Current Data on the Vitrification of Human Embryos: Which One is the Best; Zygote, Cleavage or Blastocyst Stage?

Human gametes and embryos at different developmental stages show various physiological necessities and requirements in order to survive in vivo. All laboratory procedures can cause damage to human gametes and embryos. Cryopreservatio n procedures are such unphysiological circumstances mainly on account of severe temperature and osmotic alterations. Since the early 1980s, two common methods of cryopreservation have been used. Both of these methods have finally depend on the freezing and solidification of cell or tissue. Recently, the one known as vitrification has been claimed as 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 is a more vigorous mean of all possible cell damage except the formation of intracellular ice crystals that i s totally prevented by vitrification. Nevertheless, there is no adequate cumulative data on the outcomes of vitrification performed at different stages of human embryos. The aim of this review is to assess the possible differences of outcomes of vitrification performed at different stage of human embryos.