What happens as a result of fertilization comes together. Fertilization during sexual reproduction. What problems may arise during the fertilization process?

In the process of evolution flora In flowering plants (and only in them) the phenomenon of double fertilization has appeared, as a result of which a seed is formed. Gymnosperms also produce seeds, but there is no double fertilization. Fertilization is preceded by pollination, that is, the transfer of pollen from the stamens of one flower to the pistil, most often of another flower. During double fertilization, two sperm penetrate the ovule, one of which fuses with the egg, and the second with the large central cell.

Pollen grains Different flowering plants have different shapes. In this case, most often the surface of pollen grains is rough, which allows them to stick to the body of pollinating insects and then to the stigma of the pistil. In addition, the stigma secretes a sticky liquid that retains pollen. On the stigma of the pistil, the pollen grain forms pollen tube, which grows between the cells of the stigma and the style of the pistil, after which it grows into the cavity of the ovary of the pistil.

The cavity of the ovary may contain one ovule, several or many. Their number depends on the type of plant. Ovules are called differently ovules. If there are several ovules in the ovary, then each of them is pollinated by its own pollen grain (sperm contained in it), i.e. in this case several pollen tubes will grow through the pistil.

The ovules grow from the inner surface of the walls of the ovary into the cavity of the ovary. The ovule consists of a cover and tissue of the central part, where eight haploid cells (having a single set of chromosomes) are formed. Two of these cells fuse to form a large central cell, in which a double set of chromosomes is restored.

From the side of the ovule, opposite place attachment to the ovary, located pollen passage, which is a small hole leading to the central part of the ovule.

At the tip of the growing pollen tube are two sperm. Sperm, unlike spermatozoa, do not have a tail, and therefore sperm are immobile. When the tube grows into the ovule through the pollen passage, one sperm fuses with one of the haploid cells, which plays the role of eggs. As a result of this fertilization, zygote with a double set of chromosomes. Subsequently, it develops from seed germ.

The second sperm fuses with the central cell. As a result of this fertilization, the so-called endosperm. It is characterized by a triple set of chromosomes, which is unique, since the body cells of angiosperms and many other organisms have a double set of chromosomes.

Endosperm is a tissue containing a supply of nutrients. The embryo uses these substances during seed development or during seed germination. In the first case, instead of the endosperm in the mature seed, the bulk of the embryo is occupied by the organs of the embryo (most often large cotyledons); in the second case, the endosperm remains.

When the seed ripens, the integument of the ovule turns into seed coat.

Fertilization, the initial moment of the emergence of a new genetic individuality, is the process of combining female and male gametes.

As a result of fertilization, a one-cell embryo with a diploid set of chromosomes appears and a chain of events underlying the development of the organism is activated.

The biological significance of fertilization is enormous: being a prerequisite for the development of a new individuality, it is at the same time a condition for the continuation of life and the evolution of the species.

It should be emphasized that fertilization is not a one-time act, but rather a process that takes a more or less long period of time. This is a multi-stage process, which distinguishes the following stages: attraction of sperm by the egg, binding of gametes and, finally, fusion of male and female reproductive cells. IN scientific literature events associated with the convergence of gametes are sometimes called insemination, distinguishing between external and internal insemination, depending on whether male reproductive cells are released into environment or into the female genitals. External insemination is typical for animals living in an aquatic environment. Internal insemination is characteristic mainly of terrestrial animals, although it is quite common among inhabitants of the aquatic environment. Insemination can be free, in which all areas of the oocyte are accessible to sperm, but it can also be limited, when there is a dense membrane with a micropyle on the surface of the egg. During internal insemination in a number of animals, male gametes are transferred to females in the form spermatophores, special capsules containing sperm. Spermatophores are first released into the environment and then transferred in one way or another to the female’s reproductive tract.

The connection of gametes determines the possibility karyogamy, or nuclear fusion. Thanks to karyogamy, the union of paternal and maternal chromosomes occurs, leading to the formation of the genome of a new individual. As a result of the fusion of gametes, a diploid zygote appears, the ability for DNA replication is restored, and preparation for cleavage divisions begins. The mechanisms of egg activation for development are relatively autonomous. Their inclusion can be carried out in addition to fertilization, which occurs, for example, during natural or artificial virgin development, or parthenogenesis.

