Effect of 2-mercaptoethanol and cysteine supplementation during in vitro maturation on the developmental competence of oocytes from hormone-stimulated lambs

The objective was to determine the effect of 2-mercaptoethanol and cysteine on in vitro developmental competence of oocytes from lambs (4-8-week old) stimulated with eCG and pFSH. Oocytes were matured in medium (TCM199) with no supplement (Control group) or with 100muM 2-mercaptoethanol and 600muM cysteine (GSH group). Oocytes from adult sheep were also included (Adult group). The addition of 2-mercaptoethanol and cysteine did not improve nuclear maturation or microtubule configuration 12, 15, 18, or 24h after placement in maturation medium. Sperm head decondensation and male pronucleus formation were evaluated at 6, 12, and 18h after commencement of IVF; sperm decondensation appeared earlier in the GSH group (6h after the start of IVF). There were differences (P<0.05) between the Control group and the GSH and Adult groups for: fertilization rate at both 12h (55.4, 77.0, and 80.6%, respectively) and 18h (67.9, 86.9, and 88.7%); parthenogenesis rate at both 12h (25.0, 10.8, and 5.6%) and 18h (28.3, 9.8, and 4.5%); and polyspermy rate at 18h (26.4, 4.9, and 5.7%). Blastocyst rate at 7d was higher in the GSH group than the Control group (23.9% vs. 14.9%, P<0.05), but both were lower (P<0.05) than the Adult group (38.3%). The addition of 2-mercaptoethanol and cysteine improved sperm decondensation and rates of fertilization and the blastocyst development to 7d, with no effect on blastocyst rate at 9d.