Testes

The egg shaped testes are the center of sperm production. Their position outside the body makes them highly vulnerable to injury. Nature is usually a more cautious caretaker. Yet even in this seemingly irresponsible maneuver, biological reasoning can be detected.

Theoretically, the location away from the body cavity helps keep the temperature inside the testes slightly cooler (34.8C) than that of the rest of the body (37C). This coolness is vital for good sperm production. From moment to moment, the temperature is carefully maintained. When the outside temperature is cold, scrotal muscles contract and bring the testicles up closer to the body for warmth; when it's hot, the muscles relax and the testes drop away from the body.

To offer some cushioning against injury, the testicles are wrapped in membranes and suspended in a pouch of tissue called the scrotum. More help comes from nerve fibers that run through the pouch and make the organs so pain sensitive that men instinctively try to protect them. During sports or other strenuous activity a jockstrap gives some added protection; even so, any hard blow to the scrotum may damage the sperm production center and destroy a man's fertility.

The testes vary in size and weight, depending on a man's overall body build. There is no such thing as a "right" size, nor does size have anything to do with the number or quality of sperm produced. However, if the testicles fail to mature because of a chromosome abnormality or a severe illness, they will be unproductive and remain quite small.

The testes are filled with hundreds of tightly wrinkled threads, each one up to a yard long. These are the seminiferous tubules, the birthplace of new sperm. Three or four microscopic tubules coil together into a lobule, and the few hundred lobules roll up into tiny balls that pack the testicles.

Under the influence of hormones, tiny cells planted in the tissue lining of the tubules begin to produce sperm. The initial development of sperm takes about 64 days. At that point, the new sperm, which cannot yet swim, drop into the hollow center of the tubule. These cells have the characteristic head and tail of sperm but none of the power. They are pushed along by muscle contractions and fluid pressure through small canals (rete testis) into another tightly coiled structure, the epididymis.