How a sperm wags its tail

The electric activity that spurs sperm to make a final dash to, then into, a female egg has been measured for the first time. To produce this all-important fertility sprint, sperm tails must switch from an easy, symmetrical beating to a frenetic whiplike lashing. This switching slows down sperm cells but gives them the extra force they need to penetrate an egg's protective coating.

Researchers at Harvard Medical School and Children's Hospital in Boston actually measured the tiny, tiny current in sperm tails that triggers this hyperactivity. The experiments were done with mice, but David Clapham, the Harvard professor of cardiology and neurobiology who supervised the research, says, "I expect that what's true in mice is true in humans. Their genes are very similar to ours."

The key to this seminal stamina is a protein known as CatSper, which both men and mice possess. The protein forms a channel in the middle of the sperm's relatively long tail, allowing electrically charged calcium molecules called cations to flow from body fluids into the sperm. (The name CatSper is short for cation channel in sperm.) These cations cause other fiberlike proteins to contract rapidly and incite the energetic tail whipping.

Although they display normal mating behavior, male mice without CatSper are totally ineffective at impregnating females.

You might guess that knocking out CatSper with a drug would make a great contraceptive. "Yes, it would," agrees Clapham. "It would block the channel that makes males fertile. What's more, it would only be temporary. And, since CatSper is confined to mature sperm, a drug targeted specifically for it would have no side effects."

What you probably wouldn't guess is that such a pill could also work for women. "Sperm swim in the female reproductive tract for at least 15 minutes before they reach an egg." Clapham notes. "Ideally it should be taken just before sex, but it might be made to work after sex."

Such a contraceptive might satisfy the concerns of anti- abortionists, because it would prevent sperm from ever uniting with an egg. Children's Hospital has negotiated a license with a Boston biotech company, Hydra Biosciences, to pursue this approach to pulling the tails of sperm.