Because if two sperms manage to enter and fuse with the egg, a phenomenon called polyspermia, having two male genetic material – and thus 23 extra chromosomes – will stop the fetus developing prematurely!
Therefore several protective mechanisms exist in order to prevent the sperm attacking the egg, once the first fertilization has begun. The most that is known concerns the zona pellucida, that is, the envelope made up of glycoproteins that protect the egg from the outside, and through which the sperm must first pass.
When the two gamete membranes fuse, the female cell releases the contents of small vesicles, called cortical granules, which modify glycoproteins in the zona pellucida.
Then the latter becomes rigid and impermeable to the late spermatozoa. This phenomenon is not immediate: so many male gametes can cross the envelope and end up in direct contact with the egg, Pointe Christine Jurier is a researcher at the National Center for Scientific Research and a fertility specialist in the Laboratory of Physics at the Ecole Normale Supérieure in Paris.
However, polyspermia remains very rare. So there is necessarily a second mechanism that prevents them from merging with the egg cell. “
This time, the membrane of the egg will be directly involved. The process remains a mystery, but according to one hypothesis, two proteins located on this membrane, which are necessary for fusion, will be released after fertilization. “Release it makes it less present on the egg membrane, which may limit the egg’s ability to fuse.” , the specialist assumes.
According to Science & Vie No. 1252
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