Zona pellucida

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Main organs of the female reproductive system.

The zona pellucida (ZP) is an extracellular matrix layer that forms between the cell membrane of the oocyte and the first layer of granulosa cells during primary ovarian follicle development.

This layer measures about 5 to 10 μm in thickness and is composed of four main glycoproteins: ZP1, ZP2, ZP3 and ZP4. These glycoproteins, produced by the oocyte, form an irregular meshwork structure.

The filopodia (cytoplasmic processes) of granulosa cells in contact with the zona pellucida, penetrate this layer and interdigitate with microvilli of the oocyte, establishing a pathway for communication between the cells via gap junctions.

The zona pellucida plays important roles in sperm binding and fertilization. ZP3 and ZP4 serve as primary sperm receptors within the zona pellucida. These receptors bind to specific proteins on the sperm's surface, triggering a process called acrosomal reaction. This reaction enables the sperm to release hydrolytic enzymes, which then degrade the zona pellucida, allowing access to the oocyte.

Once the first sperm fuses with the oocyte plasma membrane, a process called cortical reaction ensues, resulting in the release of enzymes (proteases) from the cortical granules. These enzymes induce biochemical changes in the zona pellucida, rendering it resistant to further sperm binding and impermeable to additional sperm in a process termed the zona reaction. This mechanism effectively prevents polyspermy, ensuring that an oocyte is fertilized by a single sperm.