Spermatogenesis

Takes place in seminiferous tubules of the testes. The seminifirous tubules are lined by germinal epithelium. The latter consist largely of cuboidal primary germ cells but contains certain tall somatic cells called sertoli; or nurse cells here and there. Spermatogenesis is a continuous process which divided into two stages: formation of spermatids from the undifferentiated germ cell and formation of spermatozoa from the spermatids.

1.Formation of spermatides: This process has 3 phase:

1) Multiplication phase. The undifferentiated germ cells present in the sminiferous tubules of the testes are called the spermatogonia or sperm mother cells. They are proliferated by mitotic division from the primary germ cells of the germinal epithelium lining the seminiferous tubule. The spermatogonia have prominent nuclei that contain 2n or diploid number of chromosomes.

The spermatogonia increase their population by repeated mitotic division so that each newly-formed spermatogonium process the same number of chromosomes, i.e. four.

2) Growth phase. On sexual maturity of the animal some spermatogonia stop dividing and grow in size by accumulating cytoplasm and replicating DNA and are then termed the primary spematocytes. The latter are about twice the size of the spermatogonia. Other spermatogonia continue to divide mitotically and produce more spermatogonia for later spermatogenesis.

3) Maturation phase. The primary spermatocytes undergo first meiotic division and produce secondary spermatocytes. This division is reductional so that each secondary spermatocyte contains half or haploid number of chromosomes.

4) Spermiogenesis. Formation of spermatozoa. Spermiogenesis is the transformation of an unspecialized spermatid into a sperm by losing a great deal of cytoplasm condensation of the nucleus into a head and by formation of a flagellated tail. The mature spermatozoon or the sperm finally gets detached and is released in the lumen of the seminiferouse tuble.

The secondary spermatocytes soon undergo the second meiotic division and produce spermatides. This division is equalization. The spermatids do not divide further and modify or metamorphose into spermatozoa, which are also haploid.

A spermatogonium, thus produces four spermatozoa. The transformation of spermatides into spermatozoa is known as spermiogenesis. It involves the movement of cell organelles to position characteristic of mature spermatozoa, certain changes in the cell organelles and loss of a good deal of Golgi apparatus, cytoplasm and water. Development of a sperm cell takes about 2 month. Sperm formation starts at puberty (13 to 16 years of age).

From one diploid primary spermatocyte, four haploid sperms are formed.

Oogenesis

Takes place in ovaries. Unlike sperm formation that starts at puberty egg formation begins before birth but is completed only after fertilization. Oogenesis begins early in born with set number of primary oocytes already in prophase I in the ovary.

1) Multiplication phase. The cells of the germinal epithelium of the ovary undergo number. These cells called as oogonia contain the diploid number of chromosomes.

2) Growth phase. Only some of the oogonia increase in size and undergo a growth phase to form the primary oocyte.

3) Maturation phase. The primary oocyte undergoes the first meiotic division to produce two haploid cells. Since there is unequal division of the cytoplasm, one of the cells the secondary oocyte is large and receives most of the cytoplasm, the other one is very small and is called the first polar body.

In the second meiotic division, the secondary oocyte divides to form a large ootid or ovum and a small polar body. the first polar body also undergo the division simultaneously to form two small polar bodies.

As a result of all these division, a primary gives rise to one ovum and a total of three polar bodies. The polar bodies ultimately disintegrate due to lack of cytoplasm and food.

The ootid grows into a functional haploid ovum. Development of an ovum takes about 13 years. In human, ova are released from the ovary in the secondary oocyte stage. Their maturation is completed in the mother s genital tract, usually after the sperm has entered for fertilization.