How do sea urchin eggs form a fertilization membrane?

In sea urchin eggs, the formation of the fertilization membrane is a rapid response to fertilization that occurs within approximately one minute.1

When a spermatozoon attaches to the egg surface, the thin vitelline membrane detaches from the egg surface. Simultaneously, cortical granules—specialized components located beneath the plasma membrane—fuse with the egg's plasma membrane and extrude their contents into the perivitelline space.12

The fertilization membrane is formed as these extruded granule materials merge with the detached vitelline membrane, causing it to thicken and develop a structured molecular organization. If this fusion process is inhibited, the membrane remains thin and soft. Additionally, other materials from the cortical granules form a transparent hyaline layer over the egg surface, which helps hold together the cells produced during cleavage. These events are accompanied by changes in the egg's electrical potential and an increase in membrane permeability.1