Effects of aging on the zona pellucida surface of mouse oocytes

In a scanning microscopy study of mouse oocytes from immature, young, and aged females and of oocytes aged in vivo or in vitro, we have observed four types of zona pellucida, which we classify as types A, B, C, and D. Oocyte aging gives rise to a significant increase in predegenerative (type C) oocytes; this type of zona pellucida surface could result from a zona hardening effect and decrease the rate of fertilization of the oocytes affected.