Survival rate of frozen-thawed bovine IVF embryos in relation to exposure time using various cryoprotectants.

The relationship between cryoprotectant and exposure time on the development of IVF-derived embryos in culture after freezing and thawing was examined. Good to excellent quality Day 7 to 8 blastocysts and expanded blastocysts were suspended in 1.6 M propylene glycol (PG), 1.8 M ethylene glycol (EG), 1.1 M diethylene glycol, and 1.3 M ethylene glycol monomethyl ether (EME). In Experiment I, after 30 and 120 min of exposure, embryos were placed directly into culture medium and washed three times. Embryos were cocultured with cumulus cells in culture medium. After a 48-h culture, there were no significant differences in fully expanded, hatching, and hatched rates between the 30- and 120-min exposure to all four cryoprotectants except for the fully expanded rate with PG (55 vs 83%). In Experiment II, to compare the cryoprotective effects of the four cryoprotectants, embryos were exposed to each cryoprotectant for variable times from 10 to 120 min, seeded, and cooled to -30 degrees C at 0.3 degree C/min, plunged into liquid nitrogen for storage, and then thawed. Survival was measured by coculturing the embryos with cumulus cells in culture medium. There were no significant differences in development to hatched blastocysts of embryos exposed to either PG (27-42%) or EG (37-56%). The number of embryos developing after exposure to EME was similar to that in PG or EG when exposure time did not exceed 40 min (43-49%). Our findings suggest that the toxicity of different cryoprotectants is related to the exposure time and that PG and EG were relatively nontoxic as is exposure to EME for short periods.