ABSTRACT
INTRODUCTION In assisted reproductive technology (ART), cryopreservation of embryos has become important for the best use of supernumerary embryos. During the steps of 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. e rst strategy for preventing intracellular ice from forming was to adopt a lower concentration of cryoprotectants and a long slowcooling stage. is slow-freezing method has proven eective for the embryos of a wide range of mammalian species. Unlike embryos of laboratory animals and domestic animals, in which dimethyl sulfoxide (DMSO), glycerol, or ethylene glycol (EG) are commonly used as the cryoprotectants (cryoprotective agents [CPAs]), human embryos at early cleavage stages have most oen been frozen in a solution of propanediol (PROH) 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 dicult to eliminate injuries occurring from ice formation completely. Furthermore, the slow-freezing method requires a relatively long period to be undertaken before the embryos are nally stored in liquid nitrogen (LN2).
