spermatogenesis

Spermatogenesis is the body's continuous assembly line for making sperm. Starting from a reservoir of stem cells in the testis, it produces millions of new sperm every day throughout adult life, an output far more relentless than the once-a-month egg release in females.

The process unfolds inside the seminiferous tubules. Stem cells divide and mature through several stages, then undergo meiosis to halve their chromosome number, and finally reshape into streamlined cells with a head and a tail. The whole journey takes roughly two to three months. It depends on hormones: FSH and high local testosterone (from Leydig cells) acting on the supporting Sertoli cells are both required, and the cooler scrotal temperature is essential too.

Because the cycle is long and depends on a delicate hormonal and thermal environment, sperm counts respond slowly to changes. After an insult such as illness, fever, or starting a fertility treatment, several months may pass before any improvement shows up in a semen analysis. This lag is why fertility assessments are repeated over time rather than judged from a single sample.