A recovery time after warming restores mitochondrial function and improves developmental competence of vitrified ovine oocytes

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The aim of the present study was to assess the ability of vitrified/warmed oocyte to recover from vitrification-induced damages after warming. In vitro matured, vitrified/warmed ovine oocytes were assessed for developmental competence, mitochondrial activity and distribution, ATP, ROS and catalase levels during 6 h of in vitro culture using fresh oocytes as control. ATP content in vitrified oocytes was lower than control during 4 h of post warming culture (p < .01). Vitrified oocytes were able to fill this gap only after 6 h of post-warming incubation. Moreover, mitochondrial activity was significantly lower (p < 0.01) in vitrified oocytes compared to controls, and this difference was maintained up to 2 h of incubation. Then the activity increased and at 4 h it was higher compared to controls (p < 0.01). These oocytes showed an increasing rate of clustered distribution of mitochondria which was lower than controls during the first 4 h of post warming culture (p < 0.01). ROS level was significantly higher at 0 h in vitrified compared to control oocytes and this difference was maintained also at 2 h and 6 h of incubation (p < 0.01). Catalase level was higher in vitrified oocytes than controls (p < 0.01) during the entire culture period. Cleavage and blastocyst rates were lower in vitrified oocytes compared to control ones during the two first time point of incubation period (p < .01), indeed they increased significantly from 0 to 4 h of incubation post warming (p < 0.01). The study demonstrated that vitrified/warmed oocytes need an extra time to restore damage due to cryopreservation procedures and to increase their developmental potential. Thus, time of damage recovery after vitrification could be used to standardize the vitrification protocols and to improve the developmental competence of vitrified/warmed oocytes.