Since the advent of medicine, there has been a vast and wild array of theories about what, exactly, sperm is. The ancient Greeks believed that semen was a vital life force made partly of brain fluid, while 17th-century physicists thought that each individual sperm contained a “little man.” Today, we know that the male reproductive cell is neither of these things. The smallest cell in the body, at 0.002-inch long, a sperm consists of a cell nucleus encased in a bulletlike or oval head and a whiplike tail, or flagellum.

Every day in a man’s testes, about 300 million to 400 million sperm reach maturity. A steady stream of male sex hormones, including testosterone, triggers the sperm’s production, which takes place over 72 days. During the process, male sex cells compact 23 chromosomes—half of a person’s full number—into a single nucleus. (The woman’s egg provides the other half of the chromosomes, but it’s the sperm that carries the X or Y chromosome that determines whether a child will be male or female.) Surrounding the head of each sperm is a protective cap called an acrosome, which contains special chemicals that help the sperm penetrate the egg. After the sperm are fully formed, they’re stored in the vasa deferentia (singular: vas deferens), the pair of sperm ducts that connect the testes to the penis, and in seminal vesicles that lie alongside the prostate gland.

During ejaculation, muscles in these structures power hundreds of millions of sperm—along with seminal fluid from the prostate—into and through the urethra, in the penis. Then the flagella power the sperm through the female reproductive tract in search of an egg to fertilize. Once released, a sperm can live for up to 7 days.

ADDITIONAL FACTS

  1. The sperm of crawfish, millipedes, mites, and worms do not have flagella, or tails.
  2. Sperm that have been frozen for months or even years can be thawed and still effectively fertilize an egg.
  3. One milliliter of average semen contains 50 million to 200 million sperm cells.