Interest in the problem of fertilization goes far beyond the scope of embryology itself. Gamete fusion is a fruitfully used model for studying the fine molecular and cellular mechanisms of specific interactions cell membranes; to study the molecular basis of metabolic activation and proliferation of somatic cells. It is also of general biological interest that fertilization is a striking and, perhaps, unique example of a complete reversal of cell differentiation. Indeed, highly specialized germ cells are not capable of self-reproduction. They are haploid and cannot divide. However, after fusion they turn into a totipotent cell, which serves as the source of the formation of all cell types inherent in a given organism.

The history of the discovery of fertilization is lost in the mists of time. In any case, in the 18th century, the Italian naturalist Abbot Lazzaro Spallanzani (1729-1799) experimentally proved that fertilization depends on the presence of sperm, and for the first time carried out artificial insemination of frog eggs, mixing them with sperm obtained from the testes. Nevertheless, the meaning of the events occurring in this case remained unclear almost until the last quarter of the 19th century, when Oscar Hertwig (1849-1922) in the late 1870s, studying fertilization in sea urchins, came to the conclusion that the essence of this process is the fusion of the nuclei of germ cells. Together with the works of the Belgian Eduard van Beneden (1883, roundworm), the German scientist Theodor Boveri (1887, roundworm), and the Swiss zoologist Hermann Fohl (1887, starfish), O. Hertwig’s research laid the foundation for modern ideas about fertilization. It should be emphasized that it was these works that served as a strong basis for the assumption that the nucleus is the bearer of hereditary properties. It was T. Boveri (1862-1915), in a series of brilliant cytological studies, who substantiated the theory of chromosome individuality in the late 1880s and created the basis of cytogenetics.

Soon after the essence of fertilization was elucidated, researchers focused their attention on the mechanisms underlying this process. This area of research remains relevant today. The lead in developing the theory of fertilization belongs to the American researcher Frank Lilly (1862-1915). While studying the properties of “egg water,” that is, sea water in which unfertilized eggs of the sea urchin Arbacia or the polychaete Nereis had been present for some time, Lilly discovered that a substance was released from the eggs that had the ability to glue sperm into lumps. The observed agglutination turned out to be species specific, and Lilly called the agglutination factor secreted by the unfertilized egg the fertilization substance, or fertilizine(from the English fertilization - fertilization). The essence of Lilly's theory of fertilization is the recognition that in the peripheral region of the egg there is fertilisin, which has an affinity for the surface receptors of sperm (sperm antifertilisin). Thanks to this affinity, fertilizin binds, according to Lilly, sperm. However, in order to claim universality and explain not only the mechanism of gamete union, but also the reasons for sperm agglutination, the possibility of preventing polyspermy, the high specificity of the fertilization process, etc., the fertilizin theory needed numerous assumptions, under the yoke of which it eventually died out.

Already in the course of early studies of fertilization, the idea of gamons arose - substances that provide activation or blocking of individual stages of fertilization. In accordance with their origin, they distinguished gynogamones, secreted by eggs, and androgamones, produced by male reproductive cells. Thus, it was believed that gynogamon 1, diffusing from the egg, activates the movement of the sperm, overcoming the action of androgamon 1, which inhibits the movement of the sperm. Gynogamon 2 is a synonym for fertilisin, and androgamon 2 is a sperm antifertilisin.

In the fifties of the 20th century, the idea of the interaction of fertilisin with antifertilisin was transformed into the hypothesis of specific phagocytosis. According to this concept, the presence of interacting molecules on the surface of the egg and sperm provides a complementary zipper reaction that allows the sperm to be absorbed into the egg.

Despite a certain speculativeness, these and many other similar hypotheses about the mechanisms of interaction between sperm and eggs played a positive role, revealing, firstly, the existence of a whole family of specific molecules on the surface of interacting gametes and, secondly, initiating systematic research into the nature of these molecules .

The second half of the last century was the heyday of ultrastructural and molecular biological research, which revealed a wide variety of specific forms of cellular interaction during fertilization. It became clear that a universal theory of fertilization, if it could exist, would be only as a set of some of the most general principles for organizing this process.

The specific mechanisms of fertilization depend on many factors. Suffice it to say about the uniqueness of fertilization in animals with external and internal insemination. Obviously, certain differences in the fertilization process are also due to the fact that in different animals the penetration of sperm into the egg occurs at different stages oogenesis. In many annelids, mollusks, nematodes and crustaceans, sperm penetrates first-order oocytes at the prophase stage. In other annelids, mollusks and insects - at the metaphase stage of the primary oocyte. Many vertebrates are characterized by insemination at the metaphase stage of the secondary oocyte. In some coelenterates and sea urchins, fertilization occurs at the stage of a mature egg after the completion of maturation divisions and the release of directional or reduction bodies. Finally, one cannot help but recall the variety of types of sperm, among which there are flagellar forms and sperm without flagella (for example, amoeboid sperm nematodes), with and without an acrosome, with and without an acrosomal thread. Naturally, in each such case, the specific mechanisms that ensure the subtle interaction between germ cells differ.

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Fertilization in plants, animals and humans is the fusion of male and female reproductive 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 transfer of hereditary information from parents to offspring.

Fertilization is common to most plants. It is usually preceded by the formation of gametangia (genital organs), in which gametes develop. If a plant undergoes a sexual process during its development cycle, then meiosis also occurs, i.e., a change in nuclear phases is detected (see Alternation of generations).

The types of sexual process in lower plants are varied. 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 the gametes are called isogametes. Thus, many unicellular algae are isogamous, for example some Chlamydomonas; being single-celled, they themselves become gametangia, forming gametes. In the multicellular alga Ulothrix, some cells that are no different from others become gametangia. In some isogamous brown algae, the gametangia are distinct 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 indicated in isogamy by the signs + and -. Only gametes of different signs, formed by physiologically different (+ and -) individuals of the algae, can merge.

The fusion of gametes with flagella that differ physiologically and in size is called heterogamy, and the gametes are female (larger) and male (smaller). For example, some Chlamydomonas are heterogamous. Fusion of flagellated large female gamete(ovum) with a small, male, usually having 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. For example, many green and brown algae, as well as red algae, are oogamous.

In iso-, hetero-, and many oogamous lower plants, gametes emerge from the gametangia into the water, where fertilization occurs. In some (for example, in the green alga Volvox), the egg remains in the oogonia, where the sperm released into the water penetrate and where the fusion of gametes occurs.

All higher plants are oogamous. Their typical gametangia - antheridia (male) and archegonia (female) - are multicellular. The archegonium produces one egg, and the antheridium produces many sperm. In bryophytes and pteridophytes, spermatozoa released from the antheridia swim in the water to the opened archegonia and merge with the eggs inside the archegonia. In pteridophytes and seed plants, fertilization occurs on (or in) shoots (gametophytes), which develop independently in the former, and on sporophytes in the latter (see Alternation of generations). Thalli of homosporous fern-like ferns are bisexual, while heterosporous and all seed ferns are dioecious (see Disputes). The strong reduction of the male prothallus of seed plants has led to the fact that antheridia are not formed in it (i.e., in the pollen grain) in either holosperms or angiosperms. In the female prothallus (primary endosperm) of almost all gymnosperms, archegonia are still developing, but in the female prothallus - the embryo sac - of angiosperms they are no longer present.

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 in a few gymnosperms (cycads, ginkgo) multiflagellate spermatozoa are formed in the male prothalla, while in the rest, for example, in conifers, and in all angiosperms, male gametes - sperm - do not have flagella.

Spermatozoa reach the archegonia, moving in the liquid produced by the plant itself. In seed plants that have sperm, the latter go to the eggs through pollen tubes formed by male prothellae. In angiosperms, after pollination, the pollen grain forms a pollen tube, which, lengthening, grows between the cells of the stigma and style, enters the cavity of the ovary and, having passed through the pollen passage of the ovule, grows with its end into the embryo sac. Here, sperm cells emerge from the opened pollen tube (see figure). One sperm fuses with the egg to form a diploid zygote, which gives rise to an embryo. The second 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 the 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. From the triploid cell, multicellular storage tissue develops - the secondary endosperm, the nutrients of which are used by the embryo for early stages 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 involves the fusion of two gametes of different sexes - a sperm and an egg. The sperm introduces into the egg the hereditary material contained in its nucleus. The location of sperm penetration into the egg can determine the location of parts of the future organism. For example, in amphibians, the part of the egg into which the sperm entered will, during development, turn into the anterior end of the body.

Until the moment when one of the sperm touches the surface of the egg, the latter influences their behavior by releasing certain substances. They make sperm move faster or, on the contrary, glue 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 part of the sperm 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.

Violent changes immediately begin in the egg, which are externally manifested in the formation of a membrane on its surface that prevents the penetration of other sperm. In addition, rapid rearrangements of the cytoplasmic structures responsible for protein synthesis occur in the egg: the synthesis processes are immediately and many times accelerated. Only after this the hereditary material of the sperm that has entered the egg is combined with the hereditary material of the egg nucleus. Maternal and paternal chromosomes (carriers of hereditary material) are distributed equally throughout all cells of the embryo formed from the zygote - a fertilized egg.

PhaseI I – the acrosomal reaction occurs in it.

PhaseThe outer membrane of the sperm ruptures, releasing proteolytic enzymes and dissolving the membrane of the egg. Plasma membranes fuse, cytoplasms are connected. The nucleus and centriole of the sperm pass into the cytoplasm of the egg.The tail part is resorbed. Then the egg is activated, its potential changes and its vitelline membrane peels off and the fertilization membrane is formed (cortical reaction). Activation ends with the beginning of protein synthesis.

III – syngamy.

It distinguishes:

Stage 2

pronuclei - the male nucleus swells, takes on a prophase appearance, during this time the DNA doubles and the male pronucleus receives a haploid set of reduplicated chromosomes (n2c). When the egg meets the sperm, it is in the stage of meiosis, blocked by a special factor. After meeting the sperm, the egg is activated and the block is removed. The nucleus of the egg, having completed meiosis, turns into the female pronucleus, also acquiring a set of n2c chromosomes. The synkaryon 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 embryonic cells (blastomeres) with a set of chromosomes 2n2c in each. Parthenogenesis the daughter organism sometimes develops from an unfertilized egg. This phenomenon is called virgin development or parthenogenesis

Fertilization is the process of fusion of female (egg) and male (sperm) germ cells, which leads to the formation of a new single-celled organism (zygote). It is this moment that many consider the beginning of a new life and the starting point of pregnancy. Let's find out in more detail how fertilization occurs and at what stages the risk of death of the unborn fetus may arise.

The fusion of egg and sperm is called the process of fertilization.

The structure of male reproductive cells

Normally, the formation of sperm capable of fertilization begins in a person during puberty (12-13 years). A mature sperm consists of a head, neck and tail. The most important part is concentrated in the head, where the nucleus is located, which delivers the father's genes to the egg.

The function of the tail is movement; it is this part of the sperm that allows it to move at a speed of 2-3 mm per minute and reach the uterus and fallopian tubes. Sperm are found in semen. It is a viscous whitish liquid, where, in addition to the germ cells, the secretion of the seminal vesicles and prostate is determined.

During sexual intercourse, 3-5 ml of sperm enters the vagina, where there are about 300-400 million sperm. Normally, most of them have normal mobility and correct structure. In the vagina they die within a few hours, but, having reached the fallopian tubes, they can remain viable for another three days.

A man produces sperm throughout his life. Their complete renewal in the human body occurs approximately once every 2-2.5 months.

The sperm nucleus contains the father's genetic information.

Female reproductive cells

A woman is born with a certain supply of eggs. When the supply of eggs is depleted, menopause occurs. Therefore, if a man is theoretically able to conceive a child at any age, then a woman is given a limited amount of time.

During puberty, a girl's follicles acquire the ability to mature and rupture in order for the egg to be released into abdominal cavity and could enter the fallopian tube for fertilization.

