From stem cell to mature sperm over about ten weeks
Spermatogenesis is the production of sperm in the seminiferous tubules of the testes, beginning with spermatogonial stem cells that divide and differentiate through spermatocyte and spermatid stages into mature spermatozoa. The full cycle takes roughly 74 days, with additional time for transit and maturation in the epididymis, so the sperm ejaculated today reflect conditions in the testes more than two months ago.
Sertoli cells inside the tubules nurse the developing sperm, while Leydig cells outside produce testosterone. This timeline explains why a fever, illness, or change in lifestyle shows up in semen quality only weeks later, and why any intervention needs a similar lag before its effect can be judged.
The hormonal signals that keep it running
Two signals are essential. Follicle-stimulating hormone from the pituitary acts on Sertoli cells, and a high local concentration of testosterone within the testis (intratesticular testosterone), far above blood levels, supports the maturing sperm. This is a key reason that raising blood testosterone with external hormones can paradoxically suppress sperm production, because it switches off the brain signals that maintain that high local level.
This is general educational information, not medical advice. Fertility concerns should be evaluated by a qualified clinician.