Zona Pellucida

The zona pellucida (ZP) is a glycoprotein layer surrounding the oocyte. Sperm must bind to and penetrate the zona for fertilization — a process that requires acrosome reaction, whereby the sperm releases enzymes to dissolve the ZP. ICSI bypasses this step entirely by direct injection into the egg.

As an embryo develops, it must "hatch" out of the zona before it can implant in the uterine lining. Assisted hatching — using a laser or acid to create a small hole in the zona — was historically used to facilitate this process, but evidence that it improves outcomes in unselected patients is limited. The zona also prevents polyspermy (fertilization by more than one sperm) after the first sperm penetrates.