For more than a decade, scientists have known of about a thousand genes that code for odorant, or chemical "scent", receptors in vertebrates, including some that function in the unlikeliest of places: on sperm cells. But only recently have they begun to unravel how nature programs sperm to turn the scent of an egg into a fertilization victory. Work described in the current issue of the journal Science provides some of the best evidence yet that human sperm are chemically attracted to the egg.

Marc Spehr and his colleagues at Ruhr University Bochum in Germany found a previously unknown odorant receptor in human testicular tissue called hOR17-4. To figure out how it worked, they cloned the receptor and transplanted it into human embryonic kidney cells to look for chemical compounds, or ligands, that would activate it. Out of a mixture of some 100 synthetic compounds, the team found one that worked best: bourgeonal.

The researchers then employed computer tracking technology to probe how sperm use odorant receptors to track down an egg. Using a technique similar to time-lapse photography, they recorded sperm racing toward a very small concentration of bourgeonal. In the presence of another compound, undecanal, the sperm were unable to find the egg.

A better understanding of what compounds work to guide the fertilization process could increase the success rate of fertility treatments and provide new methods for contraception. But at the moment, scientists don't know whether bourgeonal is produced in the female body at all. Team member Richard K. Zimmer of the University of California at Los Angeles hopes that scientists will be able to use the chemical structure of bourgeonal to identify potential sperm attractants that do exist in the environment of a human female's egg. Because the sperm reacted to a tiny dose of bourgeonal, Zimmer believes that the same process is likely to occur naturally. "Bourgeonal isn't necessarily the natural ligand, but something it mimics probably is," he says. "And that it's effective at a low dose is pretty important because an egg can't release that much stuff because it's so small."