How do cortical granules change an egg’s surface after fertilization?

Upon fertilization, cortical granules play a critical role in transforming the egg's surface to prevent further sperm entry and support early development. When a spermatozoon contacts the egg, the membranes of these granules fuse with the egg’s plasma membrane, extruding their contents into the perivitelline space—the area between the egg surface and the vitelline membrane.1

This process results in two primary structural changes:

• Formation of the Fertilization Membrane: Part of the extruded granule contents merges with the vitelline membrane, causing it to thicken and develop a robust molecular structure known as the fertilization membrane.1
• Creation of the Hyaline Layer: Other materials from the granules cover the egg surface to form a transparent hyaline layer. This layer is essential for holding together the blastomeres (cells) that form during the subsequent cleavage of the egg.1

Additionally, the egg's plasma membrane becomes a mosaic structure, incorporating patches of the original membrane and areas derived from the cortical granule membranes.1