Sperm cells. Colored scanning electron micrograph (SEM) of human sperm cells (spermatozoa). These tiny male sex cells are produced in the testes, and are responsible for fertilization of the female egg (ovum). Each sperm has a rounded head which contains the male hereditary material (DNA) and a long whip-like tail that propels it along. Sperm production commences at puberty and is dependent on the male sex hormone testosterone. Some 300 million sperm are released by the male during ejaculation, while the female usually produces one large egg at a time. Only one sperm cell can fertilize the egg. Magnification unknown.