Spermatogenesis

Spermatogenesis is the production of sperm, the male gametes. In humans it takes place in the walls of the seminiferous tubules that fill each testis, beginning at puberty and continuing for life, though output slowly declines with age. Stem cells called spermatogonia, near the outer edge of the tubule, divide by mitosis; some daughter cells stay behind to renew the stem cell pool, while others become primary spermatocytes. Each primary spermatocyte passes through meiosis I to form two secondary spermatocytes, and meiosis II then yields four haploid spermatids, each with 23 chromosomes.

In the final stage, spermiogenesis, the round spermatids reshape into sperm: they condense the nucleus, build an enzyme-filled acrosome over the head, grow a flagellum and shed most of their cytoplasm. Throughout, the developing cells move toward the tubule's center, supported by Sertoli cells, which nourish them and form the blood–testis barrier. One full round takes about two months, and many millions of sperm are made each day. Sperm then mature further in the epididymis.

Hormones control the process: FSH from the pituitary acts on Sertoli cells, and LH stimulates Leydig cells to make testosterone, with negative feedback from testosterone and inhibin. It works best a few degrees below core body temperature, which is why the testes sit in the scrotum.