Scientists have made a breakthrough that could be used to design a new class of contraceptive pills.

A sperm's path to an egg is more a deadly obstacle course than a track sprint. The one ejaculated sperm cell in a million that is lucky enough to reach the fallopian tubes, where eggs await fertilization, must conquer thick, gelatinous layers of mucus and cells surrounding the egg to reach its prize.

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Fortunately for the sperm, there is help. Two studies have shown how sperm sense progesterone, a female sex hormone that has been released by cells surrounding the egg. The hormone may guide the sperm towards the egg as well as giving it a final push to get there, the research suggests. The findings could be used to design a new class of contraceptive drug.

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The latest studies, led by independent teams in Germany and the United States, have shown that progesterone activates a molecular channel called CatSper, which floods sperm cells with calcium.

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Mice without the channel are infertile, as are some men with mutations in the genes that make it, says Polina Lishko, a reproductive biologist at the University of California, San Francisco, who led one of the studies2. Sperm that don't make CatSper cannot become hyperactive.

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Lishko and Kirichok's team developed a way of measuring the electrical currents within sperm that are created by ions like calcium, similar to how neuroscientists record the electrical activity of neurons.

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They found that adding progesterone to ejaculated human sperm boosts the current, and that treating sperm with drugs that block CatSper reduces it. Putting the cells into high-pH environments, like those found around the egg, also activated CatSper. A combination of high pH and high progesterone had an even stronger effect.

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A second team, led by Benjamin Kaupp, a biophysicist at the Center of Advanced European Studies and Research in Bonn, Germany, came to the same conclusion in their own experiments1.

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"The consequence for humans is that if you could block CatSper it would be an ideal contraceptive," the Nature quoted David Clapham, a biochemist at Children's Hospital Boston in Massachusetts, whose team discovered the channel and is looking for drugs that inhibit it, as saying.