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Super sniffers. The same receptors that help sperm locate the egg also function in our sense of smell.

Smelling Like Sperm

An inability to appreciate the smell of a certain flower may indicate that a man is infertile, a new study suggests. Scientists have found that one of the receptors sperm use to locate an egg is also present in our nose, where it allows us to detect certain scents. Men who can't smell these scents may produce sperm that can't find the egg, making "sniffing tests" an attractive future tool for judging male fertility.

Swimming blindly through the twists and turns of the female reproductive tract, a sperm cell fights an uphill battle in its quest for the egg. A 2003 study (Science, 28 March 2003, p. 2054) indicated that sperm may get some help from a newly identified cell surface protein that draws sperm toward chemical signals. The protein, hOR17-4, is an olfactory receptor--a class of proteins involved in the mammalian sense of smell. But whether this particular receptor was also found in the human nose and functioned in our sense of smell remained an open question.

To address this issue, a team of U.S. and German scientists examined nasal cells from a male volunteer. As reported in this week's Current Biology, the researchers discovered that these cells express the same hOR17-4 receptor found in human sperm. The researchers then had both male and female students sniff two different odorants specific for the sperm hOR17-4 receptor: bourgeonal, which smells like the lily of the valley flower and attracts sperm, and undecanal, which has a glue-like smell and blocks sperm from responding to bourgeonal.

Students who sniffed bourgeonal, undecanal, and bourgeonal in succession had trouble smelling the second round of bourgeonal. This suggested that undecanal can block the effect of bourgeonal in the nose just as it does in sperm. In other smell tests with the students, undecanal was unable to block the smell of odorants such as vanilla that activate other olfactory receptors, suggesting that its effect is specific for hOR17-4.

Given the similarities between the sperm and nose hOR17-4 receptor, it's tempting to speculate that men who have trouble smelling bourgeonal may produce sperm that have trouble locating the egg, says study leader Marc Spehr, a physiologist at the University of Maryland, Baltimore.

"This is a useful step forward in trying to address the function of olfactory receptors in sperm," says Gordon Shepherd, a neuroscientist at Yale University. Whether this work can be applied to fertility detection remains to be seen, says Marc Parmentier, an olfaction expert at the University of Brussels in Belgium. "The data look strong enough," he says. "So it's definitely possible."

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