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A sperm cell, or spermatozoon, is a sex cell produced by male organisms. In humans and other vertebrates (animals with a backbone), sperm are produced in great numbers by the male gonads or testes (sex organs). The job of a sperm is to swim to the female egg, penetrate its surface, and deliver its package of genetic material. This is called fertilization and usually results in the production of an offspring.

Sexual reproduction involves the fusion, or uniting, of male and female gametes (sex cells) to produce a new individual. It also usually involves a male parent that produces sperm and a female that produces an egg, or ovum. The egg is large and does not move, while sperm are usually small and highly mobile. When a sperm contacts and penetrates an egg, fertilization occurs. This fertilization produces a zygote that will develop into a genetically unique offspring. All animals that engage in sexual reproduction manage to unite a sperm with an egg.

For some species, the sperm fertilizes the egg externally (as do frogs or fish in water). The method of external fertilization used by aquatic animals would barely work if only a few sperm and eggs were released each season because the eggs and sperm are unprotected and left to unite by chance. That is why each aquatic parent produces very large quantities of sperm or eggs. Land animals, like mammals, are more complex, and their reproductive systems are more specialized. They engage in internal fertilization where the sperm it directly deposited in the female's body. While the male produces sperm in great numbers, the female makes only a very small number of eggs available for fertilization at any one time.

Nearly all male animals produce sperm as their reproductive cell. With the exception of certain worms, decapods (like crayfish), diplopods (like millipedes), and mites, all sperm have two main parts. These include a head and some form of a whiplike tail. The tail, or flagellum, whips side-to-side and gives the sperm cell its movement. The head of a sperm is different for each species and is made up mainly of a nucleus (the cell's control center). It is there that the sperm carries chromosomes, the important genetic material responsible for transmitting certain characteristics to the new individual it helps create.

It is the sperm (and not the egg) that also carries the chromosome that determines the sex of the offspring. The head of the sperm is covered with a cap called the acrosome. This cap contains chemicals that eat through the egg's protective covering and help the sperm burrow into the egg and fertilizes it. When the sperm nucleus and egg nucleus meet, their membranes fuse together and form a new nucleus. This new nucleus receives twenty-three chromosomes from the sperm and twenty-three from the egg to make a complete set of forty-six. Once an egg is fertilized by one sperm, it prevents any others from penetrating its outer layer. Sperm that do not fertilize an egg keep swimming either in water or in the female's reproductive tract until they die.

Most animals release sperm in large numbers, and a healthy human male can release 250,000,000 sperm in a single ejaculation (the ejection of sperm and fluids from the penis). Human males do not start producing sperm until puberty begins (between the ages of twelve and fourteen when they evolve from children into adolescents). The male gonads or testes where sperm are produced are housed outside the body in a bag of skin called the scrotum. Since sperm are unable to survive at normal body temperature, they are housed outside the body where it is at least 5°F (−15°C) cooler.

[See alsoFertilization; Human Reproduction; Reproduction, Sexual; Reproductive System ]