How do egg membranes protect animal eggs?

Egg membranes and envelopes serve several critical protective functions for animal eggs, primarily focusing on fertilization control and environmental defense.

Prevention of Polyspermy

A vital role of the egg surface and its associated membranes is the prevention of polyspermy, ensuring that only one spermatozoon successfully fuses with the egg nucleus. In many species, this is achieved through a reaction triggered upon the entry of the first sperm:12
* Zona Reaction: In some mammals, the zona pellucida undergoes a change once a sperm begins penetration, effectively blocking additional spermatozoa from entering.1
* Surface Modification: In other animals, the egg surface itself undergoes a molecular rearrangement that prevents further penetration.1

Environmental Protection

Egg membranes also provide physical and chemical protection against external hazards:
* Drying and Damage: For terrestrial animals, membranes like the chorion or calcified shells are essential for preventing desiccation. In higher vertebrates, the amnion provides a fluid-filled environment that protects the embryo from physical jolts and prevents adhesion to the shell.34
* Trapping and Interaction: Gelatinous coats surrounding the eggs of many animals, such as echinoderms and amphibians, help trap spermatozoa, increasing the likelihood of successful interaction. These coats also play a role in eliciting the acrosome reaction in sperm, which is necessary for fertilization.25