Sperm

Sperm is the male reproductive cell (gamete) of most animals and some plants. Its primary function is to deliver genetic information from the male parent to the egg, so that both cells contribute half their genetic material to the new fertilized cell.

Sperm is very small and cannot store large amounts of food; the egg is the larger cell, containing food and machinery for early development. For this reason, sperm is adapted for movement: its structure resembles an elongated form with a tail.

Structure

Human sperm has three main parts. The head contains the nucleus with genetic material, and on its surface is a cap called the acrosome, which contains enzymes that help penetrate the membrane surrounding the egg. The middle section is packed with mitochondria, which are the sites where the cell's energy is produced. The tail (flagellum) is a whip-like structure that propels the sperm.

Formation

Sperm is produced in the testes, specifically in tubes called tubules. The process called spermatogenesis begins with stem cells that divide, and then undergo meiosis to produce cells with half the chromosome number of a normal cell. After this, they continue to mature in the epididymis before being released.

Fertilization

When sperm reaches the female reproductive tract, it moves toward the egg with the help of chemical signals and contractions of the surrounding muscles. Normally only one sperm enters the egg; after the membranes fuse, the egg changes its surface to prevent other sperm from entering. The fusion of the two nuclei creates the first cell of a new embryo.

Variation among species

Sperm structure is not uniform across animals. Many fish and aquatic organisms release sperm into water outside their bodies, while other species fertilize internally. Some plants, such as bryophytes, have swimming sperm with tails that move through thin films of water; flowering plants, meanwhile, use pollen tubes produced by pollen grains instead of tailed sperm.