Molecular changes in the human zona pellucida associated with fertilization and human sperm-zona interactions

The zona pellucida (ZP) is a specialized extracellular matrix surrounding the mature oocyte. Specific functions ascribed to the ZP include sperm-binding, induction of the acrosome reaction, and involvement in the "slow block" to polyspermy. We have examined the ZP from a variety of human eggs recovered from follicular aspirates. Mechanically isolated ZP were heat-solubilized, iodinated, and characterized by SDS-PAGE. Under nonreducing conditions, the human ZP is characterized by two species of 92-120K (ZP1,2) and 57-73K (ZP3). After reduction, ZP1,2 separates into two components of 97K (ZP1) and 82K (ZP2), with ZP3 at 70K. Under reducing conditions, ZP specifically isolated from fertilized eggs reveals approximately equal proportions of all three iodinated ZP molecules. However, ZP specifically isolated from fertilized eggs reveals a substantial disappearance of ZP1. The ZP from unfertilized eggs does not display this modification of ZP1, even when solubilized zonae are co-incubated with sperm or sperm lysates. We suggest that this molecular change in ZP1 is a direct result of the egg cortical reaction. This demonstrates, for the first time, structural changes in the human ZP directly associated with fertilization. In additional studies, we have demonstrated that capacitated human sperm can be saturated with heat-solubilized ZP, demonstrating the presence of human ZP-binding proteins, and that the binding characteristics of zonae isolated from nonfertilized and fertilized eggs differ. Human sperm proteins having an affinity for solubilized zonae have also been identified at 16K, 18K, 19K, 35K, and 60K.