Semen is a white or grey liquid, emitted from the urethra (tube in the penis) on ejaculation. Usually, each millilitre of semen contains millions of spermatozoa (sperm), but the majority of the volume consists of secretions of the glands in the male reproductive organs. The purpose of semen is purely for reproduction, as a vehicle to carry the spermatozoa into the female reproductive tract. Although ejaculation of semen accompanies orgasm and sexual pleasure, erection and orgasm are controlled by separate mechanisms and semen emission is not essential for enjoyable sex in most people. Semen consists of the secretions of several glands but only 5 per cent comes from the testicles. The spermatozoa take over 70 days to develop and are produced solely in the testicles. Individual sperm develop within the testicles from a cell called a spermatogonium. The spermatogonium divides to produce spermatocytes, which then develop into spermatids. The spermatid develops its familiar tail and the cell gradually acquires the ability to move by beating its tail. The spermatid eventually develops into a mature spermatozoan. This process takes about 60 days and the sperm then takes a further 10 to 14 days to pass through the ducts of each testicle and its sperm-maturing tube, the epididymis, before it can leave the body in the semen, during ejaculation. The fluid produced by the testicles contains several chemicals, but is particularly rich in testosterone. The seminal vesicles usually contribute more than half of the seminal volume. Their secretions are particularly rich in a sugar called fructose, which is an essential nutrient for the spermatozoa. They also produce a substance that causes the semen to clot (become sticky or jelly-like) after ejaculation, thought useful in reproduction for keeping the semen at the neck of a woman's womb. Could environmental factors be affecting male fertility today? Many more environmental factors that can affect male fertility exist today than 50 years ago. The factors include: * endocrine disrupters * toxic pollutants * sexually transmitted infections * zinc deficiency * alcoholism * smoking * anabolic steroid use * ionising radiation. Endocrine disrupters are chemicals present in the environment that, by virtue of their ability to adversely affect the endocrine (hormonal) system, cause health consequences. Several industrial pollutants can affect fertility, as can smoking and alcohol use. Some of these toxins can cause reproductive disorders, neurological disease, immune system disorders and cancer.
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