The secretions of the cumulus-oocyte complex in relation to fertilization and early mouse embryonic development: a histochemical study.

A study has been made of the histochemical composition of the murine cumulus-oocyte complex and zona pellucida following treatment of immature females with exogenous gonadotrophins. Selected developmental stages were studied in detail, namely the ovulated and unfertilized egg, the fertilized oocyte and the preimplantation embryo. In addition, the histochemical features observed in normal fertilized embryos have been compared with those of haploid and diploid parthenogenetic embryos at comparable stages following activation. Shortly after fertilization, glycosaminoglycans, which form a major component of the extracellular matrix surrounding the cumulus cells, become incorporated into the zona pellucida of the fertilized egg. In oocytes with few or no attendant cumulus cells, there appeared to be a diminished uptake of glycosaminoglycans and a reduced intensity of the zona staining reaction to Alcian Blue. In these oocytes, uptake of glycosaminoglycans appeared to be from the secretions lining the oviduct. There was little incorporation of the glycosaminoglycans from the extra-cellular matrix of the surrounding cumulus cells into the zona pellucida in unfertilized or parthenogenetic eggs despite the activation stimulus. After fertilization or activation, the zona pellucida became increasingly PAS-positive. Enzymic studies clearly indicate that the composition of the zona pellucida of the early embryo is histochemically different from the zona that surrounds the oocyte in the preovulatory follicle. These findings are discussed in relation to the decreased viability of embryos from oocytes which have been ovulated.