The fastest sperm doesn't always win. Instead, the researchers find out that the egg determines who is in the running. Because the chemical attractants in the egg appear to be more attractive to the sperm of certain partners.
Human eggs seem to be picky about sperm - and sometimes they choose not to have their partner's. This is the conclusion reached by a research team led by John Fitzpatrick from the University of Stockholm. According to their study, which was published in the journal Proceedings of the Royal Society B, the egg cells choose which sperm they want to attract.
With sex, the sperm begins a race with many sacrifices. Because only a few of the many millions of male sperm cells reach their destination - the egg cell. Which sperm wins the race depends, among other things, on their fitness, their timing and their swimming strategy, but also on subtle signals from the egg cell. Because it releases chemical attractants.
"According to the traditional view, the chemical signals of the egg cell only serve to show the sperm the way," explains study lead author Fitzpatrick. He and his team have now conducted an experiment to determine whether there is more to it than that. To do this, the scientists observed the reaction of the sperm from a good 30 men to the attractant-containing follicular fluid from various women. In one experiment, the sperm could choose between the scents of the egg cells of two women. In a second experiment, the team tested the attractiveness of the attractants individually in comparison to a neutral control.
The result: it is obviously by no means a coincidence how strongly a sperm reacts to the attractant of an egg cell. Instead, this depends heavily on which man and woman the gametes came from. "One woman's follicular fluid attracted one man's sperm more, while the other man's attractants had a stronger effect on another man's sperm," says Fitzpatrick. "This shows that the sperm-ovum interaction depends on the specific identities of the males and females involved."
The researchers also observed that the sperm preferred by the egg cell does not always have to be that of the partner chosen by the woman. According to the researcher, this shows active selection by the egg cell. The sperm have nothing to gain from preferring certain eggs, Fitzpatrick explains: They don't have a choice. On the other hand, it could be worthwhile for egg cells to be selective and to attract sperm that are particularly genetically compatible.
"The chemical communication between eggs and sperm allows women to make a 'cryptic choice' about which man they will fertilize," the researchers said. However: This selection is unconscious and cannot be controlled by the women - it takes place at the cell level.
If the existence of this cryptic selection is confirmed, it might explain some cases of involuntary childlessness. "The fact that the chemical attractants have different effects on sperm from different partners could be decisive for the success of fertilization," Fitzpatrick and his colleagues believe. However, the experiments did not examine whether the subsequent fertilization also worked better. Fitzpatrick and his team want to start there in future investigations. They hope to improve fertility treatment with further insights into the chemical interaction between egg cell and sperm.