Bovine oocyte vitrification and recovery with ethylene glycol and either propylene glycol or dimethyl sulfoxide

Cryopreservation of female gametes allows for preservation and transportation of superior genetics; however, cryoinjury and cryoprotectant toxicity often affects the viability of oocytes that do survive the procedure due to the use of suboptimal protocols designed for embryo cryopreservation. With significant differences in cryoprotectant sensitivity and plasma membrane permeability between oocytes and embryos, there is a continued need to develop protocols accounting for the unique physiology of oocytes. In this study, oocyte recovery after vitrification with ethylene glycol and either dimethyl sulfoxide or propylene glycol were evaluated with reduced equilibration time compared to prior studies. In mature oocytes thawed after cryopreservation with the combination of dimethyl sulfoxide and ethylene glycol displayed increased mitochondrial membrane potential and reduced reactive oxygen species compared to those cryopreserved with propylene glycol and ethylene glycol (P < 0.05). The combination of propylene glycol and ethylene glycol only displayed improved recovery of the meiotic spindle measured by microtubule arrangement and chromosome distribution (P < 0.05). There were no differences in adenosine triphosphate (ATP) concentration or the number of ATP-depleted oocytes between those cryopreserved with dimethyl sulfoxide or propylene glycol (P > 0.05). Both mitochondrial and meiotic spindle functionality are vital for embryo development, and the concentration of cryoprotectants required to achieve vitrification affects both aspects of oocyte physiology. Further improvements to oocyte vitrification protocols are necessary to minimize damage induced during the procedure and improve reliability of the technology.