The structure of male, female germ cells determines the fulfillment of their most important function - the implementation of generative reproduction. It is characteristic of representatives of both plants and animals. Features of the structure of germ cells will be considered in our article.
Gametes: the relationship of structure and function
Specialized cells that carry out the process of generative reproduction are called gametes. Male and female germ cells - sperm and eggs - have a haploid, i.e., a single set of chromosomes. This structure of germ cells provides the genotype of the body, which is formed during their fusion. It is diploid, or double. Thus, the body receives half of the genetic information from the mother, and the other from the father.
Despite the common features, the structure of the reproductive cells of plants and animals differs in many respects from each other. This primarily applies to certain places of their formation. So, in angiosperms, sperm are located in the anthers of the stamens, and the egg is in the ovary of the pestle. Multicellular animals have special organs - glands in which germ cells are formed: ova in the ovaries, and sperm in the testes.
The process of formation of germ cells
The structure and development of germ cells is determined by the course of gametogenesis - the process of their formation, which proceeds in several stages. During the reproduction phase, primary gametes are divided several times by mitosis. At the same time, a double set of chromosomes is preserved. In individuals of different sexes, this stage has its own differences. So, in male mammals, it begins from the moment of puberty and lasts until old age. In females, the division of primary germ cells occurs only during fetal development. And until puberty, they remain at rest.
The growth phase is as follows. During this period, primary gametes increase in size, DNA replication (doubling) occurs. An important process is also the storage of nutrients, because they will be necessary for subsequent divisions.
The last stage of gametogenesis is called the growth phase. In its course, primary germ cells divide by reduction division - meiosis. Its result is four haploid cells formed from primary diploid cells.
Spermatogenesis
As a result of the formation of male germ cells, i.e., spermatogenesis, four identical and complete structures are formed. They have the ability to fertilize. The structure of the male reproductive cell, or rather its feature, is the emergence of specific devices. In particular, it is a flagellum, with the help of which the movement of male gametes occurs. This process occurs in the last additional phase of formation, which is characteristic only for the spermatogenesis process.
Ovogenesis
The structure of female germ cells, as well as the process of their formation (ovogenesis), has a number of characteristic features. During egg maturation during meiosis, the cytoplasm is unevenly distributed between future cells. Only one of them as a result becomes an egg, capable of giving rise to a future life. The remaining three are turned into guide bodies and, as a result, are destroyed. The biological meaning of this process is to reduce the number of mature, capable of fertilizing female germ cells. Only under this condition can a single egg get the required amount of nutrients, which is the main condition for the development of a future organism. As a result, during the time when a woman is capable of having children, only about 400 germ cells are able to form. While in men, this figure reaches several hundred million.
The structure of male germ cells
Sperm cells are very small cells. Their size barely reaches a few micrometers. In nature, such sizes are naturally offset by their quantity. The structure of the germ cells of the male body has its own characteristics.
The sperm consists of a head, neck and tail. Each of these parts performs certain functions. In the head is a constant cellular organelle of the eukaryotes - the nucleus. It is a carrier of genetic information contained in DNA molecules. It is the core that provides the transfer and storage of hereditary material. The second component of the sperm head is the acrosome. This structure is a modified Golgi complex and secretes special enzymes that can dissolve the egg shell. Without this, the fertilization process would be impossible. In the neck are mitochondrial organelles that provide tail movement. Centrioles are also located in this part of the sperm. These organelles play an important role in the formation of the division spindle during crushing of a fertilized egg. Sperm tail is formed by microtubules, which, using the energy of mitochondria, provide the movement of male germ cells.
Egg structure
Female germ cells are much larger than sperm. Their diameter in mammals is up to 0.2 mm. But the same indicator in cysterae fish is 10 cm, and in the herring shark - up to 23 cm. Unlike male germ cells, the eggs are motionless. They have a rounded shape. The cytoplasm of these cells contains a large amount of nutrients in the form of yolk. In addition to DNA carrying genetic information, the nucleus contains another nucleic acid - RNA. It contains information about the structure of the most important proteins of the future organism. The yolk may be uneven in the egg. For example, in the lancelet, it is in the center, and in fish it occupies almost the entire surface, shifting the nucleus and cytoplasm to one of the poles of the cell. Outside, the egg is reliably protected by the membranes: vitelline, transparent and external. They have to dissolve the acrosome of the sperm head to carry out the fertilization process.
