Cryopreservation of kunming mouse oocytes using slow cooling, ultrarapid cooling and vitrification protocols

The cryopreservation of oocytes has been only marginally successful with any of the current protocols, including slow cooling, rapid cooling and vitrification. We wished to test the hypothesis that oocytes from a single mouse strain would freeze successfully by 1 of the 3 mentioned protocols. Unfertilized Kunming mouse oocytes obtained 14 h after PMSG/hCG administration were randomly assigned to be cryopreserved after slow cooling, ultra rapid cooling and vitrification. Oocytes were thawed by straws being placed into 37 °C water, and their morphological appearance and in vitro fertilization capability were compared with that of oocytes that had not undergone cryopreservation. Survival of oocytes was indicated by the absence of darkened ooplasm or by broken membranes or zona pellucida. Functional integrity was evaluated by the formation of a 2-cell embryo after IVF. Survival rate of slow cooled oocytes did not differ from that seen in vitrified oocytes (55.1 vs 65.9%) but was significantly lower in the rapidly cooled oocytes (24.2%; P < 0.01). The results of IVF of slow cooled and vitrified oocytes were similar to those of the control group (72 and 73 vs 77%; P > 0.05). It appears that Kunming mouse oocytes can be successfully cryopreserved using the slow cooling method with 1,2-propanediol and vitrification, which contains both permeating and nonpermeating cryoprotectants.