Follow the heat. New research suggests that sperm like to go where it's warm.

Sperm Like It Hot

Like men drawn instinctively to the most sizzling lady in the bar, sperm home in on the heat of an expectant egg. For the first time, researchers have found a temperature gradient in the female reproductive tract that appears to guide sperm to their target. The result might suggest a way to improve in vitro fertilization.

After sperm splash down in the uterus, they swim like mad. If they're in luck, an egg is making its way nonchalantly down the fallopian tube. Just as they enter the tube at the uterus's top, the sperm stop and burrow into the wall, where they mature one last bit before resuming their quest. The flirtatious egg exudes signals to draw sperm closer. However, like perfume, the egg's chemical signals only work at close range. Scientists have wondered how sperm zero in on an egg that lies centimeters up the fallopian tube. In rabbits, the entrance to the tube is cooler than regions farther along. Michael Eisenbach at the Weizmann Institute of Science in Israel and colleagues investigated whether the "hot" makes sperm want to trot.

The team mocked-up a fallopian tube by connecting two small receptacles with a bridge. One receptacle brewed sperm cultures at the sperm-resting-spot temperature of 37°C, and the other ranged from half a degree to 2°C higher. The team then measured how eagerly rabbit sperm headed toward the warmth. They found about 60% of mature sperm swam in the general direction of the gradient, even when the temperature was only half a degree higher. Tests with human sperm showed that they also react to a temperature gradient, the researchers report in the February issue of Nature Medicine.

"The thing we've been missing is a long-range [detection system]," says cell biologist Harvey Florman of the University of Massachusetts Medical School, Worcester. "Maybe this is it." He says the result is "very exciting" and brings up many questions, such as how the temperature gradient is set up in the first place. Higher temperature, Florman speculates, may turn out to be a requirement for sperm maturation. If so, in vitro fertilization teams might improve the chances of babymaking if they incubate sperm at a higher temperature than they currently do.

Related sites
Michael Eisenbach's home page
Harvey Florman's home page