Constructing the mammalian egg zona pellucida: some new pieces of an old puzzle.

Until now, there has been relatively little information available about how the mammalian egg extracellular coat, the zona pellucida (ZP), assembles during oogenesis. Recently, two experimental approaches were taken, in part, to address this question in mice. In one case, antisense oligonucleotides directed against ZP glycoprotein mRNAs were injected into mouse oocytes to prevent synthesis of the glycoproteins (Tong et al., 1995). In the other case, the mZP3 gene was disrupted by targeted mutagenesis using homologous recombination in mouse embryonic stem (ES) cells and null female mice (mZP3−/−) were produced (Liu et al., 1995, 1996). The outcome of these experiments is described here. It is consistent with the current view of ZP structure, provides some new insight into ZP assembly, and suggests further avenues of research on the ZP.