The formation of germ cells

Gametogenesis is the process of formation and development of germ cells.
In multicellular algae, many fungi and spore plants, gametes are formed in special organs of sexual reproduction: female – in archegonia, male – in antheridia.

In most animals, gametes are formed in the gonads: spermatozoa are formed in the testes, and eggs in the ovaries.

There are dioecious and bisexual species. Dissolved organisms produce only one species of gametes, and bisexual organisms produce both species.

Hermaphrodites are bisexual organisms capable of forming both male and female germ cells.
Hermaphroditism arose as an adaptation to a sedentary, sedentary, or parasitic lifestyle. It is found in coelenterates, flat and annelids, molluscs, and most plants.

The advantage of hermaphroditism is the possibility of self-fertilization with only one individual. But in most hermaphrodite organisms, cross-fertilization occurs between different individuals.

Gametogenesis in higher animals

Spermatogenesis occurs in the testes. They contain seminiferous tubules in which sperm are formed and developed.

In the process of sperm formation, four periods (stages) are distinguished: reproduction, growth, maturation and formation.

During the reproduction period, the primary germ cells (spermatogonia) divide repeatedly by mitosis. At the same time, the diploid set of chromosomes 2n2c is preserved.

Then a period of growth begins: the formed cells slightly increase in size, DNA molecules are doubled in them. Spermatogonia develop into first-order spermatocytes with a chromosome set of 2n4c.

During maturation, two divisions of meiosis occur. After the first division, from one first-order spermatocyte, two second-order spermatocytes (1n2c) are formed, and after the second, four spermatids (1n1c).

During the period of formation, spermatids are converted into spermatozoa.

During spermatogenesis, four spermatozoa are formed from one primary reproductive cell.

Oogenesis (oogenesis) occurs in the ovaries and, unlike spermatogenesis, begins even before the birth of the female body.

In the process of egg formation, three periods (stages) are distinguished: reproduction, growth and maturation.

During the reproduction period, the primary germ cells (oogonia) divide by mitosis. In this case, the diploid set of chromosomes 2n2c is preserved, but the number of cells is much less than during spermatogenesis. The breeding period ends before the female is born. By this time, about 30 thousand primary germ cells are formed.

In a sexually mature female, the further development of individual oogonia periodically begins. During the growth period, the cell volume increases significantly due to the synthesis and accumulation of substances. Duplication of DNA occurs. A first-order oocyte (2n4c) is formed.

During the ripening period, there are two divisions of meiosis. After the first division from one oocyte of the first order, one large haploid cell (oocyte of the second order (1n2c)) and one small (polar, or directional, body) are formed.

The resulting oocyte leaves the ovary into the abdominal cavity and enters the fallopian tube – ovulation occurs.

In the fallopian tube, the cell makes a second meiotic division, as a result of which the oocyte forms an egg (1n1c) and another polar body. The first polar body, as a rule, also divides.

During oogenesis, one ovum and three polar bodies are formed from one primary reproductive cell, which are soon destroyed.