An international team of scientists found synthetic compounds mimicking the scent of lily of the valley and other flowers caused the sperm to make a beeline for a target.
"After a great deal of future research, it might be possible to design structurally related compounds that could be used to increase the success rate in in-vitro fertilization procedures," researcher Marc Spehr, a chemosensory biologist at Ruhr University in Bochum, Germany, told United Press International.
Spehr and his colleagues at the University of California at Los Angeles also identified a compound named undecanal that blocked such activity.
"One could speculate about hormone-free contraception methods that are based on an undecanal-induced blockade of sperm path-finding toward the egg," Spehr said.
Prior studies discovered chemical receptor proteins that latch onto "odorants," or scent molecules, in sperm. These are normally seen in the sensory nerves of the nose.
"Since then, there has been a lot of speculation about their functional role, but nobody has shown what these receptors are used for before. That was our initial starting point. We wanted to know what receptors are expressed in humans and what they are doing in terms of sperm behavior," Spehr explained.
Spehr and his team discovered a gene for a new smell receptor named hOR17-4 in human testicles. They copied the gene, implanted it in a lab lineage of human embryonic kidney cells and then began testing a variety of chemicals to see what activated the receptor. Promising candidates were then checked against human sperm.
In findings appearing in the March 28 issue of the journal Science, the investigators found bourgeonal, lilial and floralozone, synthetic compounds used by the perfume industry to imitate lily of the valley and other floral scents, attracted sperm to them from thin glass tubes. When these compounds stuck to the receptor, the researchers found hOR17-4 allowed calcium from outside the sperm to enter, triggering a new tail-beating pattern that drew sperm towards the bourgeonal.
"This really is a major advance in the area, and it gives us the tools to examine whether the sperm that fertilizes the egg is the one that has stopped to smell the roses," reproductive biologist Donner Babcock at the University of Washington in Seattle.
Spehr added that researchers estimate 20 to 40 different scent receptors might be found in the testicles. "It's unlikely all of those are in the sperm," Babcock said. "This is the only one that's shown to be functional in sperm so far."
If scientists take the nose as a model, Babcock added, there's only one odorant receptor for each sensory neuron. "In the data in this paper, at least one-third and probably a majority have one hOR17-4 receptor. If there is only one receptor for sperm, at most there could be two or three different receptors operating in the whole of the sperm population."
One of the next steps in research will be to find a female-produced equivalent in the egg or female reproductive tract of bourgeonal or the rest, Spehr said.
"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 treatments," Spehr said.
Success rates for such "IVF" therapies are not as high as infertile couples and physicians 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."
Undecanal, another compound often used in the perfume and chemical industries, blocked this effect. Undecanal-like compounds could also serve as non-hormonal contraceptives. "A great deal of future research is needed," Spehr cautioned.
(Reported by Charles Choi, UPI Science News, in New York.)