The efficacy and safety of human oocyte cryