Gamete formation. Reproduction. gamete development Where gametes develop in animals

Gametogenesis in humans

The process of formation of germ cells is called gametogenesis. It occurs in the genitals and gonads. Gametogenesis has certain features depending on the sex of the organism in which meiosis occurs. The formation of male reproductive cells is calledspermatogenesis, female - oogenesis.

Primary germ cells are released from the yolk sac on the 27th day of embryonic development and migrate to the site where the reproductive organs will form. On the 46th day of intrauterine development, the gonads begin to differentiate and become either ovaries or testes.

During this period, the primary germ cells in the male gonads becomespermatogonia,which continuously divide by mitosis until puberty. At approximately 15-16 years of age, spermatogonia begin to enter meiosis. They first increase in size, turning into spermatocytes of the first order , containing 46 duplicated chromosomes. Spermatocytes I order enter the first meiotic division, which ends formation of second order spermatocytes. Spermatocytes II order contain 23 chromosomes, consisting of two chromatids. After the second meiotic division of spermatocytes II order are formed spermatids containing 23 chromosomes. In spermatids, each chromosome already has only one chromatid. Spermatids subsequently acquire some structural features: the state of the cell membrane changes, a neck and tail appear, which will ensure the movement of the cell. The fully formed male reproductive cell is called sperm.

Thus, as a result of spermatogenesis from onediploid spermatogonia produces four equal haploid spermatozoa. The division of spermatogonia and the formation of spermatozoa continues until the attenuation of gonadal activity in old age. During the entire period of puberty, the male body produces at least 500 billion gametes.

The process of maturation of female germ cells (oogenesis) has its own characteristics. After primary sexual differentiation, the female body develops oogonia , which undergo several mitotic divisions. Starting from the second month of intrauterine development of the female fetus, a series of oogonium enter the first division of meiosis, turning into 1st order oocyte. Oocyte division I order stops immediately afterdiplothenes. At this stage, meiosis in the female gonads ceases at the 7th month of fetal development until puberty. After the birth of a girl, all remaining oogonia degenerate. During puberty, usually one oocyte per month I order completes the first division of meiosis, after which two different cells are formed: one large, containing almost all the cytoplasm from the oocyte I order, - oocyte II order, and small, actually including only the nucleus with 23 chromosomes, -first polar body. Then the second order oocyte enters the second division of meiosis. Meiosis II order also leads to the emergence of two unequal haploid cells: large egg, and small second polar body.In oogenesis, meiosis II order ends at the moment fertilization , i.e. penetration of the sperm into the cytoplasm of the egg.

After the sperm penetrates the egg, the fusion of the female and male haploid sets of chromosomes does not occur for several hours, although they are immediately surrounded by a common nuclear membrane. This period is very sensitive to all external damaging influences. After the fusion of the female and male sets of chromosomes, a cell called zygote. The zygote begins to divide through normal mitosis, and a new organism is formed.

GAMETES(Greek gametes husband, spouse) - specialized germ cells in organisms that reproduce sexually, which, during their fusion during the process of fertilization, ensure the transfer and unification of the gene program of the parent individuals for the development of the characteristics of the new organism.

In all multicellular organisms, mature glands develop in the sex glands - gonads (male - testes and female - ovaries) from immature germ cells - gametocytes. During the process of growth and maturation of G. - gametogenesis (see), the diploid (usual double) number of chromosomes in the nuclei of gametocytes is reduced as a result of meiosis (see) to haploid (half as much). Mature G. are haploid. When a zygote is formed as a result of fusion, the diploid number of chromosomes, characteristic of the cells of a new organism, is restored. Male and female glands differ significantly. In the vast majority of organisms, heterogamy occurs: spermatozoa (microgametes) contain a small amount of cytoplasm, a small nucleus, a “tail” (cord) and have motor activity; eggs (macro-gametes) have a large amount of cytoplasm, are large in size and are not capable of active movement.

In some organisms (roundworms, most arthropods), male spermatozoa (sperm cells) do not have locomotion organelles and move by forming cellular projections—pseudopodia. In some cases, eggs play an active role in capturing immobile sperm. In plant organisms (green and brown algae), both male and female gametes have mobility, differing in size (female macrogametes). In lower fungi and green algae, G. are not distinguishable morphologically (isogamy).

