The stage of spermatogenesis characterized by morphological differentiation of germ cells. Spermatogenesis and ovogenesis are the processes of formation and formation of germ cells. Stages of distribution of DNA molecules in germ cells

cells, is called gametogenesis. It is subdivided into spermatogenesis and oogenesis. Formation begins in the uterine period, during sex differentiation, and continues until the end of the reproductive age. Sex cells are secreted by special glands - gonads. In humans and animals, female gametes develop in the ovaries, while male gametes develop in the testes.

The process of oogenesis and its features

The development of female germ cells takes quite a long time. The beginning of the process takes place in the cortical layer of the primary ovarian follicles. Completion is observed after ovulation in the oviduct. Ovogenesis is a three-phase process that includes the stages of reproduction, growth and development.

Reproductive phase and growth phase

At the first stage of oogenesis, multiple mitotic divisions occur in the cells of the ovary wall. As a result, a large number of diploid ovogonia are formed. In the human body, reproduction of the gonads starts in embryogenesis and stops by the third year of life.

The growth period is characterized by an increase in the nucleus and cytoplasm in the cells. The substances necessary for subsequent division processes accumulate, chromosome doubling occurs. In this phase, ovogons are converted into first-order oocytes. They grow in the ovary and store nutrients. Each oocyte is surrounded by epithelial cells. It forms a bubble - a follicle.

Ovogenesis is a long process. Features of the maturation phase

The maturation phase has a number of features. Prophase I of meiosis occurs during embryonic development, and the remaining stages occur after the organism reaches puberty. One follicle matures in one of the ovaries every month. At this stage, the first division of meiosis ends, a large secondary oocyte and a small body are formed. These structures enter the second phase of meiosis. At the metaphase II stage of meiosis, ovulation occurs - the oocyte leaves the ovary, enters the abdominal cavity and passes into the oviduct.

If the egg cell fuses with the sperm cell, then further maturation of the oocyte begins. As a result of the completion of meiosis II, a mature ovotid egg and a second polar body are formed.

Ovogenesis is a complex multi-stage process, as a result of which cells with a haploid chromosome set are formed from a diploid gamete: one mature egg and three polar bodies.

The egg cell has a spherical shape and large size. Its diameter in mammals and humans varies from 0.110 to 0.140 mm. In terms of volume, the egg is 10-20 thousand times larger than the sperm and 2 times longer.

The maturation phase on the example of the human body

Maturation begins at 12-13 years, during puberty. The gonads contain multiple follicles that contain oocytes. Under the influence of follicle-stimulating hormone, one after another, they begin to develop, reaching the size of a pea. As they grow, the eggs inside these vesicles reach the lumen of the ovary. As a result, the most viable follicle is isolated here, and the rest are reduced. This usually happens on the 10th day from the start of menstruation. The follicle remaining on the surface of the ovary and the Graafian vesicle continue to grow. Having reached its maximum development, the formation bursts, and the mature egg enters the oviduct.

Ovulation occurs. Under the influence of lutein-forming hormone, the vesicle that bursts Graafian changes - now it is a corpus luteum. The cells that make up its wall acquire a yellow tint due to the fat it contains. They occupy the area in which the egg was previously located. The corpus luteum produces the hormone progesterone, the action of which is aimed at preparing the uterine mucosa for fertilization.

If the meeting of the egg and sperm did not occur, after a few days, regeneration and a decrease in the corpus luteum begin. The mucous membrane of the uterus in the absence of progesterone is destroyed and rejected. This process is characterized by bleeding from the vagina lasting 2-7 days (menstruation).

The process of spermatogenesis and its features

Ovogenesis and spermatogenesis are similar to each other, the difference lies in the fact that the maturation of male gametes occurs in 4 stages.

Spermatogenesis is the formation and formation of male germ cells - sperm. It begins from the moment of sexual differentiation and develops intensively during the period of maturity of the organism.

At the stage of reproduction, multiple mitotic cell divisions begin in the testes, resulting in the formation of numerous spermatogonia with a diploid set of chromosomes. The developmental phase in men begins at puberty and lasts almost a lifetime.

At the stage of growth, the cells are called spermatocytes of the 1st order. They gradually increase in size due to the accumulation of nutrients, duplication of DNA and chromosomes.

The maturation phase is characterized by two consecutive divisions of meiosis. As a result, 4 spermatids with a haploid chromosome set are formed from each primary spermatocyte.

Features of the development of male gonads

The maturation phase is characteristic only for spermatogenesis. Its essence lies in the fact that spermatids acquire the structure and motor function characteristic of spermatozoa.

The process of spermatogenesis from the division of the original cell to the release of the sperm into the epididymis is 35-55 days. Up to 7 billion sperm mature in the gonad per day. Male gonads retain their mobility for 2-3 months, and the ability to fertilize for more than 30 days. The formation of spermatozoa directly depends on the state of the body, nutrition and external conditions. Their viability may decrease under the influence of adverse factors, inadequate diet, internal disorders.

Spermatogenesis and ovogenesis are the most important processes that are responsible for the reproduction, development and prolongation of the genus of all living beings.

