The testicle is an oval organ surrounded by a tough, fibrous connective tissue sheath, the tunica albuginea. Internally, it is divided into several hundred compartments, called lobules, each of which contains several coiled, tightly-packed tubules called the seminiferous tubules. Each tubule can be about 50cm long uncoiled, so with 1-4 tubules in each of several hundred lobules, the total length of the seminiferous tubules is approximately 300 meters uncoiled.

Each tubule is a sperm factory.

Each tube is a multi-layered structure, with actively dividing cells in the outermost layer. Newly divided cells migrate towards the center of the tube, differentiating as they go. They shed most of their cytoplasm, reducing themselves to a thin rind wrapped around a nucleus, and they form a long flagellar tail. It takes a long time, about two months, for a newly born human germ cell to mature into a fully equipped sperm cell, at which time it is shed into the duct, ready to be cast out into the world. It's slow, but remember...you've got hundreds of meters of this stuff constantly trickling out new sperm, and it adds up.

In addition to sperm and sperm precursors, the testicle contains several other kinds of cells. The Sertoli or sustentacular cells are essential supporting cells that wrap around developing sperm cells, secrete substances to nourish them, and phagocytize the excess cytoplasm the sperm cells discard. They also produce an androgen binding protein that helps concentrate testosterone in the environment of the sperm, which is needed for maturation, and they produce hormones that feed back on the endocrine system to regulate the rate of sperm production.

Imbedded in the spaces between the tubules are another kind of cell, the Leydig cells or interstitial endocrinocytes. These are the cells that produce the hormone testosterone, which is released into the capillaries that invest the testicle, which then circulate the hormone to the rest of the body.

Once sperm complete their development and are cast off into the duct, they next migrate to the epididymis. The epididymis is another tightly coiled duct, about 6 meters worth packed into a 4cm long rope, that clings to the posterior edge of the testis. It is a sperm storage organ, and the place where the sperm acquire motility and have various other essential molecular functions finally activated.

The next step for sperm is the ductus deferens, or vas deferens, a long tube that extends from the back end of the testis, up into the body, in front of and over the pubic bone, around the urinary bladder, and finally to a region called the ampulla, were it merges with the seminal vesicle before entering the prostate gland. This roundabout route of about 18 cm is a relic of it's embryonic development, when the testicle migrated from its internal position to its final place in the scrotum. The vas deferens also stores sperm, for up to several months, and the seminal vesicle and prostate are responsible for adding seminal fluid to the concentrated sperm stored in the duct.