A sperm cell, showing it's size relative to an egg cell, courtesy Wikipedia

Sperm (from the Greek sperma for "seed") are the reproductive cells of males. Unlike most of the body's cells, they contain only one set of 23 chromosomes instead of two sets. Incredibly tiny, and much smaller than the corresponding female cell, the egg cell, over 100,000,000 of them are contained in the average ejaculate of semen. The meeting of a sperm cell with an egg cell results in fertilization and, if the fertilized egg imbeds in the female body, pregnancy.

Sperm are created and stored in the testicles, then mixed with semen before ejaculation. Men create sperm cells throughout their lives, although the quantity and quality of sperm cells decrease with age.

Sperm at Wikipedia

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