ABSTRACT
In assisted reproductive technology (ART), cryopreservation of embryos has proved important to enable the best use of supernumerary embryos. In the cryopreservation of embryos, there is a risk of various types of injury.1,2 Among them, the formation of intracellular ice appears to be the most damaging. The first strategy to prevent intracellular ice from forming was to adopt a lower concentration of cryoprotectant and a long slow-cooling stage. This slow freezing method has proved effective for embryos of a wide range of mammalian species. Unlike embryos of laboratory animals and domestic animals, in which dimethylsulfoxide (DMSO), glycerol, or ethylene glycol (EG) is commonly used as the cryoprotectant, human embryos at early cleavage stages have most often been frozen in a solution of propanediol supplemented with sucrose,3 although those at the blastocyst stage have more frequently been frozen with glycerol and sucrose.4-6 With slow freezing, however, it is difficult to eliminate completely injuries occurring from ice formation. Furthermore, the slow freezing method requires a long period of time before embryos are stored in liquid nitrogen.
