Effect of slow freeze versus vitrification on the oocyte: an animal model.

Objective: To determine whether there is a deleterious effect on dynamic events in the nucleus and cytoplasm of oocytes by using different cryopreservation protocols in an animal model.

Design: Prospective study.

Setting: University hospitals.

Patient(s): Not applicable.

Intervention(s): Immunostaining and confocal laser scanning microscope techniques were used.

Main Outcome Measure(s): The spindle and chromosomal configurations, as well as dynamic changes of the cortical granules (CGs) and mitochondria in different cryogroups.

Result(s): After thawing/warming of bovine oocytes, CGs became more dispersed in the cytoplasm, particularly in the DMSO group. A significant reduction in normal spindle and chromosomal configurations was observed in all three cryogroups, particularly in the propylene glycol (PROH) group, when compared with the fresh group. Global DNA methylation levels were significantly reduced in the slow and DMSO groups, as compared with the fresh group; however, methylation levels were significantly increased in the PROH group. The proportion of severely apoptotic oocytes was dramatically increased in all three cryogroups, compared with the fresh group.

Conclusion(s): Overall, results demonstrate that using DMSO as the cryoprotectant is better for preserving the cellular and nuclear integrity of the oocyte. The PROH method makes the oocyte more vulnerable to increased DNA methylation, which may be associated with imprinting gene alteration. This study adds to the increasing body of evidence that cryopreservation protocols vary in their impact upon the oocyte.