PhaseI I - an acrosomal reaction occurs in it. The outer membrane of the sperm ruptures, proteolytic enzymes are released and the ovum membrane dissolves. Plasma membranes merge, cytoplasms merge. The nucleus and centriole of the sperm pass into the cytoplasm of the egg. The tail is absorbed. Then the egg cell is activated, its potential changes and its vitelline membrane exfoliates and the fertilization membrane is formed (cortical reaction). Activation ends with the start of protein synthesis.

PhaseIII- syngamia... It contains:

    Stage 2 NS pronuclei - the male nucleus swells, takes the form of prophase, during this time the DNA doubles and the male pronucleus receives a haploid set of reduplicated chromosomes (n2c). The egg cell at the moment of meeting with the sperm is in the stage of meiosis, blocked by a special factor. After meeting with the sperm, the egg is activated and the block is removed. The egg nucleus, which has completed meiosis, turns into a female pronucleus, also acquiring a set of n2c chromosomes.

    The syncarion stage is the fusion of nuclear material and the formation of a zygote.

The first mitotic division of the zygote leads to the formation of two cells of the embryo (blastomeres) with a set of chromosomes 2n2c in each.

Parthenogenesis

a daughter organism sometimes develops from an unfertilized egg. This phenomenon is called virgin development or parthenogenesis... In this case, the source of hereditary material for the development of the offspring is usually the DNA of the egg - gynogenesis... Less commonly observed androgenesis- the development of a descendant from a cell with oocyte cytoplasm and sperm nucleus. The nucleus of the female gamete dies in the event of androgenesis.

Fertilization- the process of fusion of male and female gametes, leading to the formation zygotes. During fertilization, male and female haploid gametes interact, while their nuclei merge (pronuclei), chromosomes unite, and the first diploid cell of a new organism appears - zygote... The beginning of fertilization is the moment of fusion of the membranes of the sperm and the egg, the end of fertilization is the moment of unification of the material of the male and female pronuclei.

Fertilization takes place in the distal part of the fallopian tube and goes through 3 stages:

Stage I - distant interaction, includes 3 mechanisms:

· Chemotaxis - directed movement of spermatozoa towards the ovum (ginigamones 1,2);

Rheotaxis - the movement of sperm in the genital tract against the flow of fluid;

· Capacitation - an increase in the motor activity of spermatozoa, under the influence of factors of the female body (pH, mucus, and others).

Stage II - contact interaction, in 1.5-2 hours the spermatozoa approach the egg, surround it and lead to rotational movements, at a speed of 4 revolutions per minute. At the same time, spermatozoa are released from the acrosome of spermatozoa, which loosen the membranes of the egg. In the place where the shell of the egg becomes thinner, fertilization occurs as much as possible, the ovolemma protrudes and the head of the sperm enters the cytoplasm of the egg, bringing the centrioles with it, but leaving the tail outside.

Stage III - penetration, the most active spermatozoon sticks with its head into the egg, immediately after that, a fertilization membrane is formed in the cytoplasm of the egg, which prevents polyspermy. Then the fusion of the male and female pronuclei occurs, this process is called syncarion. This process (syngamia) is actually fertilization, it appears diploid zygote(a new organism, so far unicellular).

Conditions necessary for fertilization:

· The concentration of spermatozoa in the ejaculate, not less than 60 million in 1 ml;

· Patency of the female genital tract;

· Normal body temperature of a woman;

· Slightly alkaline environment in the female genital tract.

Fragmentation is a sequentially proceeding mitosis, without the growth of the formed cells, to the original size. During cleavage, a relatively rapid increase in the number of cells occurs. (blastomeres). The fragmentation continues until the ratio of the volume of the nucleus to the volume of the cytoplasm, which is characteristic of the given species, is restored. The number of blastomeres increases from 2 to about 12-16 by the third day after fertilization, when concept reaches the stage morula and goes into the uterine cavity from the fallopian tubes.

Distinguish crushing:

· Complete, incomplete;

· Uniform, uneven;

· Synchronous, asynchronous.

In humans, fragmentation is complete, asynchronous, and uneven. As a result of the first division, 2 blastomeres are formed, dark and light, light ones divide quickly and envelop the zygote from the outside - trophoblast, and the dark ones are inside and divide slowly - embryoblast. The fragmentation of the zygote in humans stops at the 107 blastomere stage.

Fertilization is the process of combining male and female gametes, which leads to the formation of a zygote and the subsequent development of a new organism. In the process of fertilization, a diploid set of chromosomes is established in the zygote, which determines the outstanding biological significance of this process.

