Practice question

Zona pellucida is transparent glycoprotein envelope surrounding mammalian oocyte, ovulated complex and early embryo until blastocyst hatching. Synthesized by oocyte and follicle cells, it comprises ZP1-4 filaments forming porous elastic matrix that supports cumulus retention, mediates species-restricted sperm binding, triggers and completes acrosome reaction, and after cortical granule release hardens preventing polyspermy. By confining blastomeres it also prevents premature adhesion and extrauterine implantation. Hence zona functions as multifunctional gatekeeper guiding sperm entry, protecting early development, ensuring implantation timing, embryo transport coordination and monospermy enforcement.