Because if two spermatozoa managed to enter and merge with the oocyte, a phenomenon called polyspermia, the presence of two male genetic materials – and therefore of 23 excess chromosomes – would stop the development of the embryo prematurely!
Several protective mechanisms therefore exist in order to prevent the ovum from being assailed by spermatozoa, once a first fertilization has been initiated. The best known concerns the zona pellucida, that is, the envelope made up of glycoproteins which protects the oocyte from the outside, and which the sperm must first pass through.
When the membranes of the two gametes merge, the female cell releases the contents of small vesicles, called cortical granules, which modify the glycoproteins in the zona pellucida.
A 2nd mechanism
The latter then becomes rigid and impermeable to late sperm. “This phenomenon is not instantaneous: several male gametes can therefore cross the envelope and end up directly in contact with the oocyte, pointe Christine Gourier, CNRS researcher and fertilization specialist at the Physics laboratory of the École normale supérieure in Paris.
Except that polyspermia remains very rare. There is therefore necessarily a second mechanism preventing them from fusing with the egg cell. “
This time, the membrane of the ovum would be directly involved. The process remains mysterious, but according to one hypothesis, two proteins present on this membrane, and essential for fusion, would be released after fertilization. “Their release makes them less present on the egg’s membrane, which could limit the egg’s ability to fuse.” , supposes the specialist.
According to Science & Vie n ° 1252
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