Structure of the Mouse Egg Extracellular Coat, the Zona Pellucida

This chapter describes zona pellucida (ZP), which is a thick extracellular coat that surrounds the plasma membrane of all mammalian eggs. It highlights mouse egg ZP structure. Either three or four glycoproteins constitute the ZP of eggs from a wide variety of mammals. ZP is well organized with primary and secondary sperm receptors (two different ZP glycoproteins), arranged periodically along ZP filaments. During the process of fertilization in mammals, sperm must bind to and penetrate through the ZP to reach and fuse with egg plasma membrane. The ZP is responsible for both species specificity of sperm binding and establishment of a slow block to polyspermy following gamete fusion. The mouse egg ZP is a relatively thick translucent extracellular coat that contains ∼3–4 ng of protein and is extremely porous, permitting relatively large macromolecules and even small viruses to pass through it. The unfertilized mouse egg ZP is composed of three glycoproteins: ZP1, ZP2, and ZP3. These glycoproteins appear either as very broad bands following one dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE), or as a very complex set of spots following high resolution two-dimensional SDS–PAGE.