The zona pellucida: using molecular genetics to study the mammalian egg coat

An extracellular matrix that mediates critical steps in fertilization and e arly development surrounds all vertebrate eggs. In mice and humans, this ma trix is known as the zona pellucida and comprises three glycoproteins: ZP1, ZP2 and ZP3. Homologues of these proteins isolated from other vertebrates have conserved protein motifs that may be important for establishing a comm on fibrillar structure. However, specific but contradictory biological role s have been assigned to individual egg coat proteins based on assays in vit ro in a wide range of species. Mouse lines lacking either ZP1 or ZP3 have b een established with abnormal or absent zona matrices and varying degrees o f infertility to examine zona structure and function in vivo. By crossing m ouse lines lacking individual zona proteins with those expressing human hom ologues, the structural integrity of the zona matrix can be restored. Becau se mouse and human spermatozoa exhibit order-specific binding to the zona p ellucida, mice with 'humanized' chimaeric zonae may provide an experimental system to elucidate the molecular basis of sperm-zona interaction.