Effects of vitrification on mitochondrial ultrastructure and membrane potential and its distribution in mouse oocytes

Abstract

BACKGROUND: Vitrification is commonly used for in vitro fertilization and has significant impact on gametes. OBJECTIVE: To investigate changes in ultrastructure, membrane potential (ΔΨm) 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 ΔΨm 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 ΔΨm 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.