In the slow block to polyspermy, sperm penetration releases an inflow of Ca2+, which in turn stimulates the cortical reaction.
A mechanism known as the cortical reaction, which is started after fertilization, stops polyspermy, or the union of several sperm with one egg. The cortical reaction occurs in two stages: the creation of the jelly layer and the dissolution of the cortical crypt, followed by the modification of the vitelline envelope into the thick fertilization envelope by the cortical vesicle reaction. The zona pellucida becomes resistant to sperm attachment and penetration after sperm and egg fusion because the contents of cortical granules (CG), a type of specific organelle in the egg, leak into the perivitelline area (cortical response).
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