Penetration of the egg' zona pellucida by the sperm cell is a process driven by hydrolytic enzymes.
The critical step in fertilization is binding between sperm receptors and glycoproteins on the zona pellucida. This is the trigger for the acrosome reaction which includes an influx of CA and a rise in pH and cAMP levels within the sperm head. As a result, the plasma membrane of the sperm fuses with the outer acrosomal membrane and the exocytosis of the acrosomal vesicle occurs. Components of the acrosome, like hydrolytic enzymes, are released and they degrade the zona pellucida allowing the sperm to move inside.
