Final answer:
The zona pellucida is a thick, gel-like glycoprotein membrane that surrounds the primary oocyte in the secondary follicle, critical for the fertilization process.
Step-by-step explanation:
In the secondary follicle, the zona pellucida is a translucent structure made of glycoproteins around the primary oocyte. This thick, gel-like membrane plays a crucial role in fertilization, as it must be penetrated by sperm for fertilization to occur. The secondary follicles contain a primary oocyte and multiple layers of granulosa cells, which, along with theca cells, are responsible for estrogen production. The zona pellucida is essential for the oocyte's interaction with sperm during the fertilization process.
Upon ovulation, when the secondary oocyte is expelled from the ovary, it is surrounded by several layers of granulosa cells forming the corona radiata, with the zona pellucida lying just outside the oocyte's plasma membrane. This structure ensures that once a sperm penetrates the zona pellucida and fertilization occurs, no other sperm can enter, which is known as the cortical reaction.