Cryobiology: preservation of mammalian embryos

The preservation of mammalian embryos has become a routine procedure. Thousands of live offspring have been produced from frozen-thawed embryos transferred into recipient foster mothers. Species whose embryos have been successfully preserved include mouse, rat, rabbit, sheep, goat, cattle, horse, antelope, baboon, and human. During the past few years, novel procedures have been introduced that permit embryos to be frozen and thawed rapidly, and to be transferred into recipients under field conditions almost immediately upon thawing. Thus, the transfer of frozen-thawed embryos of domestic animals is becoming almost as efficient as is artificial insemination using frozen-thawed semen. Because of both the inherent fundamental interest and the practical applications of embryo freezing, a substantial understanding of the mechanisms responsible for freezing damage of embryos has been achieved. To survive freezing, embryos must be exposed to protective compounds; to function after thawing, embryos must be washed free of these compounds. Based on fundamental physiology, efficient methods to accomplish such washing have been developed. Furthermore, to survive freezing, embryos must be cooled under conditions in which intracellular ice does not form. This can be accomplished either by pretreating the embryo or by cooling it in such a way as to cause it to dehydrate during freezing. Maximum survival of embryos appears to be achieved when intracellular water does not crystallize during cooling or during warming. As a result of the growing efficiency of embryo preservation, this method is being applied to a variety of practical situations. For example, large banks of frozen embryos of laboratory animals are being established to preserve valuable research resources. The freezing of cattle embryos is being used with increasing frequency as an adjunct to commercial embryo transfer. Preservation of endangered species by embryo preservation is beginning. And finally, the preservation of human embryos is finding application in the field of in vitro fertilization.