EFFECTS OF VITRIFICATION ON MITOCHONDRIAL ULTRASTRUCTURE AND MEMBRANE POTENTIAL AND ITS DISTRIBUTION IN MOUSE OOCYTES

BACKGROUND: Vitrification is commonly used for in vitro fertilization and has significant impact on gametes. OBJECTIVE: To investigate changes in ultrastructure, membrane potential (Delta Psi m) and distribution of mitochondria in mouse oocytes after vitrification. MATERIALS AND METHODS: Mouse oocytes were divided into three groups: one group as fresh control, one group for the toxicity test (treated with cryoprotectant but without vitrification), and the other for vitrification. RESULTS: Most mitochondria in oocytes were damaged after cooling and warming, being rough and fuzzy in appearance, even swollen and broken. The Delta Psi m of the toxicity test group and the vitrification group was 0.320 +/- 0.030 and 0.244 +/- 0.038, respectively, in comparison to the fresh group (0.398 +/- 0.043). The Delta Psi m of the vitrified oocytes was significantly lower than fresh oocytes and the toxicity test oocytes (P < 0.05), but there was no significant difference between fresh oocytes and the toxicity test oocytes (P > 0.05). Mitochondria in fresh oocytes were denser and strained stronger, with 59.5% distributed homogeneously and 36.4% polarized. The majority of mitochondria in the toxicity-tested oocytes were clustered (69.3%) and only a small portion were distributed homogeneously (19.6%), while mitochondria in vitrified oocytes were clustered (56.3%) and deficient (24.4%), and their fluorescent staining was weak and blurred. There was a significant disruption in mitochondrial function after vitrification. CONCLUSION: Vitrification alters the ultrastructure, membrane potential and distribution of mitochondria in oocytes, most likely caused by toxicity and mechanical injury.