Male sex cells (spermatozoa)

When studying male germ cells, one should understand the structure of flagellar (beach-shaped) spermatozoa, which are the predominant form of spermatozoa, and for comparison, get acquainted with the morphology of flagellated (non-beach-shaped) spermatozoa. It is necessary to pay attention to such physiological features as their mobility, life span, dependence of activity on the environment, etc.

Male germ cells in their structure and physiological properties differ significantly from female germ cells. Spermatozoa are much smaller than eggs. The male germ cells of a crocodile are 20 microns long, a sparrow is 200 microns, a guinea pig is 100 microns, a bull is 65 microns, and in humans, an average of 50 microns. Spermatozoa are more numerous than eggs. Their number in animals is measured in millions. For example, in a person, 1 cm 3 of sperm contains 60 million spermatozoa. Mature spermatozoa are actively mobile cells.

Among the male germ cells of various groups of animals, two significantly different types of spermatozoa are distinguished: flagellated (beach-shaped) and non-flagellated (non-flagellated). Flagellated spermatozoa are the predominant form (Figure 4).

Fig.4. Forms of human and animal spermatozoa. 1 - person; 2 - triton; 3 - crayfish; 4 - guinea pig; 5 - pigs; 6 - bull; 7 - rooster; 8 - branched cancer; 9 - tithe cancer; 10 - horse roundworm; 11 - pinworms (according to Golichenkov).

Flagellated spermatozoa, even in animals of very distant species, are built according to the same scheme, which is probably due to the similarity of their functional purpose throughout the animal world.

Four sections are distinguished in the flagellated spermatozoon: head, neck, middle part, tail (flagellum). All sections of the spermatozoon are covered on the outside by a common plasma membrane.

The head of the spermatozoon has a different shape in different classes of animals: in a newt, the head has the shape of a crochet hook; in passerine birds, it is corkscrew-shaped; in mammals, it is oval in front and pear-shaped on the side. Most of the sperm head is occupied by the nucleus. In the cytoplasm of the anterior part of the head, the acrosomal apparatus is located, which plays an important role in the dissolution of the egg membranes. Spermolysins are concentrated in the acrosome - substances related to proteolytic enzymes (Fig. 5).

Fig.5. Scheme of the structure of mammalian sperm, which shows the structures detected using an electron microscope and indicates the functions they perform (according to Alexandrovskaya)

The cervix is ​​the short, narrower part of the spermatozoon. There are two centrioles in the neck: the proximal (anterior), adjacent to the nucleus, and the distal (posterior), which is connected to the axial thread of the tail.

The middle part of the spermatozoon consists of an axial filament and the surrounding cytoplasm. In the cytoplasm there is a large number of mitochondria, which are located one after another in the form of a spirally twisted thread. Mitochondria generate the energy necessary for the movement of the male germ cell.

The tail section (flagellum) of the spermatozoon consists of an axial filament, which is covered with a thin layer of cytoplasm. The axial filament of the flagellum is represented by 2 central and 9 peripheral pairs of fibrils, which stretch without significant changes along the entire length of the tail of the spermatozoon - from the neck almost to the tip (Fig. 6).

Fig.6. Electron microscopic structure of sperm: 1 - head; 2 - neck; 3 - axial thread; 4 - mitochondria; 5 - plasmalemma. (According to Alexandrovskaya).

Thus, the general organization of spermatozoa and its departments are adapted to perform specific functions inherent in this cell. These functions are: a) ensuring a meeting with the female sex cell; b) stimulating the egg to develop; c) the transfer of paternal hereditary material into it.

In some animals, spermatozoa do not have flagella and are called non-flagellated (non-flagellated). These spermatozoa have the most varied shape: round, filamentous, biconcave, sometimes of a very unusual type (Fig. 6).

