Cryopreservation of human oocytes and ovarian tissue.

Oocyte cryopreservation has the potential to be an important adjunct to assisted reproductive technologies and bypasses some ethical, moral, and religious dilemmas posed by human embryo cryopreservation. The success of human oocyte cryopreservation depends on morphological and biophysical factors that could influence oocyte survival after thawing. Among the morphological factors, the maturity, quality, size of the oocyte, the presence or the absence of the cumulus oophorus seems to play an important role in oocyte survival after thawing. The main biophysical factor of cellular disruption during cryopreservation process in the intracellular ice formation that can be avoided by an adequate cell dehydration; thus reducing the intracellular water by increasing the dehydration process we can limit the damages of the cryopreservation procedure. The dehydration process can be affected by the presence and concentration of the cryoprotectants in the freezing solutions (equilibration and loading solutions), and by the freezing and thawing rate. Two additional properties of cryoprotectants help to protect cells during slow cooling, when the cells are very dehydrated and are surrounded by concentrated salts. The cryoprotectants appear to reduce damage caused by high levels of salt, a property known as salt buffering. Some events occurring to the oocyte during cryopreservation procedure has been found to be a premature exocitosis of cortical granules, leading to an intempestive zona hardening and consequently to a reduction of fertilization rate, and the cryoinjury to the zona pellucida leading to a polispermic fertilization. ICSI is an efficient method to by pass these two events and to achieve a satisfactory outcome in terms of normal fertilization of cryopreserved oocytes. The application of the ICSI to cryopreserved oocytes did not seem to increase the degeneration rate after insemination with respect to fresh oocytes. The increased oocyte survival rate and the use of ICSI have facilitated the recent increase in the number of pregnancies and live birth.