The main function of every person on our planet is the reproduction of their own offspring. Without reproduction, life is impossible, and therefore every living being has the ability to reproduce. In order to start the process of reproduction, it is necessary that the male and female have mature gametes. The process of formation of gametes is called gametogenesis. In this case, the production of gametes in men is called spermatogenesis, and in women - oogenesis.

Ovogenesis and spermatogenesis: the process of maturation

The process of preparing gametes for fertilization begins in the gonads, in men this organ is represented by testicles, and in women by the ovaries. In general, gametogenesis in both sexes is similar during the first three stages. However, after this part, the maturation period acquires its own characteristics in both sexes.

Already at the first stages of male and female gametogenesis, one can distinguish their own characteristics, for example, male gametes - spermatozoa are always produced much more than female ones - eggs. In addition, male gametes develop over a longer period than female ones.

A feature of the course of oogenesis is the pronounced growth of germ cells. From one cell, four new ones are formed, which are called spermatids. At the same time, out of all four cells, only one becomes an egg, and all the rest become polar bodies. Also, a feature of spermatozoa is that they have a well-defined shape, which cannot be said about eggs.

Differences between oogenesis and spermatogenesis at different stages

Gametogenesis in males and females develops gradually in several stages. Only the first three stages of spermatogenesis and oogenesis coincide, then differences appear in the process of gamete maturation. The whole process of gamete development can be divided into the following stages:

1. The first stage is the process of reproduction. This process begins in the original cells, which in men are called spermogonia, and in women ovogonia. From these cells, a large number of new cells are formed by division.
2. The next stage is the growth stage. During this period, the germ cells converted into spermatocytes and oocytes begin to increase in size. It is important to note that oocytes are much larger than spermatocytes because female cells store nutrients. Spermatocytes do not need to store nutrients because they need a high degree of mobility.
3. ripening stage accompanied by cell division. Separation occurs in two stages, which are called the first and second meiosis. Re-separation of cells implies the appearance of already mature female gametes along with polar bodies and spermatids.
4. Next, the male gametes enter the stage of formation, in which immature spermatids receive their final spermatozoon form. In female germ cells, this stage is absent, so the eggs do not have such a finished shape as male gametes.

The main features of spermatogenesis and oogenesis are that the maturation of male germ cells is mainly aimed at multiple division, in which a large number of spermatozoa are formed, while maturation in women is characterized by the formation of only one egg.

Characteristics of oogenesis and spermatogenesis

If we consider in comparison ovogenesis and spermatogenesis, then we can distinguish a number of other signs that distinguish these two processes. First, gametogenesis in males and females occurs in different gonads, which are called testes in men and ovaries in women. It is in these organs that the production and maturation of gametes occurs.

Sex cells in spermatogenesis are called spermatozoa, and in ovogenesis they are called eggs. As you know, in order for the process of fertilization to occur, it is necessary for the male gamete to get into the female.

In comparing oogenesis and spermatogenesis, it is impossible not to note the different sizes of gametes in males and females. The egg is much larger than the sperm because it absorbs many useful and nutritious substances throughout the entire period. At the same time, after maturation, male gametes become mobile, due to which they can easily overcome the genital tract of women. Female gametes, on the other hand, remain motionless at all times during which the egg is being prepared for fertilization.

Also, the sex cells of men and women differ in shape. Spermatozoa have a finished round shape with a tail, in contrast to the egg, which has a simple round shape.

If we consider oogenesis and spermatogenesis in the table, it will be possible to notice that the stages at which there is an increase in size, division and full maturation in both sexes coincide. The schemes of oogenesis and spermatogenesis are very similar, however, the development of spermatozoa also includes a fourth stage, which is the final design.

Another feature of spermatogenesis and oogenesis is the different periods of production of male and female germ cells. Eggs are formed in a woman's body cyclically, with which the menstrual cycle is associated. The formation of a new egg occurs every 21-35 days. At the end of the cycle, the egg dies, this process is accompanied by bleeding. As a result, changes in the hormonal background occur, as a result of which a new process of egg maturation is launched.

In men, the formation of spermatozoa occurs constantly, and the production of gametes occurs throughout the entire period of male maturity. A man produces about 30 million sperm per day. In a woman, the number of gametes is much less. For comparison, in a lifetime, the fair sex produces about 500 mature germ cells.

Spermatogenesis, unlike oogenesis, is more susceptible to external conditions. This is primarily due to the fact that the sex glands, or testes in men, are located outside the abdominal cavity, that is, in the testicles.

In women, the sex glands, that is, the ovaries in which the egg is formed, are located more reliably. This is due to the fact that the appearance of eggs in the body of a future woman begins even before her birth, and ends after fertilization.

The growth stage in oogenesis significantly exceeds the similar stage in spermatogenesis, which explains why the egg cell is much larger than the spermatozoa. But male gametes compensate for this stage by cell division and the formation of a large number of sperm.

Why is the egg so much larger than the sperm?

As mentioned earlier, the egg in the process of growth absorbs all the nutrients and nutrients. It is important for spermatozoa to maintain their own in order to be able to overcome the path through the female reproductive tract and fertilize the egg in the future. In addition, female gametes need nutrients in order to further nourish the fetus developing in them.

Differences in the size of male and female gametes are also caused by the fact that the duration of the existence of the egg is much longer than that of sperm. For example, male gametes, under favorable conditions, can exist for only a few days. As for the egg, it exists during the entire period of development of the fetus, until the moment of birth.

