Spermatogenesis Process in Man

 

                                          Spermatogenesis

Males start producing sperm when they reach puberty, which is usually from 10-16 years old. They are produced in large quantities (~200 million a day) to maximise the likelihood of sperm reaching the egg. Sperm are continually produced as males need to be ready to utilise the small window of fertility of the female.

Sperm production occurs in the testes of the male, specifically in the seminiferous tubules. The tubules are kept separate from the systemic circulation by the blood-testis barrier.

The blood-testis barrier is formed by sertolicells and is important in preventing hormones and constituents of the systemic circulation from affecting the developing sperm. It is also important as it prevents the immune system of the male from recognising the sperm as foreign – the sperm are genetically different from the male and will express different surface antigens. Sertoli cells also have a role in supporting the developing spermatozoa.

Spermatogonia are the initial pool of diploid cells that divide by mitosis to give two identical cells. One of these cells will be used to replenish the pool of spermatogonia – these cells are A1 spermatogonia. This replenishment of spermatogonia means that males are fertile throughout their adult life. The other cell – type B spermatogonium – will eventually form mature sperm.

Type B spermatogonia replicate by mitosis several times to form identical diploid cells linked by cytoplasm bridges, these cells are now known as primary spermatocytes. Primary spermatocytes then undergo meiosis.

• Meiosis I produces two haploid cells, known as secondary spermatocytes
• Meiosis II produces four haploid cells, known as spermatids

The cytoplasmic bridges break down and the spermatids are released into the lumen of the seminiferous tubule – a process called spermiation. The spermatids undergo spermiogenesis (remodelling and differentiation into mature spermatozoa) as they travel along the seminiferous tubules until they reach the epididymis.

From the seminiferous tubule they travel to the rete testis, which acts to “concentrate” the sperm by removing excess fluid, before moving to the epididymis where the sperm is stored and undergoes the final stages of maturation.

Spermatogenesis takes approximately 70 days, therefore in order for sperm production to be continuous and not intermittent, multiple spermatogenic processes are occurring simultaneously within the same seminiferous tubule, with new groups of spermatogonia arising every 16 days (spermatogenic cycle). Each of these populations of spermatogenic cells will be at different stages of spermatogenesis.

Note that once sperm leave the male body and enter the female reproductive tract, the conditions there cause the sperm to undergo capacitation, which is the removal of cholesterol and glycoproteins from the head of the sperm cell to allow it to bind to the zona pellucida of the egg cell.