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Spermatogenesis occurs continuously and repeatedly in the germinal epithelium of the seminiferous tubules. In human tubules areas of active spermatogenesis are interspersed with resting epithelium. As germ cells mature they move from the basement membrane of the tubule to the tubular lumen. Spermatogonia, the primordial male germ cells that are the precursors of spermatozoa multiply by mitosis throughout adult life.

Spermatogonia initiate myosis by duplicating their DNA to yield 46 chromosomes each with 2 daughter chromatids. At this stage the germ cells are called primary spermatocytes. This represents the final synthesis of DNA in the germ cell. Primary spermatocytes undergo the 1st myotic division to yield two secondary spermatocytes each having 23 chromosomes, the haploid or N number and each chromosome in turn having two chromatids or the 2N content of DNA.

Secondary spermatocytes rapidly undergo the 2nd myotic division in which the chromatids separate to yield two spermatids, each having 23 chromosomes the haploid number and the N content of DNA. The process by which spherical spermatids still engulfed by sertoli cells are transformed into elongated spermatozoa with tails is referred to spermiogenesis.

No mitosis or myosis occurs during this process. Fully developed but non-modal spermatozoa are released from Sertoli cell and washed out of the tubules into the reedy testes and then the epididymis by a fluid drive. Spermatogenesis takes 72 to 74 days only testosterone from the testicular Leydig excels is absolutely required for spermatogenesis.

However FSH greatly enhances spermatogenesis by stimulating the functions of Sertoli cells and increasing mitosis of spermatogonia. Males with either FSH receptor defects or absence of FSH can produce at least some spermatozoa and be fertile. Male athletes using androgens have low plasma FSH levels and very low plasma LH levels because of the negative feedback effect of testosterone on the hypothalamus-pituitary unit.

Spermatogenesis is reduced in those who chronically use androgens, once mitosis has been initiated in spermatogonia testosterone alone can maintain spermatogenesis.