Three-dimensional structure of the zona pellucida at ovulation

The mammalian zona pellucida (ZP) is an extracellular matrix surrounding oocytes and early embryos, which is critical for normal fertilization and preimplantation development. It is made up of three/four glycoproteins arranged in a delicate filamentous matrix. Scanning electron microscopy (SEM) studies have shown that ZP has a porous, net-like structure and/or nearly smooth and compact aspect. In this study, the fine 3-D structure of the human and mouse ZP is reviewed with the aim to integrate ultrastructural and molecular data, considering that the mouse is still used as a good model for human fertilization. By conventional SEM observations, numerous evidences support that the spongy ZP appearance well correlates with mature oocytes. When observed through more sophisticated techniques at high resolution SEM, ZP showed a delicate meshwork of thin interconnected filaments, in a regular alternating pattern of wide and tight meshes. In mature oocytes, the wide meshes correspond to "pores" of the "spongy" ZP, whereas the tight meshes correspond to the compact parts of the ZP surrounding the pores. In conclusion, the traditional "spongy" or "compact" appearance of the ZP at conventional SEM appears to be only the consequence of a prevalence of different arrangements of microfilament networks, according to the maturation stage of the oocyte, and in agreement with the modern supramolecular model of the ZP at the basis of egg-sperm recognition. Despite great differences in molecular characterization of ZP glycoproteins between human and mouse ZP, there are no differences in the 3-D organization of glycoproteic microfilaments in these species.