Sperm: the male sex cells
The mature sperm cell (see Figure 5.1) is a little swimming male sex cell. It consists of a head, body and tail. The head is covered by a cap and contains a nucleus of dense genetic material. It is attached to a region containing mitochondria, which supply the energy for the sperm's activity. The tail is able to contract and relax, producing a characteristic wave-like movement.
Look at Figure 5.1. Can you explain how sperm move up the vagina, into the uterus and along the fallopian tubes?
A sperm has a tail that can wave about, propelling the sperm along as the tail 'whips' from side to side. As you probably remember from high school biology, the mitochondria are the energy-producing 'motors' in a cell, which supply the sperm tail with the energy for its movement.
The sperm make up only about 5% of what a man ejaculates during the sex act. This represents about 100 to 400 million sperm each time, carried in a nutritious fluid (semen) which helps to keep them alive. Sperm are very, very small in size; in fact, a single sperm is the smallest cell in the male body.
From puberty onwards, new sperm develop in the testes (testicles) throughout a man's adult life. It takes about 72 days for one sperm to develop to maturity. Sperm production requires a temperature which is 3 to 5°C below body temperature -- which is why the testicles are outside the abdomen, where they can remain a little cooler. Exposure of the testicles to a higher temperature will inhibit sperm production and may lead to infertility.