A Bespoke Coat for Eggs: Getting Ready for Fertilization.

All eggs have an extracellular coat (EC) that plays unique roles during reproduction and development. ECs are designed to protect eggs and support their growth, regulate fertilization of eggs, and protect early embryos. ECs of mammalian and nonmammalian eggs consist of only a few proteins that are closely related to one another. All these proteins possess regulatory elements and a structural domain responsible for processing and assembly of the proteins into ECs. They also possess regions responsible for their functional roles during and after fertilization. Our essay addresses these and other aspects of EC biology.