Stages of distribution of DNA molecules in germ cells

At the stage of division of germ cells, upon closer examination, one can observe the process of separation of chromosomes. During cell division, all DNA molecules are duplicated in their nuclei, after which chromosomes are redistributed, which can be divided into the following stages:

1. Leptotic stage. At this stage, the nucleus and twisted strands of chromosomes can be distinguished. The paternal and maternal chromosomes are at a distance.
2. Zygotenous. At this stage, the contact of chromosomes and the exchange of genes occurs.
3. Pachytene. At this stage, the connection of chromosomes is strengthened in the cell, which are well twisted with each other.
4. Diplomatic. This stage is characterized by doubling of all chromosomes, after which they are separated into two pairs.

Summing up, we can say that spermatogenesis and oogenesis are two processes aimed at the production of germ cells, which have their similarities and differences, due to the peculiarities of the device of the male and female organisms.

MINISTRY OF EDUCATION AND SCIENCE OF THE RUSSIAN FEDERATION

FSBEI HPE "Penza State University"

medical institute

Department of Clinical Morphology and Forensic Medicine with a course of oncology.

Coursework by discipline

"Histology, Cytology, Embryology".

Spermatogenesis and oogenesis. Similarities and differences"

Completed: Art. gr. 12ll6 IzyavlevaO.V.

Checked by: assistant Yunyashina Yu.V.

Introduction

spermatogenesis

Comparison of spermatogenesis and oogenesis

Conclusion

Bibliography

Introduction

Reproduction or reproduction, the function inherent in all living beings to reproduce their own kind. Unlike all other vital functions of the body, reproduction is not aimed at maintaining the life of an individual, but at preserving its genes in offspring and procreation - thereby preserving the gene pool of a population, species, family, etc. In the course of evolution, different groups of organisms have developed - in many cases independently - different ways and strategies of reproduction, and the fact that these groups have survived and exist proves the effectiveness of different ways of carrying out this process.

With sexual reproduction, offspring usually have two parents. Each parent produces sex cells. Sex cells, or gametes, have a half or haploid set of chromosomes and arise as a result of meiosis. Thus, a gamete (from the Greek gamete - wife, gametes - husband) is a mature reproductive cell containing a haploid set of chromosomes and capable of merging with a similar cell of the opposite sex to form a zygote, while the number of chromosomes becomes diploid. In a diploid set, each chromosome has a paired (homologous) chromosome. One of the homologous chromosomes comes from the father, the other from the mother. The female gamete is called the egg, the male gamete is called the sperm. The process of formation and development of gametes in the gonads has a common name - gametogenesis. All other cells that are not directly involved in the formation of gametes are called somatic cells. Gametogenesis is a broad term that refers to the gradual "creation" of highly specialized cells capable of giving rise to a new organism.

Primary germ cells - gonocytes are descendants of embryonic totipotent cells present in the blastoderm of the embryo during the formation of the primary strip. They appear before the gonad and exist independently of it. Then they enter the posterior extraembryonic endoderm, migrating into the intestinal wall and into the surrounding mesenchyme, and then move to the dorsal mesenterium to the anlage of the gonad. Before development, the gonads actively move in the body with currents of fluids. Once near the gonad, the gonocytes approach it in an amoeboid way, attracted by a proteinaceous factor secreted by the gonad. Penetrating into the gland (ovaries in females, testes in males), germ cells are located in males in the brain, and in females in the cortical layer of the gonads. In the future, the germ cells before their maturation are in the gonads. The gonads of the embryo initially contain a relatively small number of primary germ cells that populated them. But once in the gonads, germ cells begin to divide vigorously, and their number increases dramatically. Cells divide mitotically. Mitosis ensures the transfer to two daughter cells of exactly the same sets of chromosomes containing hereditary information.

1. Spermatogenesis

In the male reproductive system, spermatogenesis occurs in the sex glands (gonads), represented by a paired organ - the testicles, which perform two important functions: - generative (formation of male germ cells); - endocrine (synthesis of male sex hormones).

These functions are interrelated, although they are provided by various structural components of the body.

Spermatogenesis includes four periods: -reproduction; -growth; - maturation; -formations.

breeding period. Spermatogenic cells are represented by spermatogonia. These are small rounded diploid cells located on the basement membrane of the seminiferous convoluted tubules. There are two types of spermatogonia: A and B. Type A is represented by light and dark slightly flattened cells with a light nucleus. Dark spermatogonia - non-dividing, resting cells, are considered stem cells; light spermatogonia - cells dividing by mitosis. Some of them support the population of cambial cells, others - in the course of successive divisions become type B spermatogonia. The latter have a pear-shaped shape, a large rounded nucleus and a centrally located nucleolus. Spermatogonia are replenished by division (complete mitosis) of stem cells of the male gonad. At a certain moment, the daughter cell (stem derivative) divides incompletely, leaving a bridge that connects the daughter cells, and enters the path of spermatogenesis. Syncytial communication, on the one hand, ensures the synchronism of the existence of cells of the clone, on the other hand (due to the mass character) - the heterogeneity and polymorphism of its constituent cells and thus high viability. The divisions of the gonias are differentiating. In the process of such mitotic divisions, the daughter cells do not fully grow to the original ones and become smaller, and as a result, they prepare the gonia to enter meiosis. The period of mitotic differentiating spermatogenesis ends with the creation of "secondary" spermatogonia, and brings the cells of the clone to the meiotic period of spermatogenesis. Cells that have completed division and entered the period of growth and maturation are called primary spermatocytes (first-order spermatocytes).