Depending on the species of organisms in sexually reproducing animals, external and internal fertilization are distinguished.

External fertilization takes place in an environment that receives male and female reproductive cells. For example, fertilization in fish is external. The male (milk) and female (caviar) reproductive cells secreted by them enter the water, where they "meet" and unite. The data on fertilization in sea urchins indicate that within 2 seconds after the contact of the sperm and the egg, changes in the electrical properties of the plasma membrane of the egg occur. Fusion of gamete contents occurs in 7 seconds.

Internal Fertilization is provided by the transfer of sperm from the male body to the female as a result of intercourse. Such fertilization occurs in mammals, and the central point here is the outcome of the meeting between the reproductive cells. It is believed that the nuclear contents of only one sperm penetrate into the egg of these animals. As for the cytoplasm of the sperm, in some animals it enters the egg in a small amount, in others it does not enter the egg at all.

In humans, fertilization occurs in the upper part of the fallopian tube, and in fertilization, like in other mammals, only one sperm is involved, the nuclear content of which enters the egg. Sometimes in the fallopian tube there may be not one, but two or more eggs, as a result of which the birth of twins, triplets, etc. is possible. For example, in the 18th century. the case of birth in Russia by one mother (the wife of the peasant Fedor Vasiliev) 16 twins, 7 triplets and 4 quadruples (69 children in total) was registered.

As a result of fertilization, a diploid set of chromosomes is restored in a fertilized egg. The oocytes are capable of fertilization within about 24 hours after ovulation, while the fertilizing ability of the spermatozoa lasts up to 48 hours.

Much remains unclear in the mechanisms of fertilization. It is assumed that the penetration of nuclear material into the egg cell of only one of the many spermatozoa is associated with changes in the electrical properties of the plasma membrane of the egg. There are two hypotheses regarding the reasons for the activation of egg metabolism by the sperm. Some researchers believe that the binding of the sperm to external receptors on the cell surface is a signal that enters the egg through the membrane and activates inositol triphosphate and calcium ions there. Others believe that sperm contain a special triggering factor.



A fertilized egg gives rise to a zygote, the development of organisms through the formation of zygotes is called zygogenesis. Experimental developments in recent years have shown that fertilization of mammalian eggs, including humans, is possible in a test tube, after which embryos that develop in a test tube can be implanted into a woman's uterus, where they can develop further. By now, numerous cases of the birth of "test tube" children are known (see section VI). It has also been established that not only sperm, but also spermatids are capable of fertilizing a human egg. Finally, fertilization of oocytes (artificially deprived of nuclei) of mammals with the nuclei of their somatic cells is possible (see § 35).

Unlike zygogenesis, many animal organisms are capable of reproduction in natural conditions by parthenogenesis (from the Greek parthenos - virgin and genesis - birth). Distinguish between obligate and facultative parthenogenesis. Obligate parthenogenesis is the multiplication of organisms from an unfertilized egg. This parthenogenesis serves as a breeding method for animals of more than 90 species, including some vertebrates. An example of obligate parthenogenesis is the reproduction of the Caucasian rock lizard, represented only by females. In contrast, facultative parthenogenesis means that the eggs are able to develop both without fertilization and after fertilization. Facultative parthenogenesis, in turn, is female and male. Female parthenogenesis is common in bees, ants, rotifers, in which males develop from unfertilized eggs. Male parthenogenesis occurs in some isogamous algae.

In plants, cases are also known when the embryo develops from an unfertilized egg. As noted above, this phenomenon is called the apomix. It is very widespread in many angiosperms, including cultivated ones, such as beets, cotton, flax, tobacco and others.

Along with natural parthenogenesis, artificial (induced) parthenogenesis is distinguished, which can be caused by irritation of oocytes using physical or chemical factors, which leads to activation of oocytes and, as a result, to the development of unfertilized eggs. Artificial parthenogenesis has been observed in the case of animals belonging to many taxonomic groups - echinoderms, worms, molluscs, and even some mammals.

A form of parthenogenesis is known, called androgenesis (from the Greek andros - man, genesis - birth). If the nucleus is inactivated in the egg and if after that several spermatozoa penetrate into it, then a male body develops from such an egg as a result of the fusion of male (sperm) nuclei. The experiments of V.L. Astaurov (1904-1974), who showed androgenesis on the silkworm, are widely known. These experiments were as follows. In the oocytes of a silkworm of one species (Bombyx mandarina), the nuclei were inactivated with the help of high temperature, and then such eggs were fertilized with the spermatozoa of a silkworm of another species (B. mori). Having penetrated into the oocytes, the latter merged with each other, which gave rise to new organisms, which in their properties turned out to be paternal organisms (B. mori). Crossings of these organisms with B. mori females produced offspring belonging to B. mori.