The viability and motility of spermatozoa in water is limited in time. In sea water, they lose their mobility after a few hours, and in fresh water, as a rule, after a few minutes. In animals with internal fertilization, male germ cells retain their viability a little longer: in a pig - 22 - 30 hours, in a sheep - 30 - 36 hours, in cattle - 25 - 30 hours, in the genital tract of a woman, the life span of sperm varies from 2 to 4 days.

In some species of animals, spermatozoa remain viable in the female genital tract for a long time. For example, in some bats, insemination occurs in the fall, but during the entire hibernation of animals, the spermatozoa are in a dormant state. Fertilization occurs only in the following spring. In chickens, spermatozoa are stored for 3 weeks. In many insects, spermatozoa are stored for a long time. For example, in bees, male germ cells persist for several years.

There have been, and still are, many misconceptions about sperm motility and fertility. It was believed that the fertilizing ability of the spermatozoon is preserved until the moment it loses the ability to move. By now, it is known that motility will last much longer than the ability to fertilize. Thus, rabbit spermatozoa lose their ability to fertilize after 30 hours in the female genital tract, while they can retain mobility for more than two days. Human spermatozoa retain the ability to fertilize for 1-2 days, and they retain mobility for up to 4 days.

In the process of fertilization, the sex cells of the paternal and maternal organism participate. Just as the physiology of men and women as a whole differs, the structure of male germ cells and female gametes is not the same. The name of male germ cells - spermatozoa - combines two Greek roots meaning "seed" and "life" and explains their function: without their participation, reproduction is impossible. The structural features of male germ cells are explained by their role in the process of fertilization.

The goal of each spermatozoon is to be the first to reach the egg (female germ cell) and break through its membrane inside. Therefore, each spermatozoon is equipped with a means for rapid movement - a tail, thanks to the vibrating movements of which the male germ cells move very quickly. in this it is completely opposite - they do not have the ability to move and overcome only 10 cm of the way along the oviduct to the place of meeting with the spermatozoon.

The structure of the male reproductive cell - sperm

The male germ cell resembles a tadpole in appearance: each spermatozoon consists of a head, tail and neck. The function of the tail is clear - it provides the ability to move. The head is a kind of container in which each spermatozoon carries the most valuable cargo - genetic information packed into chromosomes.

Ordinary cells of the human body have 46 pairs of chromosomes, and germ cells (both male and female) carry only half the set of chromosomes. Of the 23 pairs of chromosomes in a sperm cell, there is one pair of special sex chromosomes that will ultimately determine what gender the baby will be born to. Sex chromosomes can be of the X or Y type, if a spermatozoon carrying the X chromosome was involved in the process of fertilization, then a girl will be born as a result, a boy will be born on the Y chromosome. The spermatozoon and the egg, when combined, form a complete set of chromosomes, while the Y chromosome can contain only the spermatozoon.

The structure of male germ cells is such that in the head of the spermatozoon, in its front part, there is an acrosome, which provides its “penetrating ability”. The enzymes that this cell organelle secretes allow the spermatozoon to penetrate the dense membrane of the egg.

The neck of the male germ cell has a complex "stuffing". It contains organelles (structural elements) of the cell, which are responsible for its viability and activity. Protoplasm is also located here - it is she who will ensure the possibility of cell division, which is formed in the process of fertilization.

When fertilization occurs, the nuclei of the germ cells unite and a zygote is formed, giving rise to a new human life. , which leads to the impossibility of fertilization in a natural way, requires specialized treatment. In case the result remains negative, future parents turn to.

Sexual reproduction occurs in representatives of all types of flora and fauna. It is associated with the formation of special germ cells: female - eggs and male - sperm.

Sex cells (gametes) are characterized by a single (haploid) number of chromosomes (see). In addition, they differ in the ratio of the volumes of the cytoplasm and nucleus (compared to somatic cells).

The structure of the male germ cell (sperm)

Male germ cells - spermatozoa - are usually very small and mobile. Typical spermatozoa consist of a head, neck and tail.

