The mouse zona pellucida: folliculogenesis, fertility and pre-implantation development.

Perinatally, oocytes within the mouse ovary become surrounded by a layer of flattened granulosa cells and form primordial follicles. The subsequent accretion of the zona pellucida between the oocytes and granulosa cells provides a biochemical marker of folliculogenesis. In mice, the zona matrix is composed of three proteins (ZP1, ZP2, ZP3). Mouse lines lacking either ZP1 or ZP3 have been established and have abnormal folliculogenesis. Without ZP1, structurally defective zonae are formed resulting in decreased fecundity due to early embryonic loss. More strikingly, without ZP3, the zona matrix is absent, no 2-cell embryos are formed and females are infertile. The structural integrity of the zona matrix can be restored by substituting human homologues for the missing mouse protein and these 'humanized' zona matrices should prove useful in investigating the molecular basis of fertilization.