Growth period. During meiosis, complex changes in the nuclei take place, preparing the cell for the transition to the haploid state. First-order spermatocytes significantly increase in volume and become the largest spermatogenic cells, the DNA content in the nuclei doubles (2n4c). They separate from the basement membrane of the tubules and move towards the lumen of the tubule. Spermatocytes of the first order immediately enter the prophase of the first division of meiosis, the duration is about 22 days. In the prophase of meiosis I, the spermatocyte grows, and therefore such cells are also called auxocytes, that is, growing. Thus, the largest cells of spermatogenesis are spermatocytes of the first order, preparing for the first division of maturation.

Ripening period. In males, the first reduction division of meiosis ends with the formation of two second-order spermatocytes, or secondary spermatocytes. These are cells smaller than the primary ones, which are located closer to the lumen of the tubules. The second equational division ends with the appearance of 4 haploid cells - spermatids.

The period of formation (spermiogenesis). During this period, spermatids are transformed into mature germ cells - spermatozoa (sperm). During the period of formation, only structural changes in cells occur, since their chromosome set does not change, remaining haploid. At the beginning of spermatogenesis, the cells are still connected to each other by cytoplasmic bridges and continue to be part of the syncytial clone. Structural changes in spermatids are:

compaction of chromatin (due to the replacement of histones with non-histone proteins), a decrease in the nucleus, the acquisition of a pear-shaped shape:

the formation of an acrosome - a flat membrane sac containing a number of lipic enzymes necessary for fertilization. The acrosome is a derivative of the Golgi complex, which initially forms acrosomal granules, which, merging, form a bubble adjacent to the future front surface of the nucleus and gradually spreading over it in the form of a cap;

the formation of the flagellum by the distal centriole, which forms the axoneme of the tail (after the movement of both centrioles to the posterior pole of the nucleus); the proximal centriole is located in the depression of the nuclear envelope;

the formation of special elements of the cytoskeleton occurs as the tail is formed and includes the appearance of 9 longitudinally lying segmented columns around the centrioles (connecting section), which are connected in detail with 9 dense fibers located along the periphery of pairs of axonemal microtubules (intermediate section). In the main section, a fibrous sheath is formed, formed by longitudinal columns connected by ribs;

change in the shape and location of mitochondria, which, from elongated and diffusely scattered throughout the cytoplasm: spermatids, become spiral and focus around dense fibers in the emerging intermediate section, closely adjacent to each other;

removal of excess cytoplasm containing organelles and lipid inclusions from the developing sperm in the form of the so-called residual bodies, which are released into the lumen of the tubule.

A feature of spermatogenesis is the formation of functional syncytium, which unites clones of spermatogenic cells involved in this process. Intercellular connections of spermatogenic cells ensure their synchronous development, nutrient transfer and intercellular exchange of gene expression products. (Fig. 1)

Fig.1. Scheme of spermatogenesis processes.

Spermatogenesis in humans lasts 64-74 days, starting at puberty and continuing throughout life. After 50 years, its intensity decreases significantly. A person produces about 250 million sperm every day. Spermatogenesis normally proceeds at a temperature 3 degrees below body temperature (temperature in the scrotum). It is suppressed by an increase in temperature (wearing excessively warm clothes), cryptorchidism (undescended testicle into the scrotum) and pressure on it from surrounding tissues in the peritoneal cavity and inguinal canal.

2. Oogenesis

Eggs are the female gametes of animals and higher plants. As a rule, eggs are haploid cells, but may have a different ploidy in polyploid organisms. The human egg is about 150 microns in diameter.

The cytoplasm of the eggs (ooplasm) contains inclusions of nutrients - the yolk. Oocytes are produced as a result of oogenesis. After fertilization, the fertilized egg (zygote) develops into an embryo. In parthenogenesis, an embryo and then a new organism develops from an unfertilized egg.

The human egg was first described in 1827 by Baer. This increased interest in the study of the processes of gamete formation and fertilization.

The ovum differs from the sperm in:

overwhelming real estate;

respectively, a characteristic more or less spherical shape;

the presence of a variety of protective and shell-sources of nutrients;

the absence of functional organelles or formations inherent in the spermatozoon: a tail, a specialized mitochondrial complex, an acrosome, etc.;

genetic information (sex chromosomes - XX).

features of education and development, as well as life time;

a significantly smaller number of them in the body (during life, about 400 eggs are formed in the female body, while hundreds of millions of spermatozoa in the male).

a supply of nutrients for the development of the future embryo, localized in the cytoplasm;

significantly larger (a human egg is 85,000 times larger than a spermatozoon).

The process of development of female germ cells is called oogenesis. Gonocytes infiltrate the rudiment of the female genital gonad, and all further development of the female germ cells occurs in it. Once in the ovary, the gonocytes become oogonia. There is no formation period in this process.

