seminiferous tubule

The seminiferous tubules are the tiny coiled pipes inside the testis where sperm are actually made. If you could unravel all of them from both testes, they would stretch hundreds of meters, an enormous amount of production surface packed into a small organ.

Each tubule is lined by Sertoli cells, which support the developing sperm, and contains sperm cells at progressively more mature stages from the wall toward the central channel. Tight junctions between Sertoli cells form the blood-testis barrier within the tubule wall, protecting the genetically unique developing sperm from the immune system. The mature sperm are released into the central lumen and carried away to be stored and to complete maturation.

The tubules are the FSH- and testosterone-dependent sperm compartment, distinct from the Leydig cells sitting outside them that make testosterone. This anatomical separation explains a recurring clinical pattern: damage confined to the tubules (from heat, toxins, or radiation) can wreck fertility while testosterone stays normal, so a man may look and feel entirely normal yet be unable to father a child without help.