Female sex hormone controls human sperm

In some relationships, the male takes the steering wheel. At cellular level, however, when the egg encounters a sperm cell, the female is the one at the controls. Scientists from the Centre of Advanced European Studies and research (CAESAR) in Bonn are now able to explain the mechanism by which the female sex hormone progesterone acts on human sperm.

The female hormone progesterone (green) affects the CatSper ion channels (red) in the tail of the sperm, which then allow calcium ions (yellow) to flow into the sperm. Thus, the CatSper channels and progesterone form an important part of the navigation system that leads the sperm to the egg. (© CAESAR)

Benjamin Kaupp and his team have reported in the current issue of the journal Nature that the hormone activates sperm-specific CatSper (cation channels of sperm) ion channels to control the swimming behaviour of the sperm.

The egg or oocyte provides a signpost for the sperm in the Fallopian tube. To ensure fertilisation, cumulus cells, which surround the egg, release the female sex hormone progesterone. Progesterone serves as a chemoattractant and points the sperm the way to the egg.

In addition to its attracting effect, progesterone also serves another purpose: in the immediate proximity of the egg, where its concentration is highest, it triggers hyperactivation of the sperm. Hyperactive sperm beat their tails forcefully like a whiplash, and appear to mobilise their last reserves of strength – like marathon runners in the home stretch.

This enables the sperm to penetrate the thick cloud of cumulus cells and the protective egg vestments. In human sperm, progesterone evokes an increase in the intracellular calcium concentration. These calcium signals alter the beating pattern of the sperm tail and thereby control the sperm's swimming path.

The scientists from the Research Center caesar in Bonn have now discovered that progesterone acts on CatSper calcium channels. These ion channels are found exclusively in the membrane of the sperm tail. Ion channels are pore-forming proteins in the cell membrane that act as floodgates for ions.