The process of oogenesis consists of three periods: -reproduction; -growth; -ripening.

In contrast to spermatogenesis, reproduction, growth and partial maturation occur in the ovaries, ending in the oviduct. In addition, the end of the second division of meiosis occurs only as a result of fertilization, and therefore the process of oogenesis does not always reach its end.

breeding period. Diploid cells formed from gonocytes at the 8th week, oogonia (immature sex cells), repeatedly divide mitotically up to 3-4 months of intrauterine development, as a result of which their number in both human ovaries increases, reaching several hundred thousand. With such a supply of germ cells, a girl is born. New germ cells do not appear after birth and there is a massive degeneration of germ cells. After the last division in the reproduction period, the cell enters the prophase of the first division of maturation, and the cell cycle lingers on this for a long time. In prophase I of meiosis, conjugation of chromosomes, the formation of a synaptonemal complex, and crossing over occur, that is, events that determine all further meiotic processes.

Growth period. The oogonia enter a period of growth. They lose the ability to mitotic division and enter prophase I of meiosis. In prophase I of meiosis, conjugation of chromosomes, the formation of a synaptonemal complex, and crossing over occur, that is, events that determine all further meiotic processes.

There are two phases: small and large growth. Before the onset of puberty, a process of small growth takes place, when there is mainly an increase in the size of the nucleus and cytoplasm due to the accumulation of deutoplasmic substances in the form of yolk. During a period of great growth, the accumulation of nutrient material in the cytoplasm occurs, which is brought to the ovary with the blood of the mother's body. The composition of the so-called yolk inclusions includes proteins, fats, fat-like substances. On the chromosomes of the primary oocyte, a large amount of information and transfer RNA is synthesized, as well as substances of a special composition, located under the plasmolemma, forming the cortical layer. The stages of proleptotene, leptoten, zygotene, pachytene, diplotene come in succession. At the zygotenic stage of the prophase of meiosis, the formation of the synaptonemal complex and the conjugation of homologous chromosomes begin. The synaptonemal complex (SC) is a genetically determined three-membered protein structure. On pachytene, conjugation ends with the formation of a bivalent, which achieves an imaginary reduction in the number of chromosomes. This is how a primary oocyte, or first-order oocyte, is formed, initially surrounded by a layer of flat follicular cells (primordial follicle). The volumes of the nucleus and cytoplasm increase proportionally and slightly. At the same time, nuclear-cytoplasmic relations are not violated.

Forming primary follicles, in which a shiny zone appears for the first time, which has the form of an unstructured oxyphilic layer between the primary oocyte and follicular cells of a prismatic shape. It performs a number of important functions: -forms a semi-permeable barrier between follicular cells and oocyte; -increases the surface area of ​​contact between them; - provides species-specific fertilization; -provides monospermic fertilization; - protects the early embryo during its movement through the genital tract before implantation.

In the first half of a large growth, the nucleus and cytoplasm intensively increase (cytoplasmic growth). "Lampbrushes" and nucleoli reach their maximum development and actively participate in RNA synthesis. In the second half of the period of great growth, vitellogenesis (trophoplasmic growth) occurs. In the nucleus, there is a decline in RNA synthesis. Quite often, a karyosphere is formed - a special structure with pores, consisting of elements of membranes and a synaptonemal complex for isolating the diplotene chromosomes of the oocyte nucleus from the functional activity of extrachromosomal DNA and nucleoli.

At the end of a period of great growth, the “lamp brushes” lose their loops and are greatly shortened. The stage of diakinesis begins, after which the metaphase plate of the first division of maturation is formed. The nucleoli function for a short time or do not develop at all, and the karyosphere is formed early. Nuclear-cytoplasmic ratios decrease.

At the stage of diakinesis, the course of meiosis slows down until it stops completely (meiosis block). The block of meiosis in humans is removed with the onset of puberty. Prophase I can be very long, and a large growth of oocytes capable of ovulation in a person stretches for decades, that is, for the entire reproductive growth.

With each sexual cycle, a group of oocytes enters a period of great growth, but not all of them develop to the end, since most of them stop growing and die. Only one of them (very rarely several oocytes) passes to the next period of oogenesis - maturation.

Ripening period. With the accumulation of the necessary substances in the cytoplasm of the primary oocyte, the prophase is completed, and then the remaining phases of the first reduction division of maturation. As a result, two diploid, but unequal-sized cells are formed. In one of them, a large cell, called a second-order oocyte, or a secondary oocyte, almost all the accumulated substances necessary for further development remain. The other, small in size, has very little cytoplasm, and therefore is called a reduction or directional body. The formation of a secondary oocyte in a woman coincides with the moment of ovulation, when after the rupture of a mature follicle (Graafian vesicle), which usually occurs on the 14th day of the ovarian-menstrual cycle, the germ cell leaves the follicle. Following this, a second-order secondary oocyte at the metaphase stage of the second division of meiosis, surrounded by a transparent zone and follicular cells of the radiant crown, enters the funnel of the fallopian tube. (Fig.2, 3.)

Fig. 2. Scheme of oogenesis processes.

The second division of meiosis is not always completed, but only if the spermatozoon reaches the surface of the oocyte and penetrates into it. This division is also uneven, as it leads to the formation of an egg from a secondary oocyte, which retains all the substances necessary for the development of a new organism, and a new reduction body.

