Effects of slow and ultrarapid freezing on morphology and resumption of meiosis in immature cat oocytes

This study examined the ability of immature cat oocytes to survive after cryopreservation by evaluating their subsequent development following maturation in vitro. The effect of slow and ultrarapid freezing using one of two cryoprotectants dimethylsulphoxide (DMSO) or 1,2-propanediol (PROH) at different concentrations (1.5 or 3.0 mol-1) and the slow freezing with the cryoprotectant ethylene glycol (EG 1.5 mol l-1 and 3.0 mol l-1) were tested. Morphology, resumption of meiosis and metaphase II rates of oocytes were recorded after thawing. Freshly collected oocytes were used as controls. Results indicate that immature cat oocytes can survive, resume meiosis and achieve metaphase II in vitro after freezing. The highest rates of resumption of meiosis and metaphase II were achieved with slow freezing and 1.5 mol DMSO or EG l-1 (DMSO: 47.4%, 18/38 and 23.7%, 9/38 and EG: 52%, 13/25 and 20%, 5/25, respectively). The ultrarapid procedure did not result in resumption of meiosis in vitro, despite intact morphology of the oocytes after thawing. These results suggest that morphology of oocytes after freezing and thawing has no predictive value for their ability to resume meiosis.