Head almost entirely consists of a nucleus covered with a thin layer of cytoplasm. Its most anterior section is pointed, covered with a cap.

Neck narrowed, it contains the centriole (an integral part of the cell center) and mitochondria.

Tail spermatozoa consists of the thinnest fibers covered with a cytoplasmic cylinder: it is an organoid of movement.

The total length of the spermatozoon, including the head, neck and tail, in mammals and humans is 50-60 microns. It is characteristic that spermatozoa are usually formed in huge quantities (in mammals, hundreds of millions of them mature during a lifetime).

The structure of the female reproductive cell (ovum)

The female sex cells (eggs) are non-motile and usually larger than sperm. Usually they have a spherical shape and a diverse structure of the shells. In mammals, the size of the eggs is relatively small and is 100-200 microns in diameter. In other vertebrates (fish, amphibians, reptiles, birds), the eggs are large. In the cytoplasm, they contain a huge amount of nutrients.

In birds, for example, the ovum is that part of the egg that is commonly called the yolk. The diameter of a chicken egg is 3-3.5 cm, while in large birds such as ostriches it is 10-11 cm. These eggs are covered with several membranes of complex structure (protein layer, shell and shell membranes, etc.), which ensure the normal development of the embryo.

The number of eggs produced is usually much less than the number of sperm. For example, a woman will have about 400 eggs during her lifetime.

The structure of male and female germ cells of plants is described.

Development of eggs and sperm

The maturation and development of germ cells is called gametogenesis. In animals and humans, it occurs in the gonads: the eggs develop in the ovaries, and the spermatozoa develop in the testicles.

Stages of development

The processes of development of male germ cells (spermatogenesis) and female germ cells (ovogenesis) have a number of similar features. In both the ovary and the testicles, there are three different stages:

• stages of reproduction;
• growth stages;
• stages of maturation of germ cells.

On the first stage spermatogonia and ovogonia (cells - precursors of spermatozoa and eggs) multiply by and their number increases.

In men, mitotic division of spermatogonia begins at puberty and continues for decades. In women, the division of ovogons occurs only in the embryonic period of their life and ends even before birth. In animals, the division of these cells depends on the timing and periods of reproduction.

In second stage spermatogonia and ovogonia cease to multiply, begin to grow and increase in size, turning into primary spermatocytes and oocytes. The size of oocytes increases especially significantly. For example, in frogs, the linear dimensions of the oocyte are 2,000 times greater than in ovogonia. This is due to the fact that they accumulate the nutrients necessary for the development of the embryo.

The most important changes occur with future germ cells on third stage maturation. There are also significant differences between spermatogenesis and oogenesis. In this zone, primary oocytes divide twice by meiosis. During the first meiotic division, a large secondary oocyte and a small cell are formed - the primary polocyte (the first polar, or directional, body).

In the second meiotic division, the secondary oocyte divides into a large immature egg and a small secondary polocyte (second polar body). The primary polocyte can also divide into two more polocytes.

Thus, as a result of two meiotic divisions, 4 cells with a haploid set of chromosomes are obtained from one primary oocyte - an immature germ cell (which turns into a mature egg) and three polocytes, which subsequently die.

During spermatogenesis, the primary spermatocyte in the maturation zone also divides twice by meiosis. But in this case, 4 identical haploid spermatids arise. In the future, through complex transformations (changes in shape, development of the tail), they turn into mature spermatozoa.

Fertilization

Fertilization- this is the process of fusion of the nuclei of the sperm and the egg and the restoration of the diploid set of chromosomes. A fertilized egg is called a zygote. The formation of a zygote occurs only when a sperm enters the egg.

This process is carried out differently in different organisms. In mammals, the penetration of the sperm into the egg is accompanied by the dissolution of its shell with the help of various enzymes secreted by the sperm. In many insects, the eggs have a hard shell, and the sperm enters through small holes. In some aquatic organisms, a small receptive tubercle forms on the surface of the egg at the site of contact with the spermatozoon, which is then drawn inward along with the spermatozoon.

