This modification of the zona pellucida is known as the zona reaction. In mammals, however, the extracellular matrix modified by the cortical reaction is the zona pellucida. In most animals, the extracellular matrix present around the egg is the vitelline envelope which becomes the fertilization membrane following the cortical reaction. The trigger for the cortical granules to exocytose is the release of calcium ions from cortical smooth endoplasmic reticulum in response to sperm binding to the egg.
The cortical granules contain proteases that clip perivitelline tether proteins, peroxidases that harden the vitelline envelope, and glycosaminoglycans that attract water into the perivitelline space, causing it to expand and form the hyaline layer. This releases the contents of the cortical granules outside the cell, where they modify an existing extracellular matrix to make it impenetrable to sperm entry. To create this barrier, cortical granules, specialized secretory vesicles located within the egg's cortex (the region directly below the plasma membrane), are fused with the egg's plasma membrane. In contrast to the fast block of polyspermy which immediately but temporarily blocks additional sperm from fertilizing the egg, the cortical reaction gradually establishes a permanent barrier to sperm entry and functions as the main part of the slow block of polyspermy in many animals.
The cortical reaction is a process initiated during fertilization that prevents polyspermy, the fusion of multiple sperm with one egg.
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