Humans and higher animals are characterized by heterogamy—significant differences in genetic structure (Fig.) and role in the process of fertilization. The cytoplasm of eggs is rich in cytoplasmic DNA and RNA, free nucleotides, proteins and other substances necessary to ensure the development of the zygote. Thus, spermatozoa mainly carry nuclear genetic information, eggs - nuclear and cytoplasmic genetic information plus the intracellular environment for the first stages of its implementation during the development of a new organism.

In the simplest unicellular organisms during the sexual process, the function of G. is performed by the individuals themselves, in some cases undergoing morpho-physiol. changes (see Protozoa). The sexual process in some lower plants and protozoan animals occurs without the participation of genes - in the first case, physiologically different-sex areas of the body or somatic cells merge, in the second, part of the nuclei (micronuclei) is exchanged during temporary contact (conjugation) of two adult individuals (see Ciliates) .

Bibliography: Patten B. M. Human embryology, trans. from English, M., 1959; Guide to Cytology, ed. A. S. Troshina, vol. 2, p. 390, M.-L., 1966; Austin S. N. The mammalian egg, Springfield, 1961.

Yu. F. Bogdanov.

The process of formation of germ cells in plants is divided into two stages: Stage 1 - sporogenesis- ends with the formation of haploid cells - spores, during the 2nd stage - gametogenesis- a series of haploid cell divisions occur before mature gametes are formed.

The process of formation of microspores, or pollen grains, in plants is called microsporogenesis, and the process of formation of megaspores (or macrospores) is mega- or macrosporogenesis. Microsporogenesis proceeds similarly to the division of maturation in animals of male germ cells to the spermatid stage, and megasporogenesis - accordingly to the stage of an immature egg - oocyte II.

The process of gametogenesis in plants is in principle similar to that in animals, but proceeds in a slightly different way. In animals, after two meiotic divisions, gametes are formed, and no additional cell divisions occur. In plants, as a result of two meiotic divisions, a haploid spore arises, from which the gametophyte develops, which in lower plants (fungi, liverworts, mosses, a number of algae) is a whole organism and the longest stage of the life cycle. In higher plants, the haploid phase is reduced, but the nuclei of male and female spores undergo a series of mitotic divisions before gametes are formed.

Microsporogenesis and microgametogenesis

We will consider microsporogenesis and microgametogenesis using angiosperms as the most general example. In the subepidermal tissue of the young anther, a special sporogenic tissue called archesporium. Each primary archesporial cell, after a series of divisions, becomes a pollen mother cell (microsporocyte), which goes through all phases of meiosis.

As a result of two meiotic divisions, four haploid microspores arise. The latter lie in fours and are called cell tetrads.

In monocotyledonous plants, each nuclear division in meiosis is usually accompanied by cytokinesis; In dicotyledons, both cell divisions occur simultaneously at the end of meiosis.

During maturation, cell tetrads break up into individual microspores with the formation of inner (intine) and outer (exine) membranes. The outer shell, as a rule, is rough, cutinized, its surface is either smooth or rough; adapted for transferring pollen and sticking it to the stigma of the pistil. This ends microsporogenesis; after the formation of a mononuclear microspore, microgametogenesis begins. The first mitotic division of the microspore leads to the formation of vegetative and generative cells. Subsequently, the vegetative cell and its nucleus do not divide. It accumulates reserve nutrients, which subsequently ensure the division of the generative cell and the growth of the pollen tube in the pistil style.

The generative cell, containing less cytoplasm, divides again. This division can occur in the pollen grain or during its germination in the pollen tube. As a result, two male reproductive cells are formed, which, unlike animal sperm, are called sperm cells, or sperm.

Thus, from one spore with a haploid set of chromosomes, as a result of two mitotic divisions, three nuclei are formed: Two of them are sperm and one is vegetative. When a pollen tube is formed, this vegetative nucleus in a semi-liquid diffuse state passes into the pollen tube.

The process of division of the generative cell and the formation of sperm in the pollen tube were first studied in detail by S. G. Navashin in 1910 on lily plants.

Megasporogenesis and megagametogenesis

In angiosperms, the female gametophyte is the embryo sac that begins and develops inside the ovule.

