Theoretical and experimental basis of oocyte vitrification

In the last decades significant advances have been made in successful cryopreservation of mammalian oocytes. Human oocyte cryopreservation has practical application in preserving fertility for individuals at risk of compromised egg quality due to cancer treatments or advanced maternal age. While oocyte cryopreservation success has increased over time, there is still room for improvement. Oocytes are susceptible to cryodamage; which collectively entails cellular damage caused by mechanical, chemical or thermal forces during the vitrification and warming process. This review will delineate many of the oocyte intracellular and extracellular structures that are/may be stressed and/or compromised during cryopreservation. This will be followed by a discussion of the theoretical basis of oocyte vitrification and warming, and a non-exhaustive review of current experimental data and clinical expectations of oocyte vitrification will be presented. Finally, a forward-thinking vision of a potential means of modifying and improving vitrification and warming procedures and success will be proposed. This review addresses theoretical and experimental evidence accumulated over the last two decades supporting the application of vitrification and warming to oocyte cryopreservation. Issues ranging from clinical needs for oocyte cryopreservation, cryopreservation-induced stresses and normal oocyte function, practical application of vitrification-warming of oocytes, and potential future directions will be discussed. In addition, we debate commonly discussed technical methods of oocyte vitrification-warming that may not necessarily be grounded in scientific knowledge. Instead these methodologies are many times theoretical, potentially empirical and commonly lack significant testing and scientific rigor. Questions include: (i) what is the best cryoprotectant? (ii) are some cryoprotectants more toxic compared with others? (iii) how should cryosolutions be mixed with cells? (iv) is there a best container for vitrification? (v) is there a threshold cooling-warming rate or is a faster rate always better? and finally (vi) should oocytes be vitrified with or without adjacent cells? With this said, it is recognized that important advancements have been made in the past decade in oocyte cryopreservation, many times through empirical findings. Finally, we propose some new areas of research that may influence future success of oocyte vitrification and warming, fully recognizing that these theories require mechanical and biological experimental testing.