PERFUMES may not always do much for the male of the species. But there’s at least one that drives sperm wild, setting them racing off towards the source of the smell.
The work is the best evidence yet for a long-held suspicion: that human sperm, like those of sea urchins, follow a scent trail towards the egg. However, the actual chemical attractants released in women have still to be identified.
The vast majority of the body’s protein “sensors” that detect such chemicals are found in the nose, of course. But a few olfactory receptors have been found in the brain, the skin and one in sperm. By looking to see if sperm expressed any genes similar to those for known olfactory receptors, Marc Spehr’s team at Ruhr University in Bochum, Germany, managed to identify a second receptor called hOR17-4 (Science, vol 299, p 2054).
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The team then tested a range of chemicals to find out what hOR17-4 can “smell”. A substance called bourgeonal, which is found in a few perfumes, produced the strongest response. “It smells like lilies of the valley,” says Spehr.
It is not yet clear how sperm steer towards the scent. But when bourgeonal is released near them, they head straight for it, swimming twice as fast as normal. The hOR17-4 receptor triggers the opening of calcium ion channels in the sperm, thought to make them wag their “tails” harder.
The findings could have practical applications. Substances like bourgeonal could be used to select the most vigorous sperm for IVF, for instance. Or they could lead to new contraceptives. The team has already shown that a chemical called undecanal blocks the effects of bourgeonal. But human sperm are thought to have dozens of different olfactory receptors, all of which would have to be blocked to create an effective contraceptive.