Advances in oocyte cryopreservation--part II: rapid cooling using vitrification.
The need for freezing oocytes has been established for females undergoing potential therapy that could damage their ovarian egg reserve, for ethical or religious reasons (not having excess embryos frozen) or for women nearing the age of lower fecundity but not married and not ready to use donor sperm. Applying cryopreservation techniques for oocytes used for embryos resulted in very poor pregnancy results. A rapid flash freezing technique has rekindled interest in oocyte freezing known as vitrification. Certain modifications, especially minimizing the volume, have resulted in marked improved pregnancy rates with vitrified thawed oocytes. The lower volume allows decreased exposure to the toxic cryoprotection. Commercial interests have developed an effective device called cryotop but some concerns about microorganism contamination exist because it is an open system. Modifications have been made to make available the cryotip, a closed device which addresses the contamination issue. Frozen oocyte survival rates upon thawing fertilization rates and subsequent pregnancy rates after embryo transfer have been reported comparable to data with frozen thawed embryos. Because of the uncertainty of the programmable freezer used for the slow cool method and because there has been more commercial interest in the vitrification method, the "flash" freeze protocol seems to have an edge over the slow cool method for oocyte freezing.