During fertilization, the ovum guides the sperm to it, but not all of them react to the chemical signals that reach them. In the end, the ovum “chooses” the sperm most receptive to the chemoattractant molecules it secretes.

In the wild, the courtship displays of certain species are treasures of inventiveness which allow females to choose the male whose genetic heritage will give the best offspring. How females choose is a closely guarded secret.

But for mammals, this “female selection” also takes place at a cellular level. Indeed, the ova, through chemoattractant molecules, select the best sperm. This phenomenon has been described in mice or cetaceans but remains relatively unknown in humans, although we already know that the molecules secreted by the ovum guide the sperm to it.

A study published in Proceeding of the Royal Society B suggests that human ova also choose from sperm that present themselves to them by the same mechanism. And sometimes, those that respond best to chemical molecules do not necessarily belong to the woman’s love partner.

The egg attracts sperm

This experiment was carried out on couples who are followed for an in vitro fertilization. Scientists have recovered the fluid that surrounds the egg in the follicles and into which it pours its chemoattractant molecules, as well as the participants’ sperm. Then they observed the behavior of spermatozoa from several donors when faced with different follicular fluids.

The results obtained suggest that sperm respond differently to each follicular fluid. The eggs, thanks to the molecules they secrete around them, therefore attract sperm belonging to a specific man.

“The follicular fluid of a given woman was better for attracting the sperm of a given man, when the follicular fluid of another woman was better to attract the sperm of another man,” explains Professor Fitzpatrick of the Stockholm University and first author of the study. But the sperm of the man preferred by the ovum does not necessarily belong to his loving companion.

Chemistry rather than love

Indeed, scientists tested the response of sperm to follicular fluid from a couple or two strangers. Conclusion, the sperm of a given man do not accumulate more around the follicular fluid of his partner. Love therefore does not translate into a better affinity of sperm for the ovum.

And for good reason, sperm have only one goal: to swim to the egg to fertilize it. He therefore has no interest in being difficult, according to Professor Fitzpatrick. On the other hand, the ovum has every reason to choose carefully the sperm which will fertilize it for its genetic characteristics or for its affinity for the chemoattractants it secretes.

Thus, the cause of fertility problems could not be, among other things, too slow or too few sperms or a defect in ovulation, but a lack of biochemical compatibility between the ovum and the sperms.

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