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The effects of vitrification of cumulus-enclosed porcine oocytes at the germinal vesicle stage on cumulus expansion, nuclear progression and cytoplasmic maturation
Although offspring have been produced from porcine oocytes vitrified at the germinal vesicle (GV) stage, the rate of embryo development remains low. In this study, nuclear morphology and progression, cumulus expansion, transzonal projections (TZPs), ATP and glutathione (GSH) levels in vitrified cumulus-oocyte complexes (COCs) were compared to those of a control and a toxicity control (no cooling). Vitrification was performed with 17.5% (v/v) ethylene glycol and 17.5% (v/v) propylene glycol. It was shown that vitrification at the GV stage caused premature meiotic progression, reflected by earlier GV breakdown and untimely attainment of the metaphase II stage. However, cytoplasmic maturation, investigated by measurement of ATP and GSH levels, and also cumulus expansion, proceeded normally despite detectable damage to TZPs. Moreover, treatment with cryoprotectants caused fragmentation of nucleolus precursor bodies and morphological changes in F-actin from which oocytes were able to recover during subsequent IVM culture. Reduced developmental competence may be explained by premature nuclear maturation leading to oocyte aging, although other mechanisms such as initiation of apoptosis and reduction of cytoplasmic mRNA can also be considered. Further research will be required to clarify the presence and impact of these phenomena during the vitrification of immature COCs.
RD16386 Accepted 19 April 2017
© CSIRO 2017