternal organisms. A unique combination of genetic material of a new organism is formed.

The eggs of most mammals retain the ability to fertilize for a limited time after ovulation, usually no more than 24 hours. Sperm that leave the male reproductive system also live very short. So, in most fish, sperm die in water after 1-2 minutes; in the genital tract of a rabbit they live up to 30 hours, in horses 5-6 days, and in birds up to 3 weeks. Human sperm in a woman’s vagina die after 2.5 hours, but those that manage to reach the uterus remain viable for two or more days. Exist in nature and exceptional cases For example, bee sperm retain the ability to fertilize in the spermatheca of females for several years.

A fertilized egg can develop in the body of the mother's body, as occurs in placental mammals, or in the external environment, as in birds and reptiles. In the second case, it is covered with special protective shells (eggs of birds and reptiles).

Occurs in some species of organisms special shape sexual reproduction - without fertilization. This development is called parthenogenesis (from the Greek partenos - virgin, genesis - emergence), or virgin development. In this case, the daughter organism develops from an unfertilized egg based on the genetic material of one of the parents, and individuals of only one sex are formed. Natural parthenogenesis makes it possible to sharply increase the number of offspring and exists in those populations where contact of different-sex individuals is difficult. Parthenogenesis occurs in animals of different systematic groups: bees, aphids, lower crustaceans, rock lizards, and even some birds (turkeys).

One of the main mechanisms that ensures fertilization strictly within a species is the correspondence of the number and structure of chromosomes of female and male gametes, as well as the chemical affinity of the cytoplasm of the egg and the nucleus of the sperm. Even if foreign germ cells unite during fertilization, this, as a rule, leads to abnormal development of the embryo or to the birth of sterile hybrids, i.e., individuals incapable of childbearing.

Biology. General biology. Grade 10. Basic level Sivoglazov Vladislav Ivanovich

21. Fertilization

21. Fertilization

Remember!

What set of chromosomes does a zygote have?

Which animals are characterized by external fertilization?

In which organisms does double fertilization occur?

To carry out sexual reproduction, it is not enough for the body to simply form sex cells - gametes; it must be ensured that they can meet. The process of fusion of a sperm and an egg, accompanied by the combination of their genetic material, called fertilization . As a result of fertilization, a diploid cell is formed - zygote, activation and further development of which leads to the formation of a new organism. When the germ cells of different individuals merge, cross fertilization, and when combining gametes produced by one organism - self-fertilization.

There are two main types of fertilization - external (external) and internal.

External fertilization. During external fertilization, the sex cells fuse outside the female's body. For example, fish release eggs (eggs) and milt (sperm) directly into the water, where external fertilization occurs. Reproduction occurs in a similar way in amphibians, many mollusks and some worms. With external fertilization, the meeting of the egg and sperm depends on a variety of environmental factors, therefore, with this type of fertilization, organisms usually form a huge number of germ cells. For example, the lake frog lays up to 11 thousand eggs, the Atlantic herring lays about 200 thousand eggs, and the sunfish - almost 30 million.

Internal fertilization. During internal fertilization, the meeting of gametes and their fusion occurs in the female reproductive tract. Thanks to the coordinated behavior of the male and female and the presence of special copulatory organs, male reproductive cells enter directly into female body. This is how fertilization occurs in all terrestrial and some aquatic animals. In this case, the probability of successful fertilization is high, therefore such individuals have much fewer germ cells.

The number of germ cells that the body produces also depends on the degree of parental care for the offspring. For example, cod spawns 10 million eggs and never returns to the laying site, the African tilapia fish, which carries eggs in its mouth, produces no more than 100 eggs, and mammals with complex parental behavior that provides care for their offspring give birth to only one or several cubs.

In humans, as in all other mammals, fertilization occurs in the oviducts, through which the egg moves towards the uterus. Sperm travel a huge distance before meeting the egg, and only one of them penetrates the egg. After penetration of the sperm, the egg forms a thick shell on the surface, impenetrable to other sperm.

If fertilization has occurred, the egg completes its meiotic division (§ 20) and the two haploid nuclei fuse in the zygote, combining the genetic material of the paternal and maternal organisms. A unique combination of genetic material of a new organism is formed.

The eggs of most mammals retain the ability to fertilize for a limited time after ovulation, usually no more than 24 hours. Sperm that leave the male reproductive system also live very short. Thus, in most fish, sperm die in water after 1–2 minutes; in the genital tract of a rabbit they live for up to 30 hours, in horses for 5–6 days, and in birds up to 3 weeks. Human sperm in a woman’s vagina die after 2.5 hours, but those that manage to reach the uterus remain viable for two or more days. There are also exceptional cases in nature, for example, bee sperm retain the ability to fertilize in the sperm receptacle of females for several years.

A fertilized egg can develop in the body of the mother's body, as occurs in placental mammals, or in the external environment, as in birds and reptiles. In the second case, it is covered with special protective shells (eggs of birds and reptiles).

In some species of organisms, a special form of sexual reproduction occurs - without fertilization. This development is called parthenogenesis(from Greek partenos– virgin, genesis– emergence) or virgin development. In this case, the daughter organism develops from an unfertilized egg based on the genetic material of one of the parents, and individuals of only one sex are formed. Natural parthenogenesis makes it possible to sharply increase the number of offspring and exists in those populations where contact between individuals of different sexes is difficult. Parthenogenesis occurs in animals of different systematic groups: bees, aphids, lower crustaceans, rock lizards, and even some birds (turkeys).

One of the main mechanisms that ensures fertilization strictly within a species is the correspondence of the number and structure of chromosomes of female and male gametes, as well as the chemical affinity of the cytoplasm of the egg and the nucleus of the sperm. Even if foreign germ cells unite during fertilization, this, as a rule, leads to abnormal development of the embryo or to the birth of sterile hybrids, that is, individuals incapable of childbearing.

Double fertilization. A special type of fertilization is characteristic of flowering plants. It was opened at the end of the 19th century. Russian scientist Sergei Gavrilovich Navashin and received the name double fertilization(Fig. 67).

During pollination, pollen lands on the stigma of the pistil. The pollen grain (male gametophyte) consists of only two cells. The generative cell divides, forming two immobile sperm, and the vegetative cell, growing inside the pistil, forms a pollen tube. In the ovary of the pistil, the female gametophyte develops - an embryo sac with eight haploid nuclei. Two of them fuse to form a central diploid nucleus. As a result of further division of the cytoplasm of the embryo sac, seven cells are formed: an egg cell, a central diploid cell and five auxiliary cells.

Rice. 67. Double fertilization in flowering plants

After the pollen tube grows into the base of the pistil, the sperm inside it penetrate into the embryo sac. One sperm fertilizes the egg, resulting in a diploid zygote; from it the embryo subsequently develops. Another sperm fuses with the nucleus of a large central diploid cell, forming a cell with a triple chromosome set(triploid), from which the endosperm is then formed - the nutritious tissue for the embryo. Thus, in angiosperms, two sperm are involved in fertilization, i.e., double fertilization occurs.

Artificial insemination. Great importance in modern agriculture has artificial insemination, a technique that is widely used in selection for breeding and improving animal breeds and plant varieties. In animal husbandry, artificial insemination can produce numerous offspring from one outstanding sire. The sperm of such animals is stored in special low-temperature conditions and remains viable for a long time (tens of years).

Artificial pollination in plant growing makes it possible to carry out certain, pre-planned crossings and obtain plant varieties with the required combination of parental properties.

IN modern medicine infertility treatment uses artificial insemination with donor sperm and in vitro (out-of-body) fertilization - a method first developed in 1978 and known as the “test tube baby”. This method involves fertilizing eggs outside the body and then transferring them back into the uterus to continue normal development.

By 2010, about 4 million children had already been conceived using in vitro fertilization. However, the use of donor sperm donor eggs and even surrogate mothers give rise to a whole range of ethical and social problems. Many people, based on religious and moral considerations, oppose any interventions in human reproduction, including in vitro and artificial fertilization.

Review questions and assignments

1. What is fertilization?

2. What types of fertilization do you know?

3. What is the process of double fertilization?

4. What is the importance of artificial insemination in crop and livestock production?

Think! Do it!

1. What do you think is the advantage of double fertilization in angiosperms compared to fertilization in gymnosperms?

2. Is it enough to know that only one individual participates in reproduction to conclude that this reproduction is asexual?

3. Explain why twins are often born with in vitro fertilization.

4. Organize and conduct a discussion " In Vitro Fertilization: pros and cons".

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

Repeat and remember!

Plants

Pollination. Double fertilization in flowering plants is preceded by pollination– transfer of pollen (pollen grains) on the stigma. Pollination is carried out different ways. If pollen from a flower lands on the stigma of the same flower, selfing. The transfer of pollen to the stigma of another flower is called cross pollination.

Self-pollination is characteristic of a small number of flowering plants. Scientists believe that self-pollination arose secondarily when some circumstances began to prevent cross-pollination from occurring. Biologically, self-pollination is less beneficial because it does not exchange genetic information between different individuals of the species.

Cross-pollination is much more common in angiosperms than self-pollination. Biologically, cross-pollination is more beneficial than self-pollination because it allows the genetic information of different individuals to be combined. Descendants appear that differ from the parent individuals. This helps the species adapt to changing environmental conditions.

Cross pollination can be done in a variety of ways. Conventionally, they can be divided into two groups: abiotic pollination (using wind or water) and biotic pollination (using animals). Various animals can act as pollinators: insects, birds, mammals.

8.2.1.1. Artificial insemination in dog breeding The use of artificial insemination in dog breeding is currently limited mainly by the lack of appropriate regulatory documents taking into account the origin of animals and a certain conservatism