Q.What role does the zona pellucida of the ovum play at the time of fertilisation?

[!TLDR]

The zona pellucida is digested by the acrosomal reaction to let a sperm reach the oocyte, and, once hardened by the cortical reaction after fusion, blocks any further sperm -- preventing polyspermy. …

The zona pellucida is a thick glycoprotein coat surrounding the oocyte. At fertilisation, it is first a barrier that a capacitated sperm must digest through, via enzymes released in the acrosomal reaction, to reach the oocyte's plasma membrane. Once the first sperm fuses with that membrane, the resulting cortical reaction releases cortical granule contents that chemically alter and harden the zona pellucida, so it can no longer be penetrated by any other sperm still attached to it. The zona pellucida therefore plays a dual role: an initial barrier that must be overcome for fertilisation to happen …

Describe the zona pellucida's role at two different moments -- before fusion (a barrier to be digested) and after fusion (hardened to block further …

Describing the zona pellucida only as a passive barrier without recognising that its post-fertilisation hardening (the cortical re …