Human Sperm May 'Smell'

Human sperm may literally sniff their way to an egg in their quest for fertilisation.

Scientists have discovered that a smell receptor on the sperm apparently causes them to zoom in on their target.

"This receptor may be important for the path finding from sperm to egg," says study author Marc Spehr, a neuroscientist at Ruhr University Bochum in Germany.

Although very preliminary, the finding could one day lead to new forms of contraception and new fertility treatments.

"It's speculative, but it's an intriguing possibility," cautions Donner Babcock, author of an accompanying article in the March 28 issue of Science.

A scientific first

In addition to the nose, olfactory receptors are found in certain parts of the brain and, only occasionally, on the skin. Certain olfactory receptors are expressed mostly or exclusively in human spermatogenic cells, scientists have discovered. This is the first time the function of such a sperm receptor has been decoded.

Spehr and his colleagues conducted molecular, cellular and behavioural tests to try to determine the role of the receptor in question, called hOR17-4.

The tests revealed the sperm swam toward elevated levels of a substance called "bourgeonal." A compound called "undecanal," on the other hand, appeared to block the attracting effect. What this means is that signalling from hOR17-4 may control communication between the egg and sperm.

The scent of a woman

One remaining question is where the attracting compound comes from within the female. Is it the egg itself, or some other part of the female reproductive tract? "It could be that the egg is releasing an attractant that helps guide the sperm to the egg, but the problem is that we don't yet know whether, in fact, the egg is the source of that attractant," Babcock says.

Though it won't be easy finding the answers to those questions, at least researchers have a new avenue to try. "The exciting part is it gives us a way of addressing those questions," Babcock says. "A major barrier in the past was that you had to find something that was released by the egg that would attract sperm. Human eggs are few in number and almost impossible to get."

An alternative to hormone-based contraception?
In practical terms, this could one day mean an alternative to hormone-based contraceptive methods.

"It opens new doors to exploration," Babcock adds. "If this is an important mechanism to ensure fertilization, then one could think about new ways of interfering with it and thus new contraceptives."

On the other hand, if scientists discovered a bourgeonal-like compound in the female genital tract that attracts sperm, they might be able to use that to incubate an egg to make it easier for the sperm to find the egg or to increase the number of sperm that are able to fertilize the egg, Spehr says.

For now, researchers are trying to determine the function of other olfactory receptors expressed in sperm, and discovering a complementary process in the egg.

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