In biology, the male gamete of animals before fertilization in sexual reproduction. Each sperm cell has a head capsule containing a nucleus, a middle portion containing mitochondria (which provide energy), and a long tail (flagellum). In mammals sperm cells are produced in the testes of a male. They are produced by a special kind of cell division called meiosis, which halves the number of chromosomes present.
Sperm cells are highly specialized. When they are introduced into the vagina of the female by the penis of the male, they swim up the uterus and oviducts. The swimming is a result of the beating of the tail of the sperm cell. At the other end is the nucleus, but there is little else in the cell, making it fairly small and light.
The human sperm is 0.005 mm/0.0002 in long and can survive inside the female for 2–9 days. Sperm counts have fallen by 50% worldwide since 1940, according to a Danish study in 1990. This reduction may be due to increases in pollution – a number of pollutants, including some petroleum by-products and PCBs (polychlorinated biphenyls), appear to have similar effects to the female hormone oestrogen.
Structure of sperm The sperm head contains an enzyme acrosin that dissolves the ovum casing enabling the sperm to penetrate it. The discovery of a protein called protein C inhibitor (PCI) was announced by Dutch researchers in 1998. PCI blocks the activation of acrosin when sperm is stored in the epididymis until it is released by ejaculation.
In most animals, the sperm are motile, and are propelled by a long flagellum, but in some (such as crabs and lobsters) they are non-motile. Sperm cells are produced in the testes (see testis). From there they pass through the sperm ducts via the seminal vesicles and the prostate gland, which produce fluids called semen that give the sperm cells energy and keep them moving after they leave the body. Hundreds of millions of sperm cells are contained in only a small amount of semen. Mammalian sperm have receptors identical to some of those found in the lining of the nose. These may help in navigating towards the egg.
The term is sometimes applied to the motile male gametes (antherozoids) of lower plants.
Human egg fertilization
Some Special Kinds of Animal Cell
Specialized cell functions
sperm: fruit fly
development of a human fetus
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