The development of the female gametophyte in higher angiosperms is preceded by megasporogenesis. In the subepidermal layer of the young ovule, an archesporial cell is isolated, often there is only one. The archesporium cell grows, turning into a megaspore mother cell. As a result of two meiotic divisions of the megaspore mother cell, a tetrad of megaspores is formed. Each of the cells of the tetrad is haploid by the number of chromosomes. However, only one of them continues to develop, the other three degenerate (monosporic type of development), the fate of these cells resembles the fate of reduction bodies during the maturation of oocytes in animals.

The next stage involves megagametogenesis. The remaining functioning megaspore continues to grow and then its core undergoes a series of equational divisions. In this case, the cell itself does not divide, only the nucleus divides.

In different systematic groups of plants, the number of equivalence divisions of the megaspore nucleus can vary from one to three. In most plants (70% of angiosperm species), these divisions, as a rule, result in eight hereditarily identical nuclei; during these divisions, the nuclei occupy a polar position, four of them are located closer to the micropyle (the place of sperm penetration), and the other four - in the opposite end of the embryo sac, called the chalazal. Further, these nuclei are isolated into independent cells with significant amounts of cytoplasm.

Of the four cells located at the micropyle, three cells are the egg, and two so-called synergids form the egg apparatus. However, of these three cells, only one develops after fertilization, and the other two are destroyed. The fourth nucleus extends to the center of the embryo sac, where it merges with one of the nuclei that extends from the chalazal end. Two haploid nuclei fused in the central part form one diploid - secondary or central, nucleus of the embryo sac. This nucleus with the cytoplasm of the embryo sac is usually called the central cell of the embryo sac. However, often polar nuclei that have moved to the center do not fuse before fertilization. The three nuclei remaining at the chalazal end of the embryo sac are also separated into cells; they're called antipodes.

Thus, as a result of three mitotic divisions, 8 hereditarily identical haploid nuclei are formed in the embryo sac, of which only one gives rise to an egg.

The considered scheme for the formation of an eight-nucleate embryo sac from one megaspore is the most typical. However, in different groups of plants this process occurs very differently. In some cases, as we have just examined, the development of the embryo sac begins from one haploid spore (monosporic type of development), in others - from two (bisporic type) and four spores (tetrasporic type).

As we indicated, with the monosporic type, only one megaspore out of four develops, and the remaining three are destroyed in a similar way to what occurs with reduction bodies in animals. With other types of embryo sac development, a different number of megaspores are retained, resulting from meiosis and ready for further mitotic divisions.

Studying gametogenesis, one cannot help but be amazed at the parallelism that is observed during the maturation of germ cells in animals and plants, despite the fact that their divergence (divergence) in phylogenesis occurred at a very early stage in the emergence of cellular organization. This indicates the similarity of principles for constructing a number of adaptive mechanisms in both the plant and animal worlds.

So, the study of the development of germ cells in animals and plants has shown that the formation of gametes is a complex process. Before the egg and sperm are united during fertilization, they undergo a series of transformations. However, germ cells, like the cells of any other tissue, originate from somatic cells. Therefore, they cannot be considered as something separate from the body of the organism. At the same time, sex cells also have their own characteristics. The main characteristic points that distinguish them from somatic cells are the following:

1. In different animals and plants, at different stages of differentiation of embryonic tissues, germ cells separate. The process of formation and differentiation of gametes in animals is called rudimentary way.

2. In the process of development of germ cells, meiosis is of particular importance with its characteristic stages of nuclear division, namely prophase I, during which homologous chromosomes are conjugated, metaphase I and anaphase I, when the number of chromosomes is reduced and homologous chromosomes diverge to different poles.

3. The main property of germ cells is their ability to merge into one during fertilization to form a zygote, which then undergoes fragmentation and development. Somatic cells, as a rule, do not possess this ability.

(Greek gamete-wife), mature sex cells capable of fertilization.
When male and female gametes fuse, a zygote is formed, giving rise to a new organism. In men, throughout their lives, starting from puberty, a huge number of gametes (spermatozoa) are formed, carrying approximately equal numbers of X and Y chromosomes ( cm. Spermatogenesis). In women, all reproductive cells are laid in utero, and subsequently, upon reaching puberty, one egg matures monthly ( cm. Oogenesis). Due to the peculiarities of gamete maturation in women, any mutagenic effect on the body (starting from the stage of intrauterine development) can cause genetic abnormalities in her offspring. In men, on the contrary, generations of sperm replace each other quite quickly, therefore, if a man was in contact with a mutagenic factor for some time (for example, he served on a nuclear submarine), then sperm formed already 1-1.5 years after termination of the harmful factor, do not carry mutations associated with radiation.

