Cryopreserved Immature Mouse Oocytes: A Chromosomal and Spindle Study

Purpose: Cryopreservation of human oocytes might provide an alternative approach to freezing supernumerary embryos obtained during IVF. This process, performed on immature denuded prophase I mouse oocytes, was investigated. Methods: We first investigated the capacity of frozen, immature, murine oocytes to continue in vitro maturation after thawing. We then evaluated the risk to offspring from chromosomal damage by cytogenetical and cytological (spindle) analysis. Finally, we attempted to determine the reasons for and the stage of maturation failure. Results: A total of 700 immature oocytes was frozen, 629 (90%) were recovered intact after thawing, and 53% extruded the first polar body, versus 74% for the control group. Freezing was not accompanied by an increase in aneuploidy in maturing oocytes (18 and 15% for thawed and control oocytes, respectively). Consequently, the first meiotic division occurred normally, without an increase in nondisjunction. Spindle analysis demonstrated only a few abnormalities (15 and 2% for thawed and control oocytes, respectively) incompatible with further development. Oocytes arrested during in vitro maturation were mainly at the metaphase I stage (64 and 76% for thawed and control oocytes, respectively). Whereas 17% of thawed oocytes were blocked before the formation of the first meiotic spindle, this never occurred in the control group. Conclusions: Immature murine oocytes can withstand cryopreservation, which is encouraging for future human application of this technique.