Maintenance of the meiotic spindle during vitrification in human and mouse oocytes

Vitrification appears to be a viable method for the cryopreservation of human metaphase II (MII) oocytes, but concerns regarding the concentration of cryoprotectants used during vitrification have been raised. In an attempt to circumvent this potential problem, the majority of protocols are carried out at room temperature. Exposing oocytes to temperatures below 37°C, however, leads to rapid microtubule depolymerization. Polarized light microscopy was used to measure meiotic spindle retardance following exposure to cryoprotectants and vitrification in human and mouse oocytes. To quantify the extent of depolymerization, spindle retardance was determined before and after each treatment. Exposure to vitrification and warming solutions at room temperature (21–22°C) caused the spindle of mouse MII oocytes to depolymerize. In contrast, no measurable changes in the meiotic spindle were detected by maintaining the temperature at 37°C during the exposure regimen. By carrying out the entire vitrification and warming procedure at 37°C, the spindle was also unaffected. Comparable results were obtained with vitrification of human MII oocytes at 37°C. Analysis of sibling human oocytes demonstrated that slow freezing, in contrast to vitrification, was unable to preserve the meiotic spindle. Using a vitrification protocol employing 37°C impacts negligibly on the meiotic spindle. Thus, fertilization can proceed without having to await spindle reformation.