Presence of beta-mercaptoethanol can increase the glutathione content of pig oocytes matured in vitro and the rate of blastocyst development after in vitro fertilization

The present study examined the effect of beta-mercaptoethanol (BME) during in vitro maturation (IVM) of pig oocytes on in vitro fertilization (IVF) parameters, intracellular glutathione (GSH) concentration, subsequent embryo development and blastocyst cell numbers. Cumulus-oocyte complexes were cultured in North Carolina State University (NCSU)-23 medium containing porcine follicullar fluid, cysteine, hormonal supplements and 0 to 50 micromolar BME for 20 to 22 h. They were then cultured in the same medium but without hormonal supplements for an additional 20 to 22 h. After culture, cumulus-free oocytes were coincubated with frozen-thawed spermatozoa for 5 to 6 h. Putative embryos were transferred to NCSU-23 containing 0.4% BSA and cultured for 144 h (Experiment 1). In comparisons between the presence or absence of BME, no differences were observed in fertilization parameters. At 48 h, no mean differences were found in cleavage rates. However, at 144 h, compared with no addition (26%), the presence of 12.5 and 25 micromolar BME increased (P<0.05) the proportion of blastocysts in a in a dose-dependent manner (34 and 41%). Further increase from 25 to 50 micromolar BME reduced (P<0.05) the blastocyst development rate. Blastocysts derived from oocytes matured with 25 micromolar BME had the highest (P<0.05) trophectoderm (TE) and total cell numbers. No difference was found in inner cell mass (ICM) cells among treatments. In Experiment 2, after IVM, oocytes were fixed to analyze the GSH concentration. Compared to no addition, a higher (P<0.01) level of GSH was found in oocytes matured with 25 micromolar BME. Compared with 25 micromolar BME, GSH was low (P<0.05) at 50 micromolar BME. The results show that at certain concentrations BME in IVM medium has beneficial effects on subsequent embryo development, and is correlated with intracellular GSH level in pig oocytes.