The role of parthenogenesis and its forms in nature is small, since it does not provide broad adaptive capabilities of organisms. However, its use is of practical importance. In particular, B. L. Astaurov developed a method for obtaining parthenogenetic offspring in the silkworm, which is widely used in the industrial production of silk.

Unlike zygogenesis and parthenogenesis, there is gynogenesis (from the Greek gyne - woman), which is a pseudogamy in which the sperm meets the egg and activates it, but the nucleus of the sperm does not merge with the nucleus of the egg. In this case, the permissive offspring consists only of females. In some species of roundworms, fish and amphibians, gynogenesis serves as a normal form of reproduction, producing offspring consisting only of females. Gynogenesis can also be induced artificially using factors capable of destroying cell nuclei (radiation, temperature, etc.). In particular, cases of artificial gynogenesis in the silkworm, in some species of fish and amphibians are described. The production of such forms may have some practical value in the case of economically useful species.

In as a result, a diploid embryo and a triploid cell are formed, which develops into endosperm cells (see Chapter II).

Parthenogenesis, androgenesis and gynogenesis are forms of reproductive disorders. It is assumed that these forms arose in the course of evolution as a result of particular evolutionary adaptations.

Fertilization in plants, animals and humans is the fusion of male and female germ cells - gametes, as a result of which the first cell of a new organism is formed - a zygote. Fertilization is associated with sexual reproduction and the transmission of hereditary information from parents to offspring.

Fertilization is characteristic of most plants. It is usually preceded by the formation of gametangia (genitals), in which gametes develop. If a plant undergoes a sexual process in the development cycle, then meiosis also occurs, that is, a change in nuclear phases is detected (see Alternation of generations).

The types of sexual process in lower plants are diverse. Let's name only the main ones. The fusion of gametes with flagella, the shape and size of which are the same, is called isogamy, and gametes are called isogametes. Thus, many unicellular algae are isogamous, for example, some chlamydomonas; being unicellular, they themselves become gametangia, forming gametes. In the multicellular alga ulotrix, some cells that do not differ from others become gametangia. In some isogamous brown algae, the gametangia are different from the rest of the plant cells.

In many isogamous algae, not every pair of gametes can form a zygote, since the gametes are physiologically different. Outwardly identical gametes cannot be called either male or female; physiological differences are denoted in isogamy by the signs + and -. Only gametes of different signs formed by physiologically different (+ and -) algae individuals can merge.

The fusion of gametes with flagella that differ physiologically and in size is called heterogamy, and gametes are female (larger) and male (smaller). Heterogamous, for example, some chlamydomonas. The fusion of a large non-flagellated female gamete (ovum) with a small, male one, which usually has a flagellum or flagella (sperm), is called oogamy. The female gametangia of most oogamous lower plants are called oogonia, and the male gametangia are called antheridia. Oogamny, for example, many green and brown algae, as well as red algae.

In iso-, hetero-, and many oogamous lower plants, gametes emerge from the gametangia into the water, where fertilization takes place. In some (for example, the green alga volvox), the egg remains in the oogony, where the spermatozoa released into the water penetrate and where the gametes merge.

All higher plants are oogamous. Their typical gametangia - antheridia (male) and archegonia (female) - are multicellular. In archegonia, one ovum is formed, in the antheridium, many spermatozoa. In moss and ferns, the spermatozoa released from the antheridia swim in the water to the opened archegonia and merge with the eggs inside the archegonia. In fern-like and seed plants, fertilization occurs on (or in) the outgrowths (gametophytes), which develop independently in the former, and on sporophytes in the latter (see Alternation of generations). The outgrowths of homosporous fern-like bisexual, and heterosporous and all seed - dioecious (see Disputes). A strong reduction in the male overgrowth of seed plants led to the fact that antheridia in it (i.e., in the pollen grain) do not form in either the head or the angiosperms. In the female germ (primary endosperm) of almost all gymnosperms, archegonia are still developing, and in the female germ - the embryo sac - there are no angiosperms anymore.

In seed plants, fertilization is preceded by pollination - the transfer of pollen grains from microsporangia, where they began to develop from microspores, into the pollen chamber of the ovule (in gymnosperms) or on the stigma of the pistil (in angiosperms). Only a few gymnosperms (cycads, ginkgoes) develop multi-flagellate spermatozoa in male outgrowths, while in the rest, for example, in conifers, and in all angiosperms, male gametes - sperm - do not have flagella.

