Sperm Act Like Heat-Seeking Missiles

Sperm seek out warm eggs, could aid infertility treatments

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Jan. 31, 2003 -- A sperm's mysterious journey through a woman's body in search of an egg to fertilize may be a lot like a heat-seeking missile locked onto a target. New research suggests that sperm are at least partially guided on their fertilization mission by heat given off by the egg's nest.

The place where the egg lies is slightly warmer than the place where mature sperm begin the final leg of their journey through the female reproductive system, according to the researchers of a new study in the February issue of Nature Medicine. Their study shows that sperm are attracted to the warmer site and therefore swim towards it.

A similar process is known to guide microorganisms and worms, but the researchers say this is the first study to provide evidence of it in mammals, such as humans. If further research confirms this finding, they say temperature guidance may be used to improve the effectiveness of in vitro fertilization and other infertility treatments.

Prior studies have shown that once sperm pass through the uterus into the fallopian tubes, they attach themselves to the tube's wall and pause to prepare for penetrating the egg. If ovulation has occurred within the preceding 24 hours, sperm that have completed this process detach from the wall and embark on the long journey to potential fertilization.

But what guides the sperm to the egg? The researchers say it seems to be a combination of chemistry and heat between the two players.

First, previous research shows that the egg releases a chemical signal that attracts mature sperm. But like a delicate perfume in a crowded room, that chemical signal from the egg can only travel a short distance and can't completely guide the sperm's journey through the fallopian tube.

That's where heat comes into play. The researchers found the place where the sperm come to a rest in the tube is about two degrees cooler than the fertilization site.

To see if sperm were attracted to the hot spot, the researchers built a laboratory model of the resting site, fertilization site, and the tube in between and tested the behavior of rabbit sperm in response to temperature changes in the various areas.

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They found that as little as a half-degree difference in temperature was enough to attract the sperm from their resting place. In addition, only the mature, fertilization-ready sperm were affected by the temperature difference.

"Apparently, the sperm are guided by temperature when they travel through most of the fallopian tube and navigate by tuning in to the egg's chemical call when they get close to the fertilization site," says researcher Michael Eisenbach, of the Weizmann Institute in Rehovot, Israel, in a news release.

SOURCE: Nature Medicine, February 2003 • News release, Weizmann Institute.

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