Usually, only the head of the spermatozoon with mitochondrion and centriole penetrates into the cytoplasm of the egg, while the tail remains outside. The shell of the head dissolves, the nucleus begins to swell until it reaches the size of the nucleus of the egg. Then both nuclei approach each other and finally merge.

Sometimes several sperm enter the egg at the same time, but only one of them fuses with the nucleus. In the zygote, all chromosomes become paired: in each pair of homologous chromosomes, one chromosome belongs to the egg, the second to the sperm. This phenomenon is of great importance for evolution. An organism that develops from a zygote has a large range of combinative variability, and therefore more opportunities to adapt to changing environmental conditions.

characteristic of flowering plants.

Progenesis - formation, development and maturation of male and female germ cells.

sex cells- gametes, unlike somatic, have a haploid set of chromosomes. All gamete chromosomes, with the exception of one sex chromosome, are called autosomes, sexual gonosome.

male reproductive cells have X or Y sex chromosomes.

female reproductive cells only H.

Differentiated gametes have a low level of metabolism and are not capable of reproduction.

male reproductive cells

male reproductive cells - spermatozoa (sperms) develop into very few thousand millions. They are small in size (in humans, about 70 microns), have the ability to actively move at a speed of 30-50 microns / sec. The spermatozoon has a flagellar shape.

¨The process of formation and maturation of spermatozoa - spermatogenesis.

The structure of the sperm

The spermatozoon consists of two parts: 1) head; 2) tail.

Head The spermatozoon (caput spermatozoidi) contains a small dense nucleus with a haploid set of chromosomes. Humans are characterized by the presence in the nucleus of 22 autosomes and 1 sex chromosome (gonosomes). Depending on which sex chromosome the X or Y sperm nucleus has, they are divided into two types:

1) androspermia - contain Y - chromosomes,

2) gynecospermia - contain X - chromosomes.

¨The kernel is characterized by a high content nucleoprotamines and nucleohistones. The anterior part of the nucleus is covered with a flat sac that forms case sperm. On the anterior pole of the cap is located acrosome(from Greek acros - top; soma - body). Both formations (sheath and acrosome) are derivatives of the Golgi complex.

acrosome contains a set of enzymes, among which an important place belongs to hyaluronidase and proteases(trypsin), which are able to dissolve the membranes of the egg.

The head is covered with a cell membrane on the outside.

Tail(feagellum) sperm consists of:

a) the connecting part (neck) formed by two centioles - proximal and distal, the axial thread (axoneme) originates from the distal;

b) the intermediate part formed by two central and 9 pairs of peripheral microtubules surrounded by mitochondria in a spiral (mitochondrial sheath);

c) the main part, which in structure resembles an eyelash. Surrounded by a thin fibrillar sheath;

d) the terminal part, which contains single contractile filaments.

Like the head, the tail is covered with a cell membrane.

Functions of spermatozoa

1. Fertilization of the egg. With the help of the tail, the sperm is able to move in a certain direction, which is determined by specific substances secreted by the egg - gynogamons.

2. React to chemical stimuli - chemotaxis.

3. Can move against the flow of liquid - rheotaxis.

4. Retain the ability to fertilize under optimal conditions for 36-88 hours.

5. Optimal conditions are slightly alkaline environment.

female reproductive cells

female reproductive cells - eggs (oocytes). Formed in the ovaries. Quantity - for the entire life of a person and a mammal, several hundred mature. Amphibians and fish can have several tens of thousands.

Egg has a spherical shape, sizes vary from a few microns to a few cm. Characteristic of the eggs is a large volume of cytoplasm and the presence of yolk. In addition, the eggs do not have the ability to move independently.

Unlike somatic, (gametes - from the word "married") are specialized in the reproduction of generations of organisms and have a half (haploid) genetic set (lcln, or 23 chromosomes - in humans).