Rice. Scheme of the formation of human gametes (sex cells).

(Source: Sexological Dictionary)

(from Greek gamete - wife, gametes - husband) (sex or reproductive cells), female. (eggs or oocytes) and husband. (spermatozoa, sperm) reproductive cells of animals and plants, which, when fused, ensure the development of a new individual and the transmission of hereditary characteristics from parents to descendants.

(Source: Dictionary of Sexual Terms)

See what “Gametes” are in other dictionaries:

GAMETES- GAMES, sex cell elements, resp. individuals uniting during fertilization, resp. during conjugation, copulation, etc. processes in animal and plant organisms. In multicellular creatures (animals, higher plants), gametes are... ... Great Medical Encyclopedia

Modern encyclopedia

- (from the Greek gamete wife gametes husband) (sexual, or reproductive, cells), female (eggs, or oocytes) and male (sperm, sperm) sex cells of animals and plants, which, upon fusion, ensure the development of a new individual and transmission ... ... Big Encyclopedic Dictionary

gametes- Female and male reproductive cells of animals. [GOST 27775 88] Topics: artificial insemination... Technical Translator's Guide

Gametes- (from the Greek gamete wife, gametes husband), reproductive cells of animals and plants, female (eggs, or ovules) and male (sperm, sperm). When merging, they ensure the development of a new individual and the transmission of hereditary characteristics from the parents... ... Illustrated Encyclopedic Dictionary

gametes- ANIMAL EMBRYOLOGY GAMETES, GENERATIVE CELLS, GERMS CELLS – specialized cells of animals and plants that have a haploid set of chromosomes and participate in sexual reproduction. Formed during the process of gametogenesis. Gametes can be... General embryology: Terminological dictionary

Gamete; pl. (unit gamete, s; g.). [from Greek gametē wife, gametēs husband]. Male and female reproductive cells. Male, female g. ◁ Gametic, aya, oh. Gay cell (reproductive cell). * * * gametes (from the Greek gametē wife, gamétēs husband) (sexual, or reproductive ... encyclopedic Dictionary

- (gr. gametes spouse, gamete spouse) reproductive cells of animals or plants; during fertilization, two g. of opposite sexes (in humans and most animals they are called sperm and oocyte, or egg) merge into a zygote, giving rise... ... Dictionary of foreign words of the Russian language

Gametes- (from the Greek gamete wife, gametes husband) sex, or reproductive, cells with a haploid (single) set of chromosomes, female (eggs or ovules), male (sperm, sperm). Gametes ensure the transmission of hereditary information from parents... The beginnings of modern natural science

Gametes- (rp. gamete wife, gametes husband) sex or reproductive cells with a haploid (single) set of chromosomes, female (eggs, or ovules) and male (sperm, sperm, vipers). Gametes ensure the transmission of hereditary information from... ... Concepts of modern natural science. Glossary of basic terms

- (from the Greek Gamete wife, gametes husband) sexual, or reproductive, cells of animals and plants, which, upon fusion, ensure the development of a new individual and the transmission of hereditary characteristics from parents to descendants. G. have a single (haploid)… … Great Soviet Encyclopedia

Lecture No. 10.

Topic: Reproduction is a universal property of living things.

Lecture outline

1. Reproduction as a universal property of living things.

2. Forms of reproduction of organisms.

3. Evolution of the sexual process.

4. Gametogenesis. The structure of germ cells.

5. Insemination. Fertilization.

6. Features of human reproduction.

Reproduction- one of the main universal properties of living things, ensuring the reproduction of their own kind, which is based on the transfer of genetic information from generation to generation. Reproduction at the molecular level– is the replication (self-duplication) of DNA, at the subcellular level– doubling of some organelles, on the cellular– amitosis, mitosis (cell division). Cell division is the basis reproduction of organisms.