The cyclical growth and maturation of germ cells in a sexually mature female body is manifested in the fact that 5-20 oocytes are involved in the process of large growth every month, but only one of them will enter the maturation phase, and the rest will die in the process of follicular atresia. In the 5th decade, with the onset of menopause, the development of germ cells stops: in the future, they undergo degenerative changes and disappear from the ovary.

Rice. 3. Stages of human oocyte development:

A - before birth, a small proportion of primordial follicles begin to grow, and these follicles are now called developing. B - After some period of continuous growth, some of the developing follicles accumulate fluid, turning into antral follicles. C - with the onset of puberty once a month, a wave of luteinizing hormone (LH) secreted by the pituitary gland induces one antral follicle to mature: the first-order oocyte located in this follicle completes the first division of meiosis, forming a polar body and turning into a second-order oocyte. G - the second-order oocyte, together with the polar body and part of the surrounding follicular cells, is released at the moment when the follicle ruptures on the surface of the ovary. The oocyte of the second order undergoes the second division of meiosis only if it is fertilized. - primordial follicle; II - developing follicle; III - antral follicle; VI - large antral follicle (Graaf's vesicle); V - erupted follicle; 1 - oocyte of the first order, stopped in prophase I: 2 - follicular cells; 3 - cavity; 4 - oocyte of the first order; 5 - LH level rise; 6 - oocyte of the first order completes the first division of meiosis, turning into a second-order oocyte; 7 - surface of the ovary; 8 - oocyte of the second order; 9 - 1st polar body.

Ovogenesis proceeds with the constant interaction of developing germ cells with epithelial cells in the follicles.

Comparison of spermatogenesis and oogenesis

Oogenesis has a fundamental similarity with spermatogenesis, oogenesis also goes through a series of stages: reproduction, growth and maturation.

Despite this fundamental similarity of genetic processes during spermatogenesis and oogenesis, there are significant differences between them.

First, the formation stage is inherent in spermatogenesis and is absent during oogenesis.

Secondly, the growth stage in oogenesis is longer than in spermatogenesis.

Thirdly, the stage of maturation of oogenesis has its own characteristics, consisting in the uneven divisions of maturation, leading to the release of polar bodies. spermatogenesis oogenesis reproduction

Fourth, in female individuals, the first division of meiosis begins during fetal development, is completed for the first time by the time of puberty, and in the last - on the eve of menopause. In boys, meiosis begins only with the achievement of puberty and persists throughout the entire puberty of a man.

Fifth, the formation of mature germ cells in women occurs cyclically with a period of approximately 28 days, while in men it occurs continuously.

Sixth, unlike spermatogonia, each of which, as a result of meiosis, produces four functionally complete spermatozoa, only one egg cell is obtained from oogonia. After the first division of meiosis, most of the cytoplasm leaves one daughter cell, and a small part goes to the second, called the directional body. The same happens during the second division of meiosis. The directional bodies degenerate.

Seventh, the male and female sex cells are very different in structure and function: the spermatozoon is a small mobile cell, very rich in mitochondria, which supply it with energy for movement, while the egg cell is the largest cell in the human body (diameter 150 - 200 microns). ), which contains not only significant reserves of nutrients, but also messenger RNAs that will be used in the early stages of embryo development. The egg is surrounded by follicular cells that feed it and forms a specialized structure - the follicle (Graaf's vesicle).

Eighth, the course of spermatogenesis is more susceptible to the influence of environmental factors than the course of oogenesis, due to differences in the location of the genital organs (testes, as a rule, are located outside the abdominal cavity).

Rice. 4. Comparison of spermatogenesis and oogenesis.

Conclusion

Sexual reproduction is a significant evolutionary acquisition of organisms. On the other hand, it contributes to the reassortment of genes, the emergence of a variety of organisms and increase their competitiveness in a constantly changing environment. Compared to other cells, the function of gametes is unique. They ensure the transfer of hereditary information between individuals of different generations, which saves life in time.

Bibliography

1. Valkov E.I. "General and Medical Embryology". Textbook for medical schools. St. Petersburg "FOLIANT" 2003 Art. 27-34.

indicating the topic right now to find out about the possibility of obtaining a consultation.

Primordial sex cells can be found at the fourth week of development outside the embryo in the endoderm of the yolk sac. From there, they migrate during the sixth week to the sex ridges and combine with somatic cells to form primitive gonads, which soon differentiate into testes or ovaries, depending on the composition of the zygote's sex chromosomes (XY or XX).

Both during and during oogenesis, meiotic division occurs, however, there are important differences in its details and timing, which can have clinical and genetic consequences for the offspring. Female meiosis begins early during embryonic development in a limited number of cells. In contrast, male meiosis occurs continuously in many cells throughout a male's adult life.

study directly meiosis person is very difficult. In women, the main stages of meiosis occur in the ovaries of the fetus, in oocytes before ovulation and after fertilization. Although cells after fertilization can be studied in vitro, access to earlier steps is limited.

