SPERMATOCYTOGENESIS

In this process four spermatid develop from one PGC.

(i) Multiplication phase

1. The spermotogonia or sperm mother cells lie next to the basement membrane.
2. These divide repeatedly by mitosis producing new sex cells.
3. Some of these cells move towards the lumen of the seminiferous tubule and enter the growth phase.
4. These are called primary spermatocytes while others produced by the mitotic division of spermatogonia remain in the original condition and continue to produce new spermatogonia throughout the reproductive life of the animal.

(ii) Growth phase

1. The spermatogonia, formed as a result of repeated divisions of the primordial germ cells, stop dividing and prepare themselves for maturation division.
2. They increase in size by accumulating nourishing material obtained from germinal cells.
3. The enlarged cells are known as primary spermatocytes.
4. Initially, the nucleus of each primary spermatocyte is of ordinary size, but soot it imbides material and enlarges considerably.
5. At the end of growth phase, when the primary spermatocyte is ready to undergo first maturation division (i.e. the reduction division) its nucleus is much larger than that spermatogonial cell.

(iii) Maturation phase

1. Each diploid primary spermatocyte undergoes the first maturation division which is a reduction division, with its long drawn out and complicated prophase.
2. The pairing and spliting of homologons chromosomes take place during zygotene and pachytene and this leads to the formation of tetrads. Due to crossing over, the exchange of homologus chromosomes also take place.
3. According to some, all these chromosomal events are completed in the growth phase only, so that during first maturation division the chromosomes are equally distributed to the two daughter cells.
4. As a result of reduction division, the two cells formed are haploid. These haploid cells are called secondary spermatocytes.
5. The secondary spermatocytes undergo second maturation division which is a simple mitotic division.
6. As a result, each secondary spermatocyte divides into two equal cells, called spermatids.
7. In the end of maturation phase four haploid spermatids are formed from each, diploid primary spermatocyte.