Forms of reproduction of organisms

Asexual reproduction

Vegetative Sporulation

(body parts) (special cells - spores)

in unicellular organisms in multicellular organisms

in plants in animals

Vegetative reproduction of unicellular organisms:

A) bisection– the nucleus divides mitotically, then the cytoplasm is divided into two parts by a constriction; longitudinal division - in euglena, transverse - in ciliates;

b) schizogony– multiple division – the nucleus is divided into many parts, then the cytoplasm (in the malarial plasmodium);

V) budding– a protrusion with a nucleus (bud) is formed on the mother cell; the bud grows and separates from the mother (in yeast fungi and sucking ciliates).

Vegetative reproduction in multicellular organisms:

A. In plants– vegetative organs: roots, stems, leaves.

B. In animals:

a) budding (in hydra);

b) fragmentation - division of the body by constrictions into several parts (ciliated and annelid worms);

c) polyembryony – division of the embryo into several parts, each of which forms a whole organism (flukes).

Sporulation: specially formed cells - spores - give rise to a new organism (in algae, fungi, mosses, mosses, horsetails and ferns). In plants, spores are formed in special organs - sporangia.

Sexual reproduction with fertilization without fertilization (gametic copulation) (parthenogenesis) androgenesis gynogenesis

The basis of sexual reproduction is the sexual process. It can go like conjugation(exchange of genetic information between two cells) or how copulation- combining the genetic information of two cells. Conjugation is characteristic of ciliates and bacteria. During conjugation, ciliates are connected by a cytoplasmic bridge and exchange parts of the micronucleus. After this, they disperse and reproduce asexually.

At a certain period of the life cycle, protist individuals perform the function of gametes. They merge (copulation occurs) and then reproduce by division. If a fusion of cells of equal size and mobility occurs, the process is called isogamy(example: testate amoebas). The process is called anisogamy, if one cell is larger and immobile, the second is smaller and more mobile (example: malarial plasmodium).

Copulation during sexual reproduction of multicellular organisms is called gametic. Special cells called gametes are formed in the gonads (sex glands). Female gametes are produced in the ovaries, male gametes are produced in the testes.

Ovules have a round or slightly oval shape. Their sizes range from 60 microns to several centimeters in diameter. They are motionless. The eggs contain organelles and a supply of nutrients (yolk). Their cytoplasm is species specific. Eggs are covered with various membranes, and in mammals they are also covered with follicular epithelial cells.

Sperm consists of a head, neck and tail. Movable It has small dimensions (40-500 microns). The size of a human sperm is 52-70 microns. But the end of the head is located acrosome- modified Golgi complex. It ensures the penetration of sperm into the egg. The main part of the head is occupied by the nucleus, surrounded by a thin layer of cytoplasm. The neck contains the centrosome and the spiral filament, which consists of mitochondria. They produce energy to move the tail

The process of forming gametes is called gametogenesis: ovogenesis– formation of eggs, spermatogenesis– formation of sperm.

During gametogenesis, haploid gametes are formed from diploid somatic cells of the gonads.

Features of gametogenesis in humans

1. Mitotic division of oogonia ends before the birth of the organism. Mitosis of spermatogonia begins at puberty.

2. During oogenesis, the growth zone is significantly expressed; during spermatogenesis, the growth zone is almost not expressed.

3. During oogenesis, the first meiotic division stops at the prophase diakinesis stage before puberty. Second meiotic division

stops at the metaphase stage and ends after fertilization.

4. During oogenesis, the formation zone is not expressed; during spermatogenesis, the formation zone is significantly expressed.

A born girl has about 30,000 oocytes in her ovaries; 300-600 reach maturity (about 13 cells per year). During the period of sexual life, the male body produces up to 500 billion spermatozoa (several billion per one second-order oocyte).

Currently, the last stages of oogenesis are reproduced outside the body and make it possible to “conceive” in vitro. At the stage of 8-16 blastomeres, the embryo is transferred to the uterus of the recipient woman.

In lower animals, germ cells are produced throughout life, in higher animals - during the period of sexual activity.

The main advantage of sexual reproduction over asexual reproduction is the increase in genetic diversity of species and populations.

Differences in forms of reproduction

Based on the presence and functioning of the gonads in the body

distinguish between hermaphroditism and dioeciousness.

