Oocyte cryopreservation: a biological perspective.

Oocyte cryopreservation would amount to a major breakthrough in reproductive medicine. Diverse strategies have been tested to minimise cooling-induced cell injury. Nevertheless, oocytes from various species have shown a particular sensitivity to freezing, due to their unique biological characteristics. Storage of human mature oocytes with slow freezing has resulted in low survival rates, although recent studies based on modified methods have reported higher success. Survival after thawing is not necessarily a guarantee of unaltered viability. Developmental failure at pre- or postimplantation stages may originate from critical perturbations of various cell components, such as the chromosome segregation apparatus, the intracellular calcium signalling system, and the cytoskeleton. Germinal vesicle (GV)-stage oocytes have been suggested to be more amenable to freezing. But their use would require efficient in vitro maturation systems, which are not presently available. Oocyte cryopreservation remains a realistic objective, provided that more systematic approaches are applied, such as thorough analysis of the plasmalemma permeability to water and the diverse cryoprotectants.