Microscopically small sperm cells and are shaped like tadpoles with a long thin tail that propels them along at 18cm an hour. They are produced in the testicles and are divided into a head and a tail. The head contains the nucleus and its genetic material (chromosomes) and the acrosome, which is used to penetrate into an ovum (egg). The tail (flagellum) allows propulsion while the centrosome at the base of the tail plays a part in the preparation of the genetic material.
Millions of sperm cells are released by each ejaculation in the substance known as semen. Only one out of these millions is able to fertilise an egg. Every one sperm contains 23 chromosomes with the 30,000 genes necessary to create a new human. A majority do not survive the arduous trip to fertilise the egg.
Each contains half the seed of human life, about 3.5 gigabytes of information in computer terms, packed onto genes in a microscopically small space within the sperm head.
Sperm cells have a limited lifespan outside of the body and are usually dead by the time semen is dried. They can survive up to five days in the reproductive tract of women after sex.
Sperm cells need to be produced at a slightly lower temperature than that which is normal in the body, so the testicles hang down away from the body in the scrotum. The scrotum is usually about 4°C cooler than the body. Heat can slow sperm production.
The quantity and quality of sperm varies between men. Each ejaculation usually contains an average of 100 million sperm cells but anything more than 20 million is considered normal. Sperm count and quality decreases with age but unlike females, a male can be fertile throughout his life.