Egg cells send out chemical signals to entice sperm, suggests study

Progesterone released from an egg may help guide sperm towards it and assist sperm to penetrate the egg's protective layers, according two studies published in Nature. The discovery could potentially help in the development of non-hormonal contraceptives for men and women, while also explaining some cases of infertility.

For more than 20 years, scientists have known the sex hormone progesterone, released from ovulated eggs, stimulates the flow of calcium ions in sperm. This molecular interaction helps sperm identify and navigate towards the egg, as well as helping sperm break through the egg's protective covering so fertilisation can occur. However, scientists were unsure about the exact nature of this chemical relationship.

Two studies, led by separate teams in Germany and the United States, have moved closer to solving this problem after finding the CatSper, a calcium ion channel found in the sperm's tail, is triggered by progesterone.

Dr Polina Lishko of the University of California, San Francisco, and her team measured the electrical currents that drive a sperm's tail and observed that an injection of progesterone produced a stronger current and made sperm tails move faster.

Researchers discovered that placing the cells in an environment with a high pH, such as the conditions found around the egg, also triggered the CatSper.

'By studying the CatSper channel electrophysically in solution from all other ion channels or transporters, we were able to prove that the CatSper channel is activated by progesterone and is the long-sought elusive progesterone receptor of human sperm', said Dr Lishko.

The second team, led by Professor Benjamin Kaupp of the Center for the Advanced European Studies and Research in Bonn, also measured calcium levels within human sperm. Like Dr Lishko's group, administering progesterone produced almost instant results.

The discovery could potentially bring about non-hormonal birth control, which would prevent progesterone binding to CatSper and stop the egg from guiding the sperm. Such a contraceptive may have minimal side-effects as it will target a molecule only found in sperm. Increased links to certain cancers, blood clots and cardiovascular disease have all been linked to current hormone-based contraceptives.

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