“In some relationships, the male takes the steering wheel,” scientists from the Centre of Advanced European Studies and Research (caesar) wrote in a statement this week. “At a cellular level, however, when the egg encounters sperm, the female in the one at the controls.”
The egg uses the hormone progesterone to control sperm swimming behaviour by activating ion channels called “CatSper,” or cation channels of sperm, according to study results published by Bonn-based study leader Benjamin Kaupp.
“The egg or oocyte provides a signpost for the sperm in the Fallopian tube,” the team of researchers at the neuroscience centre connected with the Max Planck Institute said.
Cells surrounding the egg release progesterone as a “chemoattractant” that “points the sperm the way to the egg,” they explained.
The hormone also triggers a last-gasp jolt of hyperactivity in the sperm when they are close to the egg, where progesterone is highly concentrated, the scientists found.
“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,” they said.
This final burst of energy helps the sperm penetrate the egg.
The process occurs because progesterone encourages intracellular calcium concentration, functioning as a signal which changes the beating pattern of the sperm tail, they said.
The discovery solves a mystery long studied by scientists – how progesterone triggered the calcium signals in sperm.
While researchers knew that the CatSper channels controlled sperm swimming behaviour, the fact that they act as a receptor to progesterone is “astonishing,” the statement said.
“It was only with the help of state-of-the-art electrophysiological and optical technologies that we succeeded in resolving the 30-year-old mystery of the progesterone action,” researcher Timo Strünker said.
The researchers believe that in the distant future the discovery could lead to new forms of contraceptives with fewer side effects that work to cut off the interaction between progesterone and CatSper.