In vitro viability of mouse oocytes vitrified in an ethylene glycol-based solution.

Ovulated mouse oocytes denuded of their cumulus cells, were vitrified in a solution containing 7 M ethylene glycol as the sole cryoprotectant using one or two steps of exposure before vitrification and were diluted in 1 M sucrose solution in 5 or 10 min after warming. The results proved that the viability of oocytes are detrimentally affected by exposure to the vitrification solution even without vitrification. At 5 min dilution time, the two-step exposure was superior to the one-step in terms of the post-warming recovery rate of vitrified oocytes with normal morphology and their subsequent development to the blastocyst stage (p < 0.01) after fertilization in vitro. At 10 min dilution time, no significant difference between one- or two-step exposure was found. The effect of the addition of 0.5 M sucrose to the vitrification solution was also determined and did not result in a significant improvement in the viability of oocytes vitrified in one-step and diluted for 10 min. In conclusion, the results in this study indicate that oocytes can be vitrified with 7 M ethylene glycol as the sole cryoprotectant in the vitrification solution, and that the recovery of normal oocytes after one-step exposure in the vitrification solution can be improved by 10 min dilution time. However, the improvement in the recovery rate of oocytes with normal morphology and their subsequent developmental in vitro was not improved by the addition of 0.5 M sucrose to the vitrification solution.