Effects of cumulus cells on the ability of pig oocytes to utilize cysteine or cystine during maturation in vitro

The present study examined the effect of cysteine and cystine associated with cumulus cells in maturation medium on oocyte glutathione (GSH) concentration at the end of maturation culture (for 48 h) and male pronuclear (MPN) formation following in vitro fertilization of pig oocytes. When cumulus-enclosed oocytes were cultured in a serum-free medium containing 0.57 mM cysteine or cystine, their GSH concentration and incidence of MPN formation were higher than when they were cultured without cysteine and cystine. Supplementation with cysteine during the final 24 h of culture resulted in an increased GSH concentration of oocytes and a higher incidence of MPN formation, regardless of the presence of cumulus cells, than in those cultured without cysteine or cystine. The same high incidence of MPN formation was observed even when oocytes were denuded after 36 h of culture and exposed to cysteine for only 12 h. In contrast, when oocytes were denuded after 24 h of culture and exposed to cystine, neither GSH synthesis nor MPN formation was improved, but both parameters did improve when oocytes were not denuded. Exposure of cumulus-enclosed oocytes to cystine from 36 h of culture did not promote MPN formation. These results indicate that cumulus-free pig oocytes can synthesize GSH in the presence of cysteine and that the presence of cumulus cells is essential for maintaining a high concentration of GSH in oocytes in the presence of cystine.