Cryopreservation of oocytes and embryos: optimization by theoretical versus empirical analysis

Embryos and oocytes were first successfully cryopreserved more than 30 years ago. This procedure has come to be an important, almost essential component in the practice of assisted reproduction in animals and humans. Literally millions of animals of more than 20 species and undoubtedly hundreds of thousands of children have been born from frozen embryos. Nevertheless, there still remain shortcomings with methods used to cryopreserve oocytes and embryos. A wide variety of approaches has been used to try to improve and optimize methods of cryopreservation. These approaches range from the very basic to the completely empirical. Some investigators make use of rigorous mathematical formulations to define and describe the behavior of oocytes and embryos at subzero temperatures. Others conduct "trial-and-error" studies to improve the results by dint of many replicate experiments in which they examine the effects of various protective compounds, macromolecular supplements, and compare different cooling and warming conditions. This review considers both extremes.