Recovery of spindle morphology and mitochondrial function through extended culture after vitrification-warming of bovine oocytes.

The present study aimed to determine the effects of vitrification on the meiotic spindle and mitochondrial function of bovine oocytes submitted to different times of post-warming culture. Partially denuded cumulus-oocyte complexes were vitrified at different maturation times (18-, 20-, and 24-h) using a two-step cryoprotectant addition protocol and submitted to 6-, 4-, or 0-h of post-warming extended culture in maturation medium. Microtubule configuration and chromosomal arrangement were analyzed after 0- and 6-h of extended culture, whereas mitochondrial membrane potential and ATP content were measured at 0-, 4-, and 6-h of post-warming recovery. Results of meiotic spindle integrity revealed that vitrified-warmed oocytes that underwent 6-h of culture had similar incidence of normal microtubule configuration and chromosomal arrangement as compared to fresh oocytes, but higher than oocytes in the vitrification control group (no culture). Mitochondrial membrane potential was not different in all the vitrification groups, but the oocytes that were cultured for 4-h after warming had similar levels compared to fresh oocytes. ATP concentration in all vitrification groups was lower than the control group. However, oocytes cultured for 6-h had the lowest rate of ATP depleted oocytes among the vitrification groups. The results of this study indicate that extended culture after warming promotes the recovery of the meiotic spindle and, to some extent, mitochondrial function of vitrified-warmed metaphase II bovine oocytes.