Spermatogenesis :
* The process of spermatogenesis occurs in the male gonads or testes.
* The testes of the vertebrates are composed of many seminiferous tubules which are lined by the cells of germinal epithelium.
* The cells of the germinal epithelium form sperms by the process spermatogenesis.
* But in certain animal, e.g., mammals and Mollusca, etc there are somatic cells lying in between germinal cells.
* These somatic cells are known as sertoli cells.
* The sertoli cells anchor the differentiating cells and provide nourishment to the developing sperms.
* The insects do not possess sertoli cells.
* The spermatogenesis is a continuous process and for the sake of convenience this process can be studied in two different stages :-
1) Formation of spermatids
2) Spermiogenesis
Formation of spermatids :
The male germinal cells which produce the sperms are known as the primary germinal cells or primordial cells.
The primordial cells pass through following three phases for the formation of spermatids :
i) Multiplication phase
ii) Growth phase
iii) Maturation phase
i) Multiplication phase :
* The undifferentiated germ cells or primordial cells contain large-sized and chromatin-rich nuclei.
* These cells multiply by repeated mitotic divisions and produce the cells which are known as the spermatogonia (Gr., sperma = sperm or seed; gone = offspring).
* Each spermatogonium is diploid and contains 2X number of chromosomes.
ii) The growth phase :
* In the growth phase, the spermatogonial cells accumulate amount of nutrition and chromatin materials.
* Now each spermatogonial cells is known as the primary spermatocyte.
iii) The maturation phase :
* The primary spermatocytes are ready for first meiotic or maturation division.
* The homologous chromosomes start pairing (synapsis), each homologous chromosome splits longitudinally and by the chiasma formation the exchange of genetic material or crossing over takes place between the chromatids of the homologous chromosomes.
* The DNA amount us duplicated in the beginning of the division.
* By first meiotic division or homotypic division two secondary spermatocytes are formed.
* Each secondary spermatocyte passes through the second maturation or second meiotic or heterotypic division and produces two spermatids.
* Thus, by a meiotic diploid spermatogonium produces four haploid spermatids.
* These spermatids cannot act directly as the gametes so they have to pass through the next phase, the spermiogenesis.
Spermeogenesis :
* The process by which rounded nonmotile spermatids are converted to active and mobile sperms is called spermeogenesis.
* 1. The nucleus shrinks, chromosome become closely packed.
* 2. All nuclear material (except DNA) get lost.
* 3. The sperm nucleus become elongated and narrow.
* 4. The spermatid contain two centriole in the centrosome which move to lie behind the nucleus.
* 5. All mitochondria spirally arranged in the middle piece of spermatozoa.
* 6. The Golgi apparatus from acrosome.
* 7. Most of cytoplasm are eliminated and remaining forms a condensed layer surrounding the middle piece.