Sperm are male gametes, reproductive cells that contain half a set of chromosomes that can be joined with a female gamete to form a zygote. If allowed to develop, the zygote will turn into an embryo and eventually will mature into a baby. Male gametes are found in semen, a fluid produced by men for the purpose of reproduction. The composition of semen varies, but sperm as a general rule make up only a very small percentage, usually less than five percent, of the volume.
The groundwork for the production of these unique cells is laid during fetal development. Specialized cells known as germ cells migrate to the reproductive tract and settle in the gonads. During puberty, the cells start to divide, producing sperm. Over the course of a man's lifetime, he produces billions of these cells, with around 50 million being released with each ejaculation. Of these millions, in sexually active men, hundreds generally manage to make their way into the female reproductive tract, where they compete to fertilize the waiting egg.
Once sperm have been produced in the gonads, they take approximately seven days to work themselves into a position for ejaculation. Along the way, they become highly concentrated, with fluids being absorbed by the male reproductive tract to leave tightly packed gametes behind. Glands produce the other constituents of semen, including compounds that make the semen clot on release to ensure that it stays in the female reproductive tract along with sugars that provide nutrition to the sperm as they work their way towards the egg.
The structure of these cells includes a head that contains enzymes designed to help the cell penetrate the egg. While the human egg is very small, male gametes are even smaller, and the outer wall of the egg is essentially an impenetrable wall without enzymes to help the cells wriggle their way in. An extension called the flagella trails from the head and acts to animate the cells as they move through the reproductive tract. This tail will later drop off.
A number of problems can occur during spermatogenesis, when the male body makes more sperm. Errors during division can lead to chromosomal abnormalities, some of which may be fatal. Production and the health of the gametes can also be limited by environmental factors. When couples experience infertility, medical tests are conducted to learn more about the causes and to identify a potential treatment plan. In men with a low sperm count or limited sperm motility, it can be difficult to successfully impregnate a woman without assistance.