Illustration showing the structure of a sperm cell (spermatozoon), the male sex cell. Each sperm cell consists of a head (bottom left), mid-piece (upper left) and tail (right). The head contains the acrosome, which will pierce the egg's membrane during fertilisation, and the nucleus, within which is the genetic material. The mid-piece contains the centriole and numerous mitochondria (disc-shaped) that provide the energy necessary for the cell to swim. The tail contains microtubules arranged in a central axial filament (axoneme, upper right) that spins in a twisting motion to propel the sperm towards the egg for fertilisation.