ASSOCIATION OF ULTRARAPID FREEZING OF MOUSE OOCYTES WITH INCREASED POLYPLOIDY AT THE PRONUCLEATE STAGE, REDUCED CELL NUMBERS IN BLASTOCYSTSAND IMPAIRED FETAL DEVELOPMENT

Mature mouse oocytes were frozen ultrarapidly with a cryoprotectant so lution consisting of 3.5 mol dimethylsulfoxide l-1 and 0.5 mol sucrose l-1 or exposed to the freezing solution and recovered. Freshly collec ted oocytes were used as controls. After ultrarapid freezing and thawi ng, 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 decr eased (59% versus 69%). Examination of the chromosome complement of fi rst-cleavage embryos revealed that the incidence of polyploid embryos was four times that of the control group (23% versus 6%). Fewer fertil ized eggs progressed to the blastocyst stage (49% versus 81%) and the mean number of cells per blastocyst was decreased. Furthermore, the ca pacity of transferred blastocysts to develop in vivo was reduced. Auto psy at day 17 of gestation revealed that the proportion of embryos imp lanting was lower in the embryos derived from ultrarapidly frozen-thaw ed 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 c ontrol group. Although the study needs further results to draw definit e conclusions, our findings question whether the applied protocol is s uitable for mouse oocytes.