Cryopreservation of oocytes

Introduction Oocyte cryopreservation is a major issue in human reproductive medicine. The need to cryopreserve oocytes rather than embryos is not limited to countries with restrictive legislation. In fact, all the major clinical applications rely on fertility preservation for both medical and non-medical reasons [1–4] and for oocyte donation programs [5–7]. The first success in freezing human oocytes was reported by Chen [8] using a slow-cooling method in dimethyl sulfoxide (DMSO) as the cryoprotectant. The technique was essentially a modification of that used for cryopreservation of human embryos, which in turn had evolved from methods applied for freezing mouse and cattle embryos. Out of 40thawed oocytes, the reported survival, fertilization, and cleavage rates were 80%, 83%, and 60%, respectively. Despite the technique resulting in a twin pregnancy, its adoption in human reproductive medicine was delayed, due to the failure to reproduce the results reported. Concerns were expressed about the difficulty in dehydrating and cooling human oocytes due to their large volume, the sensitive nature of the metaphase nucleus, premature cortical granule release, and interruption to intercellular ultrastructure [9, 10].