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Association of ultrarapid freezing of mouse oocytes with increased polyploidy at the pronucleate stage, reduced cell numbers in blastocysts and impaired fetal development.

Mature mouse oocytes were frozen ultrarapidly with a cryoprotectant solution consisting of 3.5 mol dimethylsulfoxide l-1 and 0.5 mol sucrose l-1 or exposed to the freezing solution and recovered. Freshly collected oocytes were used as controls. After ultrarapid freezing and thawing, a high mean percentage of oocytes (78%) survived. The incidence of parthenogenetic activation in frozen-thawed oocytes was not increased. After insemination in vitro, the rate of two-cell formation was decreased (59% versus 69%). Examination of the chromosome complement of first-cleavage embryos revealed that the incidence of polyploid embryos was four times that of the control group (23% versus 6%). Fewer fertilized eggs progressed to the blastocyst stage (49% versus 81%) and the mean number of cells per blastocyst was decreased. Furthermore, the capacity of transferred blastocysts to develop in vivo was reduced. Autopsy at day 17 of gestation revealed that the proportion of embryos implanting was lower in the embryos derived from ultrarapidly frozen-thawed oocytes. Furthermore, some of the living fetuses were abnormal, but our sample size is too small for the effect to be significant. In the solution control group, no differences were found compared with the control group. Although the study needs further results to draw definite conclusions, our findings question whether the applied protocol is suitable for mouse oocytes.