Human sperm may sniff out the path to an egg. (By a Nose?).

A man's sperm seek out a variety of floral scents, suggesting that these microscopic swimmers possess a primitive kind of nose that enables them to navigate to a woman's egg. This discovery could inspire new forms of contraception or improvements in in vitro fertilization, say researchers.

It's long been suspected that human sperm sense chemicals secreted by an egg. More than a decade ago, for example, investigators found that human sperm sport proteins called olfactory receptors, the same molecules that nerve cells in the nose use to detect smells.

Yet it's been difficult to identify what binds to these receptors or to confirm that human sperm sense and swim toward any chemical. In the March 28 Science, however, Marc Spehr of Ruhr University Bochum in Germany and his colleagues describe a new olfactory receptor on human sperm and report that its response to several chemicals can direct a sperm's swimming behavior.

After identifying the new sperm protein, known as hOR17-4, the researchers added its gene to human kidney cells growing in a lab dish. By monitoring the flow of calcium ions within the modified kidney cells, the scientists could determine whether a substance triggered the new olfactory receptor.

After testing hundreds of synthetic compounds, many of which provide floral scents to commercial products, the researchers homed in on a chemical called cyclamal. They then found similar compounds that also activate the receptor. One of these, a floral scent called bourgeonal, is the most potent signal identified so far.

Human sperm were even more sensitive to this compound than were the kidney cells used in the original experiment, the researchers report.

"Bourgeonal has two effects on sperm behavior. It speeds up sperm and changes undirected swimming behavior to direct movement toward the compound," says Spehr. In fact, at certain concentrations of bourgeonal, sperm speed almost doubled--to 30.1 micrometers per second from a normal 18.3 [micro]m/sec.

Since the scientists don't think that eggs make bourgeonal, they're now searching the fluid from women's reproductive tract for a natural stimulus for the new sperm receptor. "The natural sperm attractant might be structurally related to bourgeonal," says Spehr. "One can only speculate if it is secreted by the egg itself or the cumulus cells surrounding the egg or even by cells lining other parts of the female genital tract."

Bourgeonal or other sperm attractants might someday be used in a fertility test to identify sperm with the best chance of artificially inseminating an egg, the researchers suggest. The investigators also identified a compound, called undecanal, that somehow blocks bourgeonal's attractiveness to sperm. They speculate that undecanal could provide a lead for a new form of contraception that prevents sperm from sniffing their way to the egg.

The identification of a novel sensory receptor on human sperm, along with compounds that trigger it or block its action, constitute a "landmark event," says Donner F. Babcock of the University of Washington in Seattle in a commentary published with the new report.