Spermatogonia to spermatozoa over about 70 days
Expansion
Stages
- Spermatogonia (diploid stem cells) divide by mitosis
- Primary spermatocytes undergo the first meiotic division
- Secondary spermatocytes undergo the second, giving haploid cells
- Spermatids undergo spermiogenesis: acrosome formation, nuclear condensation, flagellum development and loss of cytoplasm
- Spermatozoa released into the tubule lumen
Timing: about 70 days for the cycle, plus 12 to 21 days of epididymal transit where motility and fertilising capacity are acquired.
Sertoli cells support the process: they form the blood-testis barrier, provide nutrition, secrete androgen-binding protein and inhibin, and phagocytose residual cytoplasm.
Requirements: FSH and testosterone (in high local concentration, far above plasma levels), and a temperature about 2 degrees below core.
Practical implications: a semen analysis reflects events of three months ago, so illness, fever, drugs or heat exposure in that window explain an abnormal result; and after vasectomy, clearance of stored sperm requires about 20 ejaculations or 12 weeks before a confirmatory sample.