Hermaphrodite- an organism that has male and female gonads that form sex cells in one individual. Such hermaphroditism occurs in flatworms and annelids. This - true hermaphroditism. A variety of it may be the hermaphroditism of mollusks, the sex gland of which, depending on age and living conditions, periodically produces either male or female gametes. When false hermaphroditism in one individual the external genitalia and secondary characteristics of both sexes develop, and the gonads develop of one sex (male or female). A person may have signs of false hermaphroditism.

Dioecious organisms have either female or male gonads. Their genital organs are formed during embryogenesis. Males and females are characterized by signs of sexual dimorphism: differences in body size, color, structure, vocal characteristics, behavior and other characteristics. Signs of sexual dimorphism in humans are: features of the musculoskeletal system; distribution of subcutaneous fat; degree of hair development; voice timbre; features of the nervous system and behavior, etc. A number of processes that ensure the meeting of female and male gametes is called insemination. In most aquatic animals external insemination: gametes are released into the external environment, and their fusion occurs in water. At internal insemination(in terrestrial animals) male gametes are introduced into the female's reproductive tract during sexual intercourse. The process of insemination is followed by the process of fertilization - the fusion of gametes to form a zygote. The meeting of gametes is ensured by:

Different charges of gametes;

Movement of sperm and contraction of the walls of the female genitalia

The release of special chemicals by the egg - gamons, to which sperm exhibit positive chemotaxis.

During the fertilization process, external and internal phases are distinguished. The external phase of fertilization is the activation of the egg and the penetration of the sperm into it. There is a hole in the membrane of some eggs - micropyle through which the sperm enters the egg. In most cases, its penetration into the egg occurs with the help of acrosome reaction. Upon contact with the egg, the acrosome membrane is destroyed and an enzyme is released hyaluronidase. It dissolves the egg membrane, the acrosome filament is released from the acrosome and penetrates the egg membranes and fuses with the egg membrane. In this area of ​​the egg cell is formed sensory tubercle, which captures and introduces the head, centriole and mitochondria of the sperm into the cytoplasm of the egg. An egg can contain one sperm (in mammals) and the process is called monospermia. If several sperm enter (in insects, fish, birds), the process is called polyspermy. Activation of the egg involves complex structural and physicochemical changes: restructuring of the cytoplasm, changes in membrane permeability and metabolism. After penetration of the sperm, a fertilization membrane is formed on the surface of the egg, and other sperm cannot enter. This ends the external phase of fertilization.

The second important process is associated with the internal phase of fertilization - synkaryogamy– fusion of haploid gamete nuclei and formation of a diploid zygote nucleus. The colloidal properties of the egg cytoplasm change, and its viscosity increases. Male pronucleus(sperm nucleus) swells to the size of the female pronucleus (egg nucleus), rotates 180° and moves forward with the centrosome to the side female pronucleus. The pronuclei meet and their fusion occurs. The diploid set of chromosomes is restored and a zygote is formed. Fusion of gametes in humans occurs in the upper third of the oviduct.

A special form of sexual reproduction is parthenogenesis and its varieties: gynogenesis and androgenesis - the development of organisms from unfertilized eggs.

Parthenogenesis (Greek) partenos– virgin, genos- birth) was described in the middle of the 18th century by the Swiss naturalist C. Bonnet. Natural parthenogenesis found in lower crustaceans, bees, butterflies, and rock lizards. The nuclei of somatic cells of such individuals will be haploid. The diploid set is sometimes restored when the nucleus of the egg cell fuses with the nucleus of the guide body. In 1886 A.A. Tikhomirov described artificial parthenogenesis. He caused the crushing of unfertilized silkworm eggs, acting on them with physical

or chemical irritants. B.L. Astaurov developed an industrial method for obtaining parthenogenetic offspring from the silkworm.

Gynogenesis(Greek gyne– woman) – the development of the body occurs on the basis of information from the female pronucleus. The sperm is an activator of development. The sperm nucleus does not participate in fertilization. If it enters the egg, it is destroyed. Gynogenesis occurs in some fish species (for example, silver crucian carp). Their offspring consists of only females.

Androgenesis- (Greek) andros- man, genesis– birth) – the development of the embryo occurs due to the nuclei of one or two male gametes that penetrated into the egg with a destroyed nucleus. Such individuals were obtained from silkworms and some wasps. All of them had only paternal characteristics.