Effects of cysteine in serum-free maturation medium on male pronuclear formation of maturing pig oocytes penetrated in vitro

Pig immature oocytes were cultured for 24, 36 and 48 h in serum-free maturation medium with or without 0.57 mM cysteine. The addition of cysteine to the medium was associated with increased glutathione synthesis by oocytes but did not promote nor inhibit nuclear maturation, sperm penetration in vitro, and decondensation of sperm nuclei in penetrated oocytes. The incidence of activation of penetrated oocytes 14 h after insemination in vitro was lower in those cultured both in the presence and absence of cysteine for 24 h than 36 and 48 h. The lower ability of oocytes cultured for 24 h to be activated was not improved by neither the addition of cysteine (0.57 mM) in fertilization medium nor prolonged culture time after insemination. However, male pronuclear formation in activated oocytes after sperm penetration at any stages of maturation was largely accelerated when cysteine was added to maturation medium. An increased concentration of glutathione may induce full decondensation of sperm nuclei in immature pig oocytes penetrated in vitro ensuring transformation of the decondensed sperm nuclei to male pronuclei only in synchronization with oocyte activation.