ASSESSMENT OF THE DEVELOPMENTAL POTENTIAL OF FROZEN-THAWED MOUSE OOCYTES

Mouse oocytes were cryopreserved by a protocol shown previously to min imize damage to the zona pellucida and cytoskeletal system. After thaw ing, the incidence of fertilization did not differ from that in contro l groups of oocytes, and after fertilization, the ability of the ferti lized frozen-thawed oocytes to develop to the blastocyst stage in vitr o was only slightly less (77%) than that of the controls (87 and 89%). Transfer of frozen - thawed and fertilized oocytes after their cultur e to the blastocyst stage in vitro resulted in a lower implantation ra te (46%) than for the controls (68 - 73%), but of the implanting embry os the same proportions in experimental and control groups survived to yield viable fetuses. In contrast, transfer after culture in vitro to the 2- to 4-cell stage resulted in similar implantation rates for con trol and frozen-thawed fertilized oocytes (70-84%), but the spontaneou s abortion rate was higher for the embryos derived from frozen-thawed oocytes. Overall the cumulative survival rate for frozen oocytes trans ferred at the 2-cell stage (36%) was better than after transfer at the blastocyst stage (30%), but both were less than for the transfer at a ny stage of the control oocytes (47-55%). The cumulative survival of c ryopreserved oocytes to viable fetuses was 30-40% less than that of th e control oocytes. These results are compared with those from previous studies and the main remaining obstacles to completely successful cry opreservation are identified.