At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize. The sperm must undergocapacitationin the female's reproductive tract over several hours, which increases its motility and destabilizes its membrane, preparing it for theacrosome reaction (the enzyme releasing process), the enzymatic penetration of the egg's tough membrane, thezona pellucida, which surrounds the oocyte.
After binding to the corona radiata the sperm reaches thezona pellucida, which is an extra-cellular matrix of glycoproteins.This binding triggers the acrosome to burst, releasing enzymes like hyaluronidase and acrosome that help the sperm get through the zona pellucida.
Once the sperm cells find their way past the zona pellucida, thecortical reactionoccurs. Cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco-proteins in the zona pellucida to cross-link with each other making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm. The cortical reaction and acrosome reaction are both essential to ensure that only one sperm will fertilize an egg.