Distinguish male and female sex cells, which carry genetic information along the paternal and maternal lines. In human germ cells, there are 22 autosomes and 1 sex chromosome, which is designated as X or Y for men and X for women. When the male and female germ cells merge during fertilization, a diploid cell is formed - a zygote, which gives rise to all the cells of a new organism. In human embryogenesis, germ cells are first found in the yolk endoderm, i.e. extragonadally, and then migrate to the anlage of the gonads.

At the first acquaintance with the structure of mature male and female reproductive cells, draws attention to their high functional expediency. Spermatozoa, which should be highly mobile in the female genital tract, are small cells, practically devoid of cytoplasm and consisting of a nucleated part, or head, carrying genetic material, and a locomotion organ - a tail, or flagellum. Spermatozoa do not have any other subcellular elements, except for mitochondria, which provide energy for movement, an acrosomal vacuole with proteolytic enzymes for dissolving egg membranes, and a proximal centriole. The total length of the sperm is about 60 µm, of which 55 µm is in the tail.

At the ultramicroscopic level in the head sperm acrosomal and postacrosomal zones are distinguishable, in the tail - intermediate, main and terminal sections. Most of the head is occupied by the nucleus filled with condensed chromatin. The space between the nucleus and the anterior section of the sperm plasmalemma is occupied by the acrosomal vacuole, a specialized lysosome containing a group of enzymes-lysins of the egg membranes: acrosin (destroys the transparent zone of the oocyte), penetrase (dissociates cells of the radiant crown), hyaluronidase (breaks down hyaluronic acid), acid phosphatase (destroys forscholin during the passage of sperm through the oocyte plasmalemma). The sperm neck is a short section containing the proximal centriole and 9 segmented columns.

The intermediate section contains axoneme and 9 longitudinally oriented elements of the cytoskeleton, consisting of keratin-like proteins and being a continuation of segmented columns. Outside of these fibers are mitochondria. The axoneme is built on the principle of a cilium, the beating of which is initiated by calcium cations in the environment and mitochondrial ATP of sperm. In the terminal section of the tail, the elements of the cytoskeleton disappear, and the axoneme is covered only by the plasmolemma. In humans, the axoneme doublets in the terminal section break up into 18 single microtubules.

After leaving the testis, the spermatozoa are morphologically formed, but functionally immature- they are immobile, incapable of fertilization and, in addition, are carriers of a number of specific antigens, the inactivation of which (as well as the acquisition of mobility and fertilizing ability - capacitation) occurs in the system of the vas deferens of the male genital tract.

The ovum, unlike the spermatozoa, is large and motionless. In most mammals and humans, its size reaches 100-200 microns. The female germ cell is the custodian of the supply of trophic compounds necessary for the early development of the embryo, and the "reserve" of cytoplasmic structures for the formation of the first generations of germ cells - blastomeres. In humans, the large size of the egg is not associated with large reserves of yolk - the human egg is poor in yolk (oligolecital).

At the ultramicroscopic level egg human has a large rounded nucleus, in which euchromatin predominates, and a cytoplasm with a moderate amount of mitochondria with a few lamellar cristae. The system of protein synthesis is well developed due to the high content of ribo- and polysomes, short tubules of the granular endoplasmic reticulum. In the cytoplasm, a few accumulations of vitellin granules - collectors of trophic inclusions - are rarely and diffusely located.

It should be especially emphasized that in higher placental mammals and human female reproductive cells do not exist in isolation. With the exception of the earliest stages of development (primary germ cells, ovogonia), they are in close contact with the somatic cells of the ovary (follicular epitheliocytes and connective tissue cells), which form an epithelial and connective tissue membrane around each germ cell. The complex "sex cell - somatic membranes" is called the ovarian follicle, or ovo-somatic (follicular) histion. The structure of the ovarian follicle becomes more complex during oogenesis.