testis

The testis is the male gonad, a paired organ that runs two factories at once: one makes sperm and the other makes the hormone testosterone. It hangs outside the body in the scrotum, an arrangement that keeps it a few degrees cooler than core body temperature, which sperm production requires.

Inside, the testis is divided into two functional compartments. The seminiferous tubules are coiled tubes lined by Sertoli cells that nurture developing sperm; this is where spermatogenesis happens. Between the tubules sit the Leydig cells, which secrete testosterone into the blood. The pituitary hormones FSH and LH control these compartments respectively: FSH supports the Sertoli cells and sperm production, while LH stimulates the Leydig cells to make testosterone.

Because the testis carries out both functions, damage can affect them separately. For example, certain infections, chemotherapy, or heat can impair sperm production while leaving testosterone output relatively intact, so a man can have a normal hormone level yet poor fertility. The testis also descends from the abdomen into the scrotum before or shortly after birth; failure of that descent (undescended testis) impairs sperm production and raises later cancer risk if uncorrected.