Final answer:
The zona pellucida prevents polyspermy, which is the fertilization of an oocyte by more than one sperm, by forming a hard barrier after the first sperm has fertilized the egg.
Step-by-step explanation:
The zona pellucida is a thick, gel-like glycoprotein membrane that surrounds the oocyte. It acts as a barrier that must be penetrated by sperm for fertilization to occur. This membrane plays a crucial role in preventing polyspermy, which is the fertilization of an oocyte by more than one sperm. Once a sperm penetrates the zona pellucida and fuses with the oocyte's plasma membrane, a sequence of events called the cortical reaction occurs. This reaction leads to the formation of a fertilization membrane, solidifying the zona pellucida and ensuring that no additional sperm can penetrate and fuse with the oocyte, preventing any further fertilization beyond the first successful sperm.
After fertilization, the resultant zygote undergoes several cellular divisions as it travels towards the uterus. The zona pellucida remains intact during these initial stages to protect the developing embryo. Eventually, the conceptus, now known as a blastocyst, hatches from the zona pellucida, allowing for implantation into the uterine wall to occur.