Types of Fertilization
The structure and function of germ cells determine the implementation of the fertilization process - the fusion of gametes. As a result of this process, the genetic material of gametes combines in a single core, and a zygote is formed. She is the first cell of a new organism.
Depending on the place of passage of this process, external (external) and internal fertilization are distinguished . The first type is carried out outside the body of a female individual. This usually occurs in an aquatic habitat. Examples of organisms in which external fertilization occurs are representatives of the fish class. Their females spawn in the water, where the males and water it with seminal fluid. The number of eggs of such animals reaches several thousand, of which not many individuals survive and grow. Most of them eat aquatic animals. But for all mammalian animals, internal fertilization is characteristic, which occurs inside the female body with the help of specialized male copulative organs . Moreover, the number of eggs ready for fertilization is small.
The structure of the male and female reproductive cells and the reproductive system of plants is significantly different from that of animals. Therefore, the process of fusion of gametes occurs differently. Male germ cells of plants do not have a tail and are not capable of movement. Therefore, fertilization is preceded by pollination. This is the process of transferring pollen from the anther of the stamen to the stigma of the pestle. It occurs with the help of wind, insects or humans. Having appeared in this way on the stigma of the pestle, sperm descend along the embryonic tube into its expanded lower part - the ovary. There is an egg. When gametes merge, a seed germ is formed.
The concept of parthenogenesis
The structure of germ cells, in particular female cells, makes possible one of the unusual forms of generative reproduction. It is called parthenogenesis. Its biological essence lies in the development of an adult organism from an unfertilized egg. Such a process is observed in the life cycle of Daphnia crustaceans, during which sexual and parthenogenetic generations alternate. The female reproductive cell contains enough nutrients to give rise to a new life. However, during parthenogenesis, new combinations of genetic information do not occur, which means that the appearance of new traits is also impossible. However, parthenogenesis is of great biological importance, since it makes possible the process of sexual reproduction even without the presence of an individual of the opposite sex.
Menstrual phase
In the female body, germ cells are not always ready for fertilization, but only in certain phases of the menstrual cycle. During this physiological process, cyclical regular changes in the functions of the reproductive system occur in the body. This process is regulated by the humoral system. The duration of this cycle is 21-36 days with an average of 28. This period can be divided into three phases. In the first (menstrual), which lasts approximately the first 5 days, the uterine mucosa is rejected. This is accompanied by rupture of small blood vessels. On the 6-14th day, under the influence of the pituitary gland, a follicle is secreted, in which the egg matures. The uterine mucosa during this period begins to recover. This is the essence of the postmenstrual phase. From the 15th to the 28th day, the formation of fatty connective tissue - the corpus luteum - occurs. It plays the role of a temporary gland of internal secretion, which produces hormones that delay the maturation of follicles. In the period from the 17th to the 21st day, the probability of fertilization is highest. If this does not happen, the germ cell is destroyed and the mucous membrane exfoliates again.
What is ovulation
On the 14th day of the menstrual cycle, the structure of the female reproductive cell changes slightly. The egg breaks the follicular membrane and leaves the ovary into the fallopian tube. It is there that its maturation ends. This process is called ovulation. This is a very important period during which the uterus acquires the ability to accept a fertilized egg.
Chromosomal germ cell set
Ovum and sperm cells have a single set of genetic information. For example, in a human sex cells contain 23 chromosomes each, and a zygote contains 46. When fusing gametes, the body receives half of the genes from the mother, and the second from the father. This also applies to gender. Among chromosomes, autosomes and one pair of genital are distinguished. They are indicated by Latin letters. In humans, female cells contain two identical sex chromosomes, and male cells are different. Sex cells contain one of them. Thus, the sex of the unborn child is completely dependent on the male body and on the type of chromosomes that carries the sperm.
Germ cell function
The structure of the female reproductive cell, as well as the male, is interconnected with the functions that they perform. Being part of the reproductive system, they carry out the function of generative reproduction. Unlike asexual, in which the integrity of the body’s genetic information is maintained, sexual reproduction provides the creation of new traits. This is a prerequisite for the occurrence of adaptation, and hence the entire existence of living organisms.