Cryopreservation of metaphase II human oocytes effects mitochondrial membrane potential: Implications for developmental competence

Abstract

Background: Current outcome results with embryos derived from thawed MII human oocyes are significantly lower than with embryos cryopreserved at the pronuclear stage. Here, we investigated whether freezing-thawing was associated with changes in oocyte mitochondrial polarity (ΔΨm) that could influence competence by altering ATP levels or the ability of the cytoplasm to regulate intracellular Ca2+. Methods: Fresh and thawed uninseminated and unfertilized MII oocytes were stained with the ΔΨm-specific probe JC-1 to detect clusters of high-polarized mitochondria (J-aggregate positive) and with the Ca2+- specific probe Fluo-4 to measure changes in intracellular levels of this cation. ATP content per oocyte was measured directly and cortical granules were visualized with a cortical granule-specific probe. Results: A significant difference between fresh and thawed MII oocytes existed for pericortical J-aggregate fluorescence and for the ability of the cytoplasm to increase free Ca2+ in response to ionophore exposure. No significant difference in ATP contents was measured and cryopreservation was not associated with an apparent release of cortical granules. Conclusion: Irreversible loss of high ΔΨm in thawed oocytes may be associated with defects in Ca2+ signalling after insemination and could have downstream consequences for normal embryogenesis. © European Society of Human Reproduction and Embryology 2004; all rights reserved.