Contribution of zona proteins to oocyte growth.

The zona pellucida is an extracellular matrix surrounding growing oocytes, ovulated eggs and preimplantation embryos. In mice, the zona pellucida is composed of three glycoproteins designated as ZPA, ZPB and ZPC. ZpA and ZpC knockout mice are infertile due to incomplete folliculogenesis. In this article, the role of ZPA in the growth and survival of oocytes and the mechanism by which it is incorporated into zona structure is described. Oocyte-granulosa cell complexes (OGCs) were injected with short interfering RNAs (siRNAs) of mouse ZpA and cultured in alpha-MEM for 5 days. The survival rate of these oocytes and the diameter of surviving oocytes were significantly lower as compared to the control oocytes. The zona pellucida was also abnormal in morphology. These observations suggest that mouse ZPA protein contributes to oocyte growth as well as zona pellucida formation. In addition, a minigene construct carrying pig ZpA was transferred into mice to examine whether the gene is expressed and the protein product incorporated into the mouse zona pellucida. Pig ZPA was synthesized and secreted by the mouse oocytes but not used to assemble a chimeric zona pellucida. These results suggest that there are different requirements for zona assembly in pigs and mice.