Embryo development in vitro of cat oocytes cryopreserved at different maturation stages

The purpose of this study was to evaluate the ability of cat oocytes, at different stages of maturation, to survive after cryopreservation and to assess their subsequent development following IVM and IVF. In the initial toxicity trial, immature oocytes were exposed to different concentrations of DMSO and ethylene glycol (EG). Resumption of meiosis and metaphase II were evaluated after removal of the cryoprotectant and IVM. The highest rates of resumption of meiosis (51.4%) were achieved after exposure to 1.5 mol l(-1) of cryoprotectants, and no difference was observed with control oocytes. Metaphase II was obtained in 25.7% (P<0.01) and 22.9% (P<0.005) of oocytes exposed to 1.5 mol l(-1) of DMSO and ethylene glycol, although at lower rates than in control oocytes (54.4%). On the basis of this finding, 1.5 mol l(-1) of cryoprotectant was chosen for freezing cat oocytes at the germinal vesicle stage (immature) or at metaphase II stage (mature). Post-thaw viability was assessed by the evaluation of the embryo development in vitro. After fertilization, mature oocytes frozen in ethylene glycol cleaved in better proportions (38.7%) than immature oocytes (6.8%, P<0.001), and no differences were observed in the cleavage rate of oocytes frozen at different maturation stages with DMSO (immature 12.8%; mature 14.1%). Embryonic development beyond the 8-cell stage was obtained only when mature oocytes were frozen with ethylene glycol (11.3%). This study suggests that cryopreserved cat oocytes can be fertilized successfully and that their development in vitro is enhanced when mature oocytes are frozen with ethylene glycol. The stage of maturation may be a key element in improving cat oocyte cryopreservation.