Get testicular material for the study of male meiosis is less difficult, since testicular biopsy is included in the examination of many men attending fertility clinics. A lot of information has been obtained about the cytogenetic, biochemical and molecular mechanisms of normal meiosis and the causes and consequences of meiotic disorders.

spermatogenesis

spermatozoa formed in the seminiferous tubules of the testicles after puberty. The tubules are filled with spermatogonia at different stages of differentiation. These cells originate from primary germ cells as a result of a long series of mitoses. The last cell type in the sequence of spermatogenesis are the primary spermatocytes, which enter the first division of meiosis, forming two haploid secondary spermatocytes.

Secondary spermatocytes quickly pass the second division of meiosis, forming two spermatids, which, without further division, mature into spermatozoa. In humans, this process takes about 64 days. Huge numbers of spermatozoa are formed, usually about 200 million per ejaculation, and during a lifetime about 1012, which requires many hundreds of previous mitoses.

Ovogenesis

Unlike spermatogenesis, which begins at puberty and continues throughout life, oogenesis begins during the prenatal period. The ovum develops from oogonia, cells in the ovarian cortex derived from primordial germ cells in about 20 mitoses. Each oogonium is the central cell in the developing follicle.

About third In the first month of intrauterine development, the ovogons of the embryo begin to form primary oocytes, most of which enter the prophase of the first division of meiosis. The process of oogenesis is not synchronized, and both its early and final stages coexist in the fetal ovaries. At the time of the girl's birth, there are several million oocytes, but they are mostly degraded.

Ripen and ovulate eventually only about 400 eggs. Primary oocytes practically complete the prophase of the first division of meiosis by the time of birth, and those that do not undergo degeneration remain in this phase for quite a long time, until ovulation during the menstrual cycle.

With the onset of sexual maturity individual follicles about once a month begin to grow, mature and ovulate. Before ovulation, the oocyte quickly completes the division of meiosis I in such a way that one descendant cell becomes a secondary oocyte (or egg) containing almost the entire cytoplasm with organelles, and the second turns into the first directional (polar) body.

The second division begins quickly meiosis, during ovulation, it reaches the metaphase stage, where the division process stops, ending in the event of fertilization.

Fertilization in humans

Fertilization of the egg usually occurs in the fallopian tubes within about a day after ovulation. Although there may be many sperm around the egg, the entry of the first sperm into the egg sets off a chain of biochemical events that prevent the entry of other sperm.

Behind fertilization This is followed by the completion of the second division of meiosis with the formation of the second polar body. The chromosomes of a fertilized egg and sperm form pronuclei surrounded by a nuclear membrane. Shortly after fertilization, the chromosomes of the zygote double and divide by mitosis, forming two diploid daughter cells. This is the first mitosis in a series of cleavages that initiates the development of the embryo.

While development starts with education zygotes(conception), in clinical medicine, the onset and duration of pregnancy is usually calculated from the start of the last menstrual cycle, approximately 14 days before conception.

The conception of a child occurs due to the course of certain preparatory processes in the male and female body, namely the maturation of germ cells, which is called gametogenesis. Comparative characteristics of spermatogenesis and oogenesis makes it possible to detect similarities and differences.

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The production and maturation of male sex gametes occurs in the testicles, and this process is called spermatogenesis.

The development of the female germ cell is called oogenesis.

The stages of maturation in the female and male bodies have many similarities, but there are also differences.

The formation of spermatozoa occurs from the primary germ cells, which are called gonocytes. This complex process occurs during the growing up of a young man and continues until old age. It is believed that the full cycle of maturation and development of seminal fluid takes at least three months.

The process of sperm formation in the male body is quite complex and subject to many negative external influences, therefore, from an early age, it is recommended that a young man take his health seriously in order to maintain fertility and transfer personal genetic material to the unborn child at the right time.

Every day, two hundred million spermatozoa are formed in the testicles of a man, but no more than half remain viable.

The maturation of spermatozoa in the male body is divided into three main stages.

At the first stage, the division of stem cells occurs, which are located in the male testes. This stage of sperm development is called mitosis.

The second stage is meiosis of the first order. During this period, some of it is converted into spermatocytes, after which their number grows.

At the final stage of formation, spermatocytes are reborn:

• first into spermatids;
• after which the spermatozoa themselves are formed.

During the maturation of seminal fluid, male gametes come into close contact with Sertoli cells, which are necessary to create special favorable conditions for spermatozoa.

Sertoli cells create the following conditions:

• form a favorable environment with optimal characteristics for spermatozoa;
• produce a special fluid that is required for the normal functioning of the testicles;
• provide transportation of oxygen and nutrition to cells;
• participate in the synthesis of a binding protein that delivers testosterone.

The maturation process is very complex and depends on many external factors and the normal functioning of the body as a whole. A healthy man produces good quality seminal fluid, which contains the maximum number of viable spermatozoa, ready to participate in the conception of the fetus.

The process of their formation depends on environmental factors. At each stage of maturation of gametes, they can be negatively affected, and this feature is inherent only in the male body.

The temperature of the environment and the body negatively affects the process of spermatogenesis. The required value for normal maturation is 34 ° C, it is this level that is maintained in the scrotum due to the uninterrupted functioning of blood flow.

The quality of seminal fluid changes significantly with an increase in body temperature. Even small changes in this value as a result of a cold lead to a loss of sperm viability. Therefore, the use of heated seats in modern cars often does not become a boon for a man.

