**5. Technical aspects of oocyte cryopreservation**

Oocyte cryopreservation is a delicate and complex process. Mammalian cells are generally stored at a temperature of -196°C, at which no biological activity takes place. Cryopreservation must transform human oocytes from a biologically active system at 37°C to an inert structure at -196°C; oocytes are most vulnerable during this temperature transition. Membrane permeability and kinetics vary throughout the developmental cycle of the oocyte; metaphase II oocytes have demonstrated higher post-cryopreservation survival in a mouse model (Gook & Edgar, 2007). There are three main goals of oocyte cryopreservation: avoidance of ice crystal formation, avoidance of solution effect, and avoidance of osmotic shock (Jain & Paulson, 2006). As water freezes and expands to form ice, *crystal formation* causes shearing forces on organelles and increases intracellular pressure. Additionally, as water transitions from its liquid to solid form, any solutes dissolved in liquid water are excluded from the ice. This can lead to very high, if not toxic, levels of non-liquid solutes and electrolytes, known as *solution effect*. Further damage to intracellular proteins can occur in the presence of these toxic levels of intracellular substances during cryopreservation. Finally, *osmotic shock* can occur in the setting of rapid rewarming, during which rapid free water shifts lead to cell shrinking and swelling to accommodate alterations in extracellular osmotic pressure. These three goals are achieved through the use of different cryoprotectant chemicals. Cryoprotectants facilitate oocyte cryopreservation by generating an osmotic gradient by which water can exit the oocyte. Permeating cryoprotectants are able to enter the oocyte, thereby preventing cell shrinkage during osmosis of water to the extracellular space.

Two protocols for oocyte cryopreservation exist, slow-freeze methods and vitrification. While slow-freezing is the most widely used and has been studied more in the literature, recent studies in embryos suggest that vitrification may have improved post-thaw survival rates, though it is still not clear whether there are significant differences in clinical pregnancy rates. These methods are discussed here and are analyzed in light of recent evidence of comparative efficacy. In addition, methods for ovarian tissue cryopreservation are briefly discussed.
