Come and get me (Image: Fotex/Rex Features)

WHEN a human egg is ready to be fertilised, it releases a chemical that signals “come hither” to nearby sperm. Now we know how this signal whips sperm into shape, which might make it possible to develop non-hormonal contraceptives that turn the signal off.

Although biologists have known for decades that egg cells provide sperm with a little chemical encouragement to reel them in, the molecular nature of this interaction has remained elusive.

To investigate, Polina Lishko at the University of California, San Francisco, and colleagues refined a technique to measure the electrical currents that drive the sinuous movements of a sperm’s tail. Lishko’s team found that when the sperm get a boost of progesterone – a hormone released by follicular cells surrounding the egg – the electric current increases in strength and their tails move faster.


It also turns out that progesterone binds to an ion channel on the sperm cell called CatSper, and that this causes an influx of calcium ions to propel the sperm forward (Nature, DOI: 10.1038/nature09767).

“This is one of the first times that people have figured out at the molecular level how an egg signals to a sperm,” says Dejian Ren, a physiologist at the University of Pennsylvania in Philadelphia.

The discovery offers an opportunity to create non-hormonal birth control in the form of a drug that prevents progesterone from binding to CatSper, effectively preventing the egg wooing the sperm. Current hormonal contraceptives may increase the risk of certain cancers and cardiovascular disease.

“We’ve finally solved the question of what progesterone does to human sperm,” Lishko says. “Now we need to find the exact binding site on CatSper to move forward with drug therapy.”