Methanol as a cryoprotectant for equine embryos

Abstract

Equine embryos ( n=43) were recovered nonsurgically 7–8 days after ovulation and randomly assigned to be cryopreserved in one of two cryoprotectants: 48% (15 M) methanol ( n=22) or 10% (136 M) glycerol ( n=21). Embryos (300–1000 μm) were measured at five intervals after exposure to glycerol (0, 2, 5, 10 and 15 min) or methanol (0, 15, 35, 75 and 10 min) to determine changes (%) in diameter over time (±S.D.). Embryos were loaded into 0.25-ml plastic straws, sealed, placed in a programmable cell freezer and cooled from room temperature (22 °C) to −6 °C. Straws were then seeded, held at −6 °C for 10 min and then cooled to −33 °C before being plunged into liquid nitrogen. Two or three embryos within a treatment group were thawed and assigned to be either cultured for 12 h prior to transfer or immediately nonsurgically transferred to a single mare. Embryo diameter decreased in all embryos upon initial exposure to cryoprotectant. Embryos in methanol shrank and recovered slightly to 76±8% of their original diameter; however, embryos in glycerol continued to shrink, reaching 57±6% of their original diameter prior to cryopreservation. Survival rates of embryos through Day 16 of pregnancy were 38 and 23%, respectively ( P>0.05) for embryos cryopreserved in the presence of glycerol or methanol. There was no difference in pregnancy rates of mares receiving embryos that were cultured prior to transfer or not cultured ( P>0.05). Preliminary experiments indicated that 48% methanol was not toxic to fresh equine embryos but methanol provided no advantage over glycerol as a cryoprotectant for equine blastocysts.