Influence of mouse defective zona pellucida in folliculogenesis on apoptosis of granulosa cells and developmental competence of oocytes†

Zona pellucida (ZP), which enwraps the oocyte during folliculogenesis, initially forms in the primary follicle and plays an important role in female fertility. Here, we investigated a mouse strain ("mutant mice" for short) carrying two types of ZP defects in folliculogenesis, i.e., ZP thinned (but intact) and ZP cracked, caused by targeted mutation in the Zp1 gene. Using this mutant mouse strain and wild-type mouse as control, we studied the effects of the ZP defects on the development of oocytes and granulosa cells during folliculogenesis. For each ZP defect, we examined the morphology of transzonal projections and apoptosis of granulosa cells in the corresponding growing follicles, as well as the morphology of corresponding ovulated eggs and their abilities to develop into viable individuals. Our results suggested that ZP integrity rather than thickness or porosity is crucial for preventing the ectopia of granulosa cells, maintaining adequate routine bilateral signaling between oocyte and surrounding granulosa cells, and thus for ensuring the survival of granulosa cells and the establishment of the full developmental competence of oocytes. This is the first study to elucidate the effects of different degrees of ZP defects caused by the same gene mutation, on the apoptosis of granulosa cells and developmental competence of oocytes, and to explore the potential mechanisms underlying these effects.