Spermatogenesis

Spermatogenesis is the sequence of divisions, inside the seminiferous tubules, that converts immature germ cells into sperm. During foetal development, primordial germ cells migrate into the testis and settle on the inner surface of the seminiferous tubules as spermatogonia (sperm mother cells); from puberty onward these divide mitotically, continuing throughout adult life. Some spermatogonia migrate inward among the Sertoli cells, enlarging into diploid primary spermatocytes (46 chromosomes). Each primary spermatocyte completes meiosis I to give two haploid secondary spermatocytes (23 chromosomes), and each of these completes meiosis II to give a total of four haploid spermatids. These spermatids are then remodelled, without any further division, into slender, motile spermatozoa through a process called spermiogenesis, and are finally released into the tubule's central lumen by a process called spermiation. The entire sequence takes about 64 days, and because different stretches of tubule are always at different stages simultaneously, overall sperm output stays roughly constant at around 200 million sperm per day. The whole process is switched on at puberty by a rise in hypothalamic GnRH, which stimulates the anterior pituitary to release two gonadotropins: FSH, which promotes test …