Schematic representation of the composition of the ZP in various mammals: The ZP matrix of the mammalian oocyte is composed of either three or four glycoproteins. The mouse ZP matrix is composed of three glycoproteins, namely, ZP1 (blue), ZP2 (brown) and ZP3 (green). The rat, hamster, bonnet monkey and human ZP matrices are composed of four glycoproteins: ZP1, ZP2, ZP3 and ZP4 (red). The bovine, porcine and canine ZP matrices contain three glycoproteins, with ZP4 replacing ZP1. ZP, zona pellucida.