The Evolution of the Cryopreservation Techniques in Reproductive Medicine - Exploring the Character of the Vitrified State Intra- and Extracellularly to Better Understand Cell Survival after Cryopreservation

Nowadays, cryopreservation of gametes and embryos is a fundamental, integral and indispensable part of infertility treatment or fertility preservation. Cryopreservation is not only needed for the policy of single embryo transfer and cryopreservation of surplus embryos but for deferring embryo transfer in case of ovarian hyperstimulation syndrome (OHSS), uterine pathologies and suboptimal endometrium built-up or when preimplantation genetic testing is needed. Several current strategies in ART would be inconceivable without highly efficient cryopreservation protocols. Nevertheless, cryopreservation hampered for a long time especially in terms of low survival rates after freezing and thawing. Only the technical progresses during the last decades, namely in regard to the implementation and advancement of vitrification leveraged its application and, thus, even allows the cryopreservation of human oocytes – a process which is far from being easy. This review aims to provide a deeper insight into the physical processes of cryopreservation and to explore the character of the vitrified state in the extra and intracellular milieu in order to demonstrate that the common denominator to all cryopreservation procedures is the establishment of an intracellular amorphous condition that hinder the likelihood of crystallization.