US researchers have identified a receptor on the surface of eggs that binds to a sperm surface protein.

They believe it should be possible to target the receptors so that the egg is stimulated to make changes to its outer coat that would prevent fertilisation by a sperm, New Scientist magazine reports.

Existing birth control methods, such as barrier contraceptives and hormones, are either not totally reliable or have side effects.

Scientists have already identified at least three binding proteins on the sperm.

But identifying the corresponding receptors on the egg has been difficult, partly because eggs are harder to come by.

Egg breakthrough

Sperm may be blocked from penetrating the egg

Now scientists at the State University of New York in Stony Brook have found an egg receptor for a critical sperm surface protein called fertilin-beta.

In mice with a faulty fertilin-beta gene, sperm rarely fuse with eggs. The team synthesised the part of the sperm protein thought to bind to the egg, attached a radioactive tag, and then mixed it with mouse eggs.

The tagged fragment bound only to a receptor called alpha-6/beta-1 integrin.

Evidence from frog eggs suggests that an integrin receptor can activate the egg and prompt it to change its outer coat to prevent more than one sperm from getting in.

If this happens in humans, the integrin receptor could be a target for a new contraceptive that deceives the egg.

However, mammalian eggs have yet to be successfully activated in this way.

Janice Evans, of the Johns Hopkins School of Public Health in Baltimore, Maryland, said sperm probably have several ways of getting into an egg, so researchers might have to come up with a cocktail of drugs to block all the different sperm-egg interactions.

She said: "If there was only one way to get a sperm to an egg, none of us would be here."

Richard Schultz, of the University of Pennsylvania in Philadelphia, believes targeting sperm proteins would be a better approach, because the alpha-6/beta-1 integrin receptor is also found on cells other than the egg.

Drugs that block this receptor might disrupt other cell interactions and cause side effects.