Sperm are made inside the testes in tightly coiled tubes called seminiferous tubules. The process starts with stem cells (spermatogonia) at the outer edge of each tube. These divide, and some go on to become spermatocytes, which undergo meiosis: a special kind of division that halves the number of chromosomes so each sperm carries one set.
The resulting round cells, spermatids, then go through a remarkable makeover called spermiogenesis. They grow a tail, pack their DNA tightly into a compact head and form a cap (the acrosome) that helps them get into an egg. Throughout, they are supported and fed by Sertoli cells, the "nurse" cells of the tubules.3
Newly released sperm cannot yet swim properly. They pass into the epididymis, where they mature and are stored until ejaculation. Production is continuous, so the testes are always working on many overlapping batches at different stages.
How long does spermatogenesis take?
Around 64 days on average, with real variation between men. In practice, that means roughly two months before a new batch of sperm starts to appear in the ejaculate.
The classic estimate comes from a 1963 study that labelled dividing cells and tracked them with testicular biopsies. It found one cycle of the seminiferous epithelium lasts 16 days and that the whole of spermatogenesis takes about 64 days.1
A 2006 study used a safer method: 11 men drank heavy water (a harmless labelled form of water) and gave semen samples every two weeks. Labelled sperm appeared in the ejaculate after an average of 64 days, ranging from 42 to 76 days.2 The authors noted that the store of sperm in the epididymis can affect how quickly new sperm show up.2 Our post on how long sperm take to regenerate goes into the practical side.
What controls sperm production?
Hormones from the pituitary gland in the brain. Follicle-stimulating hormone (FSH) acts on Sertoli cells, and luteinising hormone (LH) drives testosterone production, which the process also needs.
FSH works on the Sertoli cells, alone and together with testosterone, to support their growth and function.3 LH stimulates the Leydig cells between the tubules to make testosterone.3 Men born with a faulty gene for FSH or its receptor have absent or suppressed sperm production, which shows how central it is.3
For more on each hormone, see follicle-stimulating hormone and luteinising hormone.
Why can testosterone therapy lower sperm production?
Testosterone taken from outside the body signals the brain to cut back LH and FSH. The testes then lose the hormonal drive they need to keep making sperm.
This is why the American Urological Association guideline advises that men currently trying to conceive should not be prescribed testosterone therapy.4 Because sperm production depends on FSH and LH-driven signals inside the testes, switching those signals down with outside testosterone can leave men with very low counts.3,4
If you are on, or considering, testosterone replacement therapy and want children now or in future, raise it with your doctor before starting or stopping anything.
What does the timeline mean for lifestyle changes?
Anything that helps or harms sperm production takes weeks to show up in a semen sample. Give a change at least a couple of months before judging it.
Because a full cycle takes around two months, a semen test today reflects conditions in your testes weeks ago.1,2 Stopping smoking, losing weight or recovering from an illness will not show up in next week's sample. Equally, a short-lived problem can affect results for a while after you feel better.
That is also why clinics usually wait a couple of months before repeating a semen analysis after a change. Our guide to lifestyle factors that affect male fertility covers what is worth trying.
Common questions
Where does spermatogenesis happen?
In the seminiferous tubules inside the testes. Finished sperm then move to the epididymis, where they mature and are stored until ejaculation.
Does spermatogenesis stop with age?
Sperm production normally continues through adult life, unlike the fixed egg supply in women. Ongoing production is not the same as unchanged fertility, so talk to a doctor if you have questions about conceiving later in life.
Do you run out of sperm if you ejaculate often?
No. Production is continuous, with many batches of sperm developing at once, so the testes keep making new sperm whether you ejaculate often or rarely. The supply is constantly renewed.
Read more on this
How long does it take sperm to regenerate?
Lifestyle factors that affect male fertility: a practical guide
How testosterone changes after 40 (and what actually helps)
Sources
- Heller CG, Clermont Y. Spermatogenesis in man: an estimate of its duration. Science. 1963;140(3563):184-186. PubMed
- Misell LM, Holochwost D, Boban D, et al. A stable isotope-mass spectrometric method for measuring human spermatogenesis kinetics in vivo. J Urol. 2006;175(1):242-246. PubMed
- Oduwole OO, Peltoketo H, Huhtaniemi IT. Role of follicle-stimulating hormone in spermatogenesis. Front Endocrinol (Lausanne). 2018;9:763. PubMed
- Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and management of testosterone deficiency: AUA guideline. J Urol. 2018;200(2):423-432. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.