This process occurs approximately once a month in the middle menstrual cycle and is called ovulation. It is during this period that the egg can meet with the sperm to conceive.

A mature human egg does not have independent mobility, unlike a sperm. Its movement occurs under the influence of the suction peristaltic effect fallopian tube and flickering of epithelial cilia. The egg consists of a nucleus, where the genetic information of the mother is concentrated, the zona pellucida and the corona radiata.

The ability to fertilize is highest immediately after and it persists throughout the day. Subsequently, the death of the egg occurs. In women, this process is manifested by menstrual bleeding.

The egg is surrounded by a transparent membrane and a corona radiata.

Where and how does the process of human fertilization take place?

During sexual intercourse, sperm usually falls on the posterior vaginal fornix, with which the cervix comes into contact. Normally, the environment in the vagina is acidic, which makes it possible to weed out weak and non-viable sperm. The surviving male cells enter the uterus, where the environment is alkaline and begin to move more actively towards the fallopian tubes.

Important! On normal days, the cervix is covered with a dense mucus plug, but during the period the permeability of the mucus increases, which allows sperm to penetrate to the site of fertilization.

After ejaculation in the vagina, only a few minutes pass and active sperm can already be found in the uterus. After 2-3 hours they reach the end sections of the fallopian tubes, where the egg is located. They can exist there for two days, maintaining their ability to fertilize and waiting for an egg. If this does not happen, the sperm die.

The process of fertilization (fusion) itself occurs in the expanded (ampullary) part of the fallopian tube. Here thousands of sperm rush towards the egg. The transparent shell of the egg and the cells of the corona radiata allow only one or several sperm to penetrate the egg. But only one of them will participate in fertilization.

Important! In rare cases, there is a reaction disorder and the egg is fertilized by several sperm. This process is called polyspermy and leads to the formation of a non-viable zygote.

The meeting of the sperm and the egg ends with the fusion of their nuclei, where the genetic material is not simply summed up, but mutually united and the formation of a single zygote nucleus occurs. This is how genetic material is transferred to the child from both parents.

How does this process proceed day by day?

The zygote stage lasts for one and a half days. Soon it begins the process of cell fragmentation, resulting in the formation of an embryo. It moves slowly through the fallopian tube and reaches the uterus only 7-10 days after fertilization. The movement of the embryo occurs due to the flickering of the cilia and the peristaltic activity of the fallopian tube itself.

Then it is introduced (implantation) into the mucous membrane of the uterus and is immersed in its functional layer. This process takes about 2 days.

After implantation is complete, the embryo and its membranes begin to develop rapidly. It gradually becomes overgrown with vessels, which ensures its nutrition and respiration. After all these stages are completed, a fruit is formed, surrounded by amniotic fluid and three shells.

7-10 days after fertilization, the embryo implants into the body of the uterus.

What problems may arise during the fertilization process?

On the one hand, fertilization is a natural biological process that occurs on its own and, as a result, is born new life. But couples who are faced with infertility perceive it completely differently. Let's look at the reasons why we most often fail to conceive a child the first time:

sexual intercourse occurred when the woman was not ovulating, i.e. no egg in the fallopian tube;
sperm turned out to be non-viable and did not reach the egg during ovulation;
obstruction of the fallopian tubes, which prevented the sperm and egg from meeting;
the egg was fertilized by several sperm and the embryo died;
fertilization of the egg has occurred, but with a defective sperm - in such situations the zygote dies in the early stages;
the process of transport of the embryo into the uterus is disrupted and implantation occurs in the fallopian tube (ectopic pregnancy) - death of the embryo and a condition that threatens the woman’s life;
the embryo reached the fallopian tube, but could not implant due to the thin functional layer of the uterus or its absence (this happens after an abortion). A miscarriage occurs before the woman even finds out she is pregnant.

Here is only a small list of problems due to which the process of fertilization and the beginning of pregnancy may be disrupted. Some interruption mechanisms are due to nature’s protective reaction for the birth of healthy offspring, for example, the death of an embryo with defective anomalies. Others occur due to health problems in both men and women. In order not to think about how fertilization occurs, you need to monitor the state of your reproductive system and plan your pregnancy.