Current data on the vitrification of human embryos: Which one is the best; zygote, cleavage or blastocyst stage?

Human gametes and embryos at different development stage show various physiological necessities and requirements in order to survive in in vivo conditions. Therefore, all laboratory procedures can cause damage to human gametes and embryos. Cryopreservation procedures are such unphysiological circumstances mainly due to severe temperature and osmotic alterations. Since the early of 1980s, two common methods of cryopreservation were been used. Both of these methods are finally depending on the freezing and solidification of cell or tissue. Recently, the latter one that known as vitrification has claimed to be the future of cryopreservation because of increased survival and success rates. However, this method is a non-equilibrium technique of cryopreservation that shows critical requirements of much higher concentration of permeable cryoprotectants and rate of cooling. Thus, it has defined to be a more vigorous in mean of all possible cell damage except the formation of intracellular ice crystals that is totally prevented by vitrification. Nevertheless, there is no adequate cumulative data on the outcomes of vitrification preformed at different stages of human embryos. The aim of this review is to assess the possible differences of outcomes of vitrification those performed at different stage of human embryos.