Human sperm may `smell' their way to egg

A new study identifies a novel odorant receptor on human sperm and shows how activating this receptor causes the sperm to make a beeline for a target.  

A BETTER understanding of the new `smelling' capabilities of human sperm cells may lead to advances in contraception and fertility treatments.

This new receptor is a member of a family of receptors primarily expressed in the sensory neurons of the nose.

In a study appearing in the journal, Science, German and U.S. researchers report that the binding of certain compounds to the new odorant receptor (hOR17-4) found on the surface of sperm cells, triggers a series of physiological events that may result in the directed movement of human sperm.

In this chemosensory response, the sperm cells travel toward elevated concentrations of a sperm-attracting substance called bourgeonal.

"We were not expecting to uncover a receptor for chemo-attraction, this is the best we could expect to find," said author Marc Spehr of Ruhr-Universit�t Bochum in Bochum, Germany, who noted that one of the next steps is to identify a female-produced equivalent to bourgeonal.

The scientists do not yet know if the egg itself produces some sperm-attracting compound similar to bourgeonal or if some other part of the female reproductive tract makes the chemical that may bind to the new receptor.

"If a natural equivalent to bourgeonal is, at least in part, responsible for successful path finding or screening of fertile sperm, then it should be possible to use bourgeonal within in-vitro fertilization (IVF) treatments," said Spehr.

Success rates for IVF treatment are not as high as sterile couples and medical doctors would like, he added: "Some of the difficulties experienced in IVF treatments may be linked to the `quality' of sperm. Bourgeonal might be used in the future to find the motile and fast sperm cells that are needed for fertilization."

Further research is necessary before such approaches may be realised. The researchers also identified an antagonist compound, `undecanal' that appears to block the affect of bourgeonal and inhibits the chemosensory response in sperm cells.

"One of the greatest problems in contraception these days is the use of hormones. If undecanal can inhibit egg-sperm communication, this drug might be used, after a great deal of future research, to prevent undesired pregnancies. One could speculate about delivery of undecanal into the female genital tract or even about drugs containing equivalents to undecanal that could be used by men," said Spehr who cautioned against taking these ideas out of the framework of odorant-receptor research.