Eggs not silent partners in conception.

Eggs not silent partners in conception

A human egg cell does not idle languidly in the female reproductive tract, like some Sleeping Beauty waiting for a sperm Prince Charming to come along and awaken it for fertilization. Instead, new research indicates that most eggs actively beckon to would-be partners, releasing an as-yet-unidentified chemical to lure sperm cells.

A binational team of researchers, led by Michael Eisenbach of Israel's Weizmann Institute of Science in Rehovot and David L. Garbers from the University of Texas Southwest Medical Center in Dallas, have found a fluid in some women's ovaries that acts as a powerful chemical magnet for attracting sperm cells. The substance -- drawn from the ovarian follicles that nurture maturing eggs before they are released during ovulation -- has the potential for eventual use in treating some forms of infertility, the researchers suggest.

"If we succeed in isolating the attractive factor from the fluid ... this may be a way to enrich a sperm sample and select for sperm capable of fertilizing [a particular] egg," says Eisenbach. Anti-bodies to the purified factor might also serve as a contraceptive, he says.

"It's biologically plausible that there should be a chemical attraction between egg and sperm," says Robert Stillman, a reproductive endocrinologist at George Washington University in Washington, D.C. "Now a lot of work needs to go into isolating and characterizing [the attractant]."

Eisenbach and his colleagues set out to determine whether human eggs exude an attractive substance after previous studies done by others showed that female mammals can hoard dormant sperm, which later spring into action after the female ovulates. The researchers, who report their results in the April 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, collected fluid from the follicles of 40 women whose mature eggs were being removed for in vitro fertilization. Fluid from half of the women caused two to three times as many sperm to swim through a filter as did a plain salt solution. In another test, sperm also changed direction to swim toward a squirt of the fluid. Moreover, an egg from a follicle whose fluid attracted a sperm proved nearly twice as likely to be fertilized as one from a follicle whose fluid drew no sperm.

The fluid attracted only the most active sperm. Eisenbach says this finding could explain why only 200 to 300 of the roughly 280 million sperm contained in one ejaculate actually reach an ovulated egg.