Ultra-fast vitrification: Minimizing toxicity of cryoprotective agents and osmotic stress in mouse oocyte cryopreservation

Globally, women have been adopting oocyte cryopreservation (OC) for fertility preservation for various reasons, such as inevitable gonadotoxic treatment for specific pathologic states or social preferences. While conventional vitrification (C-VIT) technology has advanced the success rate of OC, challenges remain regarding the possible toxicities of high concentration cryoprotective agents and osmotic stress. To overcome such difficulties, we evaluated the ultra-fast vitrification (UF-VIT) method, which reduces non-vitrification stage exposure time compared to C-VIT, by observing mouse oocyte intracellular organelles. Consequently, compared to fresh mouse oocytes, UF-VIT had significant differences only in endoplasmic reticulum (ER) intensity and mitochondria (MT) distribution. Meanwhile, C-VIT showed significant differences in the survival rate and all ER and MT parameters. Among the two methods, UF-VIT had fewer damages than C-VIT in all MT parameters. Meiotic spindle (spindle and chromosome) morphology showed no significant changes between the groups during the vitrification/warming (VW) process, suggesting that the VW process did not harm the meiotic spindle of the oocytes. In conclusion, UF-VIT seems more effective in OC by efficient cytoplasmic water molecule extraction, osmotic stress reduction, and minimization of cell contraction and expansion amplitude, thus compensating for the drawbacks of C-VIT.