How does an egg block other sperm after the first sperm enters?

To prevent polyspermy—the entry of multiple sperm into an egg—most animal eggs utilize specific defense mechanisms that trigger upon the arrival of the first spermatozoon.1

The method of defense varies by species:

• Sea Urchins: An immediate response involves the elevation of a vitelline membrane from the egg surface. Cortical granules fuse with the plasma membrane and release their contents into the perivitelline space, which thickens the vitelline membrane into a robust fertilization membrane. This process is accompanied by a change in the egg's electric charge, known as the fertilization potential.2
• Mammals: Defense often relies on the zona pellucida. Once a sperm begins penetrating this thick outer coat, a "zona reaction" occurs, preventing additional sperm from entering. In some mammals where this reaction is weak or absent, the defense mechanism instead resides directly within the egg surface.1

While some species are naturally polyspermic, only one sperm nucleus ultimately fuses with the egg nucleus to form a zygote, while the others degenerate.1