Factors affecting nuclear maturation, cleavage and embryo development of vitrified bovine cumulus-oocyte complexes

The objective was to investigate the effects of cryodevice, vitrification solutions, and equilibration time on in vitro maturation, cleavage, and embryo development of vitrified bovine oocytes. In Experiment 1, the nuclear maturation (MII) rate of immature bovine COCs vitrified was compared between two equilibration times (0 vs 10 min) in vitrification solution 1 (VS1) and two cryodevices (cryotop vs 0.25 mL straw). The MII rate was higher in the non-vitrified control group than in vitrified groups (61 vs 16%, P < 0.0001). Equilibration time did not affect MII rate (P = 0.964); however, the MII rate was higher for COCs vitrified on cryotops than in straws (23 vs 9%, P = 0.007). In Experiment 2, bovine COCs were vitrified on cryotops using two equilibration times (0 vs 5 min) in VS1 and two kinds of vitrification solutions (freshly prepared vs frozen). Cleavage and blastocyst rates were higher (P < 0.0001) in the non-vitrified control group than vitrified groups (cleavage rate 93 vs 42% and blastocysts rate 31 vs 0.4%). Cleavage rate of COCs vitrified using frozen solutions with 5 min equilibration was higher (P = 0.05) than other treatment groups. However, blastocyst rate did not differ (P = 0.993) among treatment groups. In conclusion, cryotop was a better cryodevice than 0.25 mL straw for vitrification of bovine COCs. Furthermore, 5 min equilibration in VS1 improved cleavage. Compared with control, the vitrification procedure per se damaged bovine COCs, resulting in poor nuclear maturation and embryo development. However, vitrification did not immediately kill oocytes, as the cleavage rate was acceptable.