Why freeze embryos at the blastocyst stage?

The very first report of successful cryopreservation of the human embryo was in 1983 (Trounson & Mohr, 1983) with a pregnancy arising from the freezing in DMSO, thawing and transfer of an eight-cell embryo. Within a year or so appeared the first successes from the use of glycerol to cryopreserve human blastocysts (Cohen et al., 1985; Fehilly et al. 1985). In the same year Lassalle et al. published a simple but consistent protocol using propanediol plus sucrose (1985) that has become probably the most commonly used approach for freezing both early cleavage stage and pronucleate one-cell embryos. Attempts to improve on the consistency and convenience of cryopreserving blastocysts reappeared when, using Vero cell co-culture to enhance extended culture, Menezo et al. (1992) explored the use of a combination of glycerol and sucrose as cryoprotectants to freeze spare expanded blastocysts. Essentially all of the above protocols employed a slow freeze/rapid thaw approach, requiring the use ...  more