In addition to temperature fluctuations, the following points have a negative impact on the process of spermatogenesis:

• stressful conditions;
• the use of antibiotics and steroid drugs;
• hormonal imbalance.

A man who wants to conceive a child should avoid these factors. It is necessary to postpone the intake of any medications, try not to be nervous, lead a measured and healthy lifestyle.

In addition to the listed negative factors, the formation of healthy spermatozoa is influenced by the ecological situation in the area where a person lives. The polluted atmosphere and the presence of dangerous chemical compounds in it stimulate negative processes and contribute to the occurrence of infertility.

If it is impossible to conceive a child in a couple, first of all, the health of a man is checked, for which biological material is taken from him for research. This test is called a spermogram, it is used to determine the viability of spermatozoa.

Healthy sperm can keep a woman active for up to seven days. The head of this flagellum contains a special chemical that dissolves the protective shell of the egg and allows the gametes to penetrate inside, after which the conception of the fetus occurs.

Ovogenesis

The main difference between oogenesis and spermatogenesis is that the formation of female cells occurs even in the prenatal period, before the birth of the future mother.

The process of oogenesis is designed to perform the following functions by the female body:

• the nucleus of the egg forms a haploid set of chromosomes;
• the zygote is filled with nutrients.

The scheme of development of oogenesis, in comparison with spermatogenesis, occurs in almost the same way and can differ only in small details.

In general, oogenesis goes through the following stages of gamete development:

• reproduction;
• growth;
• maturation.

Reproduction of gametes in the female body occurs even in the prenatal period. At this time, the primary rudiments of the eggs are formed. They begin active division as early as the sixth month of pregnancy, and thus, ovogonia accumulate in the cortical layer, which concentrate the necessary nutrients (protein, glucose, fats). Owogonia formed from germ cells exceed them in size, but the genetic composition does not change.

At the second stage of egg formation, the formation of first-order oocytes from ovium occurs. This process is also recorded during the period when the fetus is in the womb.

Oocytes develop rapidly and gain a significant mass, after which the eggs are transferred to conservation mode, forming prophase gametes, which in the female body are initially about two million. In the future, there will be no more than five hundred cells.

The final phase of egg maturation occurs during adolescence. One of the oocytes begins to function, as a result of which the young girl begins her period.

A feature of oogenesis is the pronounced development of cells, when four are formed from one. They are usually called spermatids, while only one of them is reborn into an egg, while the rest are converted into polar bodies. The common developmental phases for spermatogenesis and oogenesis are: cell reproduction, their growth and maturation.

Three stages of formation occur in the testicles of a man and the ovaries of a woman, but after passing through this process, differences and features of further maturation arise.

Having studied the process of formation of female and male germ cells, it can be found that the production of sperm and eggs by the body at the initial stage occurs in the same way. The first three phases coincide, but subsequently the maturation of gametes continues according to different patterns.

The general phases of maturation in the male and female body can be defined as the following processes:

• at the first stage, reproduction occurs, by division, which begins from the original germ cells of the male spermogony, and the female - ovogony;
• the second stage is the growth of cells and their transformation into spermatocytes in men and oocytes in women.

An increase in size begins, and oocytes are significantly superior to spermocytes, since their task is to accumulate nutrients for the future egg;

• in the third phase of development, division and maturation occurs, called meiosis. At this point, eggs are formed, ready for fertilization.

After passing through the general three stages of development of germ cells, the formation of the final form of sperm occurs in the male body, and the female eggs bypass this stage, remaining in the form that they acquired in the third phase of maturation.

These features of the formation of gametes in the male and female body are of great importance for the successful conception of a child. Uneven cell division as a result of oogenesis allows the formation of one large egg, which is provided with a large amount of nutrients.

For spermatozoa, such a reserve is completely irrelevant. Its main task is to be mobile in order to achieve its goal, to fertilize the female egg and transfer its own set of chromosomes.

Main differences

With the prevailing similarity of the processes of gamete formation in the body of women and men, significant differences are noticed.

The general phases of development of spermatozoa and female germ cells include three stages: reproduction, growth and maturation.

This completes the formation of eggs, while the fourth stage begins for male gametes.

At the fourth stage of development of spermatozoa, their shape changes: they become oblong, and the movable tail provides the possibility of movement.

The next point of difference in the formation of male sex gametes and the egg is the feature of division. So four cells grow from the spermatocytes of the first order. At the same time, only one egg capable of fertilization is produced from an oocyte at the same stage of development. Its formation is subject to cycles, and the update occurs every 20 or 35 days. Bleeding occurs, as a result of which it dies, after which a change in the hormonal background gives an impulse to the maturation of the next egg. In addition, it should be noted that the first two stages of egg formation occur in the female body in the prenatal period, and at the birth of a girl, she already has a set of first-order oocytes.

The final maturation of the egg occurs at the time of sexual development, after which the process ends.

Spermatozoa are formed exclusively in sexually mature men, but continuously throughout adulthood. Including there is a quantitative difference between the values ​​of sperm and eggs. The differences between spermatogenesis and oogenesis include the process of formation of germ cells throughout life.

The male body is capable of producing about thirty million sperm per day, and in a future mother, no more than five hundred eggs are formed in her entire life, ready for fertilization.

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