The Zona Pellucida Facilitates Fertilization, Blocks Polyspermy and Protects Pre-Implantation Embryos

The zona pellucida is an extracellular matrix that surrounds eggs and early embryos. Composed of 3–4 proteins depending on the mammalian species, the zona mediates initial gamete recognition in the oviduct, provides a potent postfertilization block to polyspermy and protects the embryo as it passes through the oviduct prior to implantation. The use of molecular biology, mouse genetics and sophisticated imaging provides insight into the role of individual proteins in these processes. Future progress will depend on identification of the cognate sperm cell surface proteins and high resolution structural investigations to better understand molecular mechanisms of mammalian fertilization.