One organ, two factories
The testis does two very different jobs at once, and it is easiest to understand if you picture it as two factories sharing one building. One factory makes a hormone — the steroid testosterone that circulates through the whole body. The other factory makes cells — sperm, by the hundreds of millions. The clever part is that these two jobs are not separate; the hormone factory exists partly to keep the sperm factory running.
If you slice a testis open under a microscope you see tightly coiled tubes packed together. These are the seminiferous tubules, and they are where sperm are built. In the small spaces *between* the tubules sit clusters of cells with a different look entirely. The geography matters: hormone-making happens between the tubes, and sperm-making happens inside them.
Leydig cells: the hormone factory
The cells sitting between the tubules are the Leydig cells. Their single great task is to make testosterone — an androgen, meaning a hormone that drives masculine traits. Because testosterone is a steroid hormone built from cholesterol, Leydig cells are full of the machinery of steroidogenesis. The testosterone they release pours into nearby blood vessels and also, importantly, soaks the tubules right next door.
Sertoli cells: the nurse cells
Inside the tubules, the tall Sertoli cells stretch from the outer wall all the way to the hollow center. Developing sperm are tucked along their sides like passengers held by an usher. Sertoli cells feed the developing sperm, clear away debris, and form a tight seal — the blood–testis barrier — that walls off the maturing sperm from the bloodstream and the immune system. Without this nursery, spermatogenesis cannot proceed.
Notice the partnership: Leydig cells make testosterone, but it is the Sertoli cells right beside them that *use* the high local testosterone to nurture sperm. The whole tubule bathes in androgen concentrations far higher than anywhere else in the body. This is why a man can have plenty of testosterone in his blood yet still be infertile if the link between the two factories is broken.