CRYOPRESERVED IMMATURE MOUSE OOCYTES - A CHROMOSOMAL AND SPINDLE STUDY

Purpose: Cryopreservation of human oocytes might provide an alternativ e approach to freezing supernumerary Embryos obtained during NF. This process, performed on immature denuded prophase I mouse oocytes, was i nvestigated. Methods: We first investigated the capacity of frozen, im mature, murine oocytes to continue in vitro maturation after thawing. We then evaluated the risk to offspring from chromosomal damage by cyt ogenetical and cytological (spindle) analysis. Finally, we attempted t o determine the reasons for and the stage of maturation failure. Resul ts: A total of 700 immature oocytes was frozen, 629 (90%) were recover ed intact after thawing, and 53% extruded the first polar body, versus 74% for the control group. Freezing was not accompanied by an increas e in aneuploidy in maturing oocytes (18 and 15% for thawed and control oocytes, respectively). Consequently, the first meiotic division occu rred normally, without an increase in nondisjunction. Spindle analysis demonstrated only a few abnormalities (15 and 2% for thawed and contr ol oocytes, respectively) incompatible with further development. Oocyt es arrested during in vitro maturation were mainly at the metaphase I stage (64 and 76% for thawed and control oocytes, respectively). Where as 17% of thawed oocytes were blocked before the formation of the firs t meiotic spindle, this never occurred in the control group. Conclusio ns: Immature murine oocytes can withstand cryopreservation which is en couraging for future human application of this technique.