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, causing the contents of the cortical granules to be released outside the cell where they modify an existing extracellular matrix to make it impenetrable to sperm entry. 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. 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.