Cumulus-enclosed or cumulus-free pig oocytes were cultured for 48 h in a serum-free maturation medium, TCM-199B containing 1 mg/ml polyvinylalcohol, supplemented with gonadotropins (PMSG + hCG) and/or epidermal growth factor (EGF), and inseminated with frozen-thawed ejaculated spermatozoa. Fourteen hours after insemination, the ability of oocytes to form male pronucleus (MPN) was examined. EGF significantly increased the ability of cumulus-enclosed oocytes to form MPN irrespective of the concentrations (1–100 ng/ml) tested. Gonadotropins had a greater effect than EGF on MPN formation in cumulus-enclosed oocytes, and a synergetic effect was observed when EGF and gonadotropins were added together. However, neither EGF nor gonadotropins exhibited a stimulatory effect on MPN formation in cumulus-free oocytes. These results indicate that the role(s) of both EGF and gonadotropins in stimulating cytoplasmic maturation of pig oocytes is mediated by cumulus.
