Cryobiology: Preservation of Mammalian Embryos

Abstract

The freezing of mammalian embryos has become a routine procedure. It can be reasonably estimated that tens of thousands of live young have been born from embryos that had been frozen and stored in liquid nitrogen at -196°C. The first reports of the successful freezing of mouse embryos were published in 1972 (56,61). Since that time, embryos of 9 other mammalian species have been “successfully” frozen, “successfully” in the sense that live young have been born. As shown in Tab. 1, these species include lab oratory animals, domestic animals, wild animals, and even humans. It is somewhat ironic that the successful freezing of human embryos was accomplished before that of nonhuman primates, experimental species often touted as models for the human. There is every reason to believe that this list will grow with time, as an ever-increasing variety of species’ embryos are subjected to this procedure. Parenthetically, only embryos of the swine have been reported as being so sensitive even to chilling to around O°C that they have not been successfully frozen (35). Despite that, the reason for optimism is that the freezing of mammalian embryos has become not only a routine procedure, but one that rests on a firm mechanistic understanding of some of the factors responsible for cell injury and death caused by freezing and thawing. In many respects, the freezing of mammalian embryos has contributed to that understanding. The purpose of this chapter is to describe (a) how embryos are frozen routinely, (b) the mechanistic basis of successful embryo freezing, (c) some of the more novel methods reported recently, and (d) the degree of success that can be achieved by embryo freezing.