Cryopreservation of embryos and ova.

Development of techniques for cryopreservation of embryos of several species, principally the mouse, laid the foundation for cryopreservation of human embryos. As IVF has become more widely available and the need for the cryopreservation of human embryos has become apparent, pressure for technical development has increased. The ideal method would be simple, inexpensive, and effective. The most effective method for cryopreservation of early human embryos, such as those at the 1-cell pronuclear stage and up to the 4-cell stage, now appears to be stepwise cooling in 1,2-propanediol with sucrose in plastic ministraws. The preferred method for intermediate stage embryos uses DMSO with cooling and thawing at slow rates in a programmed biologic freezer. For the human blastocyst, slow cooling in glycerol and rapid thawing is the only method reported with survival rates comparable to those achieved for intermediate stage embryos using DMSO. The rates of survival from freezing and thawing blastocysts are not sufficiently high, however, to justify the losses associated with prolonged in vitro incubation. Even at the current level of technical achievement, cryopreservation of human embryos provides the clearest opportunity to improve the clinical results obtained with IVF. Research now underway in the modification of methods for vitrification and ultrarapid freezing holds promise for both simplification of technology and improvement of outcome. In view of legal and ethical considerations involved in embryo preservation, the desirability of ova preservation is widely accepted. Although a small number of human unfertilized mature ova have been cryopreserved using various methods, success rates are still low. Methods for the cryopreservation of eggs should be developed, but these methods probably should be proved by animal experiments to be safe, especially with regard to genetic damage, before a policy of transfer of embryos derived from frozen-thawed human ova is applied on a large scale.