Cryopreservation of human oocytes: a review of current problems and perspectives

It is well recognized that the ability to cryopreserve unfertilized human oocytes would make a significant contribution to infertility treatment. However, despite considerable interest, very few successful pregnancies have arisen from cryopreserved oocytes after thawing, insemination and transfer of the subsequent embryo. The reasons for this lack of progress may well result from a dearth of information on how the various biophysical changes during a cryopreservation regimen affect human oocyte function. Recently, fundamental studies on the effects of cooling, membrane permeability, cryoprotectant addition and ice formation have been performed on human oocytes by a number of groups, and these form the basis of the current review. It is likely that successful human oocyte cryopreservation will only follow once these factors are fully understood, but the existing base of knowledge should provide a platform for further improvements in the techniques currently employed.