Spermatozoon

A spermatozoon is a male gamete, developed by the testes.

The spermatozoon is a small cell (5 micrometres in diameter), which is far smaller than the egg but has a mobile tail (a flagellum) which is 60 micrometres long. In order to be light and mobile and reach the egg quickly, the spermatozoon is smaller in size due to its cytoplasm being minimised and its DNA compacted. Its head is covered with the acrosome, a sac full of enzymes used to pierce the membrane of the egg.

It is a haploid cell as it contains half of a person's genetic material, the other half being lost in meiosis during the spermatogenesis process which primordial germ cells undergo in the seminiferous tubules of the testes.

The spermatozoon contributes half of the future embryo's genetic pedigree. As the spermatozoon is a gamete produced by a male, its sex chromosome may be X or Y, and therefore determines the sex of the embryo (whereas the egg always contains an X).

Using its flagellum the spermatozoon swims through the vagina, which it is ejected into by the testes, to the fallopian tubes where it meets the egg. As a result of reactions which occur during fertilisation, only one spermatozoon can fertilise an egg.