THE POSTTHAW DEVELOPMENTAL CAPACITY OF FROZEN BOVINE OOCYTES FOLLOWING INVITRO MATURATION AND FERTILIZATION

The present study was designed 1) to investigate and compare the post-thaw developmental capacity of frozen bovine oocytes following in vitro maturation (Metaphase II) and fertilization (pronuclear stage), and 2) to identify the important factors of cryopreservation procedures on the post-thaw developmental capacity of those oocytes. Frozen pronuclear-stage oocytes had a higher cleavage rate (P < 0.001) and developmental capacity to the two- (P < 0.05) and eight-cell stages (P < 0.001) than frozen mature oocytes. The effects of different concentrations of cryoprotectant (1.0 M vs 1.5 M glycerol) and sucrose (0.25 M vs 0.5 M) as well as two freezing curves (A vs B) on post-thaw developmental capacity were examined. The concentration of cryoprotectant was an important factor in the post-thaw developmental capacity of the oocytes frozen at pronuclear and mature stages, and 1.0 M glycerol was a more effective concentration of cryoprotectant than 1.5 M glycerol (P < 0.001 for the cleavage rate and the developmental capacity to the eight-cell stage, and P < 0.05 for the developmental capacity to the two-cell stage). However, the effects of other factors were not significantly different. We concluded that in vitro mature bovine oocytes were more vulnerable to freezing shock than pronuclear stage oocytes and that the concentration of glycerol as a cryoprotectant was an important factor in the improvement of post-thaw developmental capacity of bovine oocytes frozen at Metaphase II and at the pronuclear stage.