..........Testicles.........

The testicles (aka balls or nuts) are remarkable organs. They live outside the body because sperm production can only take place in a slightly cooler environment than that of the body cavity. They produce sperm - about 150 million every 24 hours - and they keep the body well supplied with testosterone, the hormone responsible for men's sexiness, body hair, odor, erections, sexual libido and many of the other things that make a man a man, so to speak. The testicles are undoubtedly vulnerable, hanging outside the body in the scrotum, and it seems remarkable that they are damaged so infrequently, given the male propensity for sport and fighting. Maybe their resilience is a testimony to their great design! So, what do they do, and how do they work? What do testicles do? They make testosterone and they make sperm. They do this in different parts of the testicle - the sperm are made by "germ" cells in the seminiferous tubules; the testosterone by Leydig cells which occupy the spaces between the tubules. The sperm-producing part of the testicle occupies about 60 to 80% of its volume in the human male. So tightly twirled up are the seminiferous tubules, that if you uncoiled them, each testis would contain, on average, about 360 meters (nearly 1200 feet) of these tubules, all of which is lined with sperm producing tissue. This astonishing fact means that the average pair of testicles produces about 150 million sperm a day; overall, the testicles contain about two billion sperm at any one time. The process of sperm production - the transformation of normal cells into mobile, tail-lashing swimmers with only one set of chromosomes - is a remarkable one which involves six stages of development; perhaps even more remarkable is the fact that when a man's fertility is impaired because he cannot produce sperm able to swim, doctors can now extract sperm at an earlier stage of development from his testicle and inject them straight into the egg, thereby overcoming the capriciousness of nature. What, you may ask, happens to sperm that never make it to the outside world? The answer is that the body has a mechanism by which other cells destroy them, rather in the manner that foreign cells invading the body are destroyed by the white blood cells of the immune system. The development of the testicles There seems to be a common myth that a boy's balls drop at puberty. But this is simply wrong. The testicles begin their descent into the scrotum at week 28 of pregnancy, and the process is complete in 97% of boys by 12 weeks after birth. Testicles that don't descend can be problematic: sperm production can be disturbed, and if they remain in the abdomen, there is an increased risk of testicular cancer. This may have something to do with the temperature of the testicles: in men, they are about 3 to 5 degrees Centigrade cooler than the abdominal temperature, and about one and a half degrees warmer than the skin of the scrotum. It is not known why this should be with any certainty. Sperm production begins at puberty, not surprisingly, and depends on a complicated interrelationship of hormonal influences in the male body. One of these is the appropriate level of testosterone - too much testosterone acts as a negative feedback control on the hypothalamus and prevents the brain from producing the hormones necessary for spermatogenesis, and therefore acts like a male contraceptive. Long-acting testosterone-like compounds are indeed the basis of several products undergoing trials as male contraceptives at the moment. One of the side effects of testosterone replacement therapy, therefore, is that a man may become, at least temporarily, infertile and his testicles shrink. These changes are reversible when the testosterone regime is stopped, except when the supplement are administered in such extreme doses as to bear no relationship to normality. Thus, body builders who take testosterone in excessively high doses may find when they reduce the supplements that they have permanently damaged their body's ability to restore a natural hormonal balance. How big are adult testicles? The orchidometer is a device used by doctors to measure boys' and adult males' testicles. A grown man's testicles are generally between 16 and 27 milliliters in volume, and about an inch and a half long. If you click here, you can see an orchidometer in plan (the actual thing is a series of egg shaped plastic balls on a string, which the doctor compares to the testicle size). Apparently for some reason, in eighty percent of men the left testicle - as you face forwards - is the bigger and lower. I don't know if it produces more sperm than its smaller brother. The sperm output from the testicles is mixed and stored in the epididymis before being mixed with fluid from the seminal vesicles and prostate and launched into the big wide world. Each ejaculation, which is usually between 2 and 6 milliliters in volume contains about 100 million sperm. This means that the sperm occupy no more than 2 or 3 per cent of the volume of each ejaculation. Amazing.