Molecular dissection of the mouse zona pellucida: an electron-microscopic perspective utilizing high-resolution colloidal-gold labeling methods.

The zona pellucida (ZP) is an extracellular coat that encloses growing oocytes, ovulated eggs, and preimplantation embryos in mammals. The ZP contains receptors that mediate initial interactions between the sperm and egg and the relative species-specificity during gamete interaction. It also prevents polyspermy and protects the developing embryo prior to implantation. The current model of the organization of mouse ZP depicts this extracellular matrix as an extensive three-dimensional array of long interconnected filaments of a structural repeat. Using high-resolution colloidal-gold labeling methods in combination with specific lectins and monoclonal antibodies against three major ZP glycoproteins, we have characterized the outer and inner mouse ZP during folliculogenesis and shown the modifications of the ZP after ovulation and fertilization. Our immunogold labeling results also indicate the involvement of a vesicular aggregate, a specialized subcellular compartment in the oocyte, in the synthesis and secretion of ZP glycoproteins.