Spermatozoa reach archegonia, moving in the liquid produced by the plant itself. In seed plants with sperm, the latter go to the eggs through the pollen tubes formed by the male outgrowths. In angiosperms, after pollination, the pollen grain forms a pollen tube, which, elongating, grows between the cells of the stigma and the column, enters the cavity of the ovary and, after passing through the pollen duct, the ovule grows into the embryo sac. Here sperm emerge from the opened pollen tube (see fig.). One sperm fuses with the egg, forming a diploid zygote, giving rise to the embryo. The second one fuses with the central cell of the embryo sac, which in most angiosperms has two haploid nuclei or one diploid (if the nuclei are fused). After the fusion of the central cell with sperm, its nucleus becomes triploid. This peculiar process, characteristic only of angiosperms, was first described by the Russian scientist S.G. Navashin (1898) and called double fertilization. A multicellular storage tissue, a secondary endosperm, develops from a triploid cell, the nutrients of which are used by the embryo in the early stages of its development.

Fertilization, independent of the presence of free water, is one of the most important adaptations of seed plants to existence on land.

Fertilization in multicellular animals consists in the fusion of two gametes of different sexes - a sperm and an egg. The sperm cell introduces the hereditary material contained in its nucleus into the egg cell. The place where the sperm enters the egg can determine the location of the parts of the future organism. For example, in amphibians, that part of the egg cell into which the sperm has entered will turn into the anterior end of the body during development.

Until the moment when one of the spermatozoa touches the surface of the egg, the latter affects their behavior, releasing certain substances. They make the sperm move faster or, conversely, stick together and immobilize them (this is necessary if there are too many sperm). Particularly active interactions begin as soon as the sperm touches the surface of the egg. Within a few seconds, the front of the sperm cell turns into a tube, the tip of which sticks to the surface of the egg. Through this tube, the contents of the sperm, including its nucleus with hereditary material, are pressed into the egg cell.

In the egg, violent changes immediately begin, which outwardly manifest themselves in the fact that a shell is formed on its surface, which prevents the penetration of other spermatozoa. In addition, rapid rearrangements of the structures of the cytoplasm responsible for protein synthesis occur in the egg: the synthesis processes are immediately and many times faster. Only after this is the hereditary material of the sperm that has entered the egg is combined with the hereditary material of the nucleus of the egg. Maternal and paternal chromosomes (carriers of hereditary material) are distributed equally across all cells of the embryo, which is formed from the zygote - a fertilized egg.

Flowering (angiosperms) plants belong to seed plants (along with gymnosperms) and, therefore, sexual reproduction in them is carried out with the help of seeds. Moreover, only in flowering plants during sexual reproduction such a phenomenon is observed as double fertilization... It was discovered in 1898 by the scientist S. Navashin.

The essence of double fertilization is that in flowering plants two sperm are involved in fertilization. One of them fertilizes the egg, resulting in the formation of a zygote. The second sperm fertilizes the so-called central cell, from which storage tissue (endosperm) develops. At the same time, a double set of chromosomes is restored in the zygote, and in the future endosperm - a triple set (which is unique). Below, the process of double fertilization in flowering plants is described in more detail.

In the stamens, in their pollen sacs, they ripen pollen grains... Each pollen grain contains two cells: vegetative and generative.

In the ovary of the pistil develops ovule(one, several or many, depending on the type of plant). Inside the ovule, as a result of division, eight cells are formed containing a single set of chromosomes (gametophyte). Two of these cells fuse and form central cell... Another one of these cells becomes ovum.

When a pollen grain hits the stigma of the pistil, the vegetative cell of the grain begins to divide and forms pollen tube, which grows through the tissue of the pistil and penetrates into the ovule. For this, there is a special hole in the ovum - pollen inlet.

The generative cell of the pollen grain divides and forms two sperm... Through the pollen tube, they penetrate into the ovule. Some sperm fertilizes the egg, is formed zygote containing a double set of chromosomes. The second sperm is fused with the central cell, resulting in triple chromosome cell.

As a result of numerous divisions, the zygote develops into embryo a new plant. As a result of the division of the central cell, endosperm(nutrient tissue for the embryo). The walls of the ovule become seed skin... Thus, the ovule becomes seed.

The pistil ovary is converted to fetus... Sometimes, not only the ovary, but also other parts of the flower is involved in the formation of the fetus. The fruit is a kind of adaptation of flowering plants to the spread of seeds. The variety of possible distribution methods (using animals, wind, water, self-spreading) has given rise to a huge variety of angiosperm fruits.