Sperm cell. Artwork of a spermatozoon, a human male sex cell. At left is the 'head', containing the genetic material in the nucleus. To its right is the midpiece, which has a central core with energy-rich mitochondria forming the spiral around it. At centre and right is the 'tail' (flagellum), used to propel the sperm cell along the reproductive tract. During intercourse, millions of sperm cells are deposited in the female reproductive tract. They use their tails to move towards the egg cell (female reproductive cell), where they will attempt to fertilise it. Only one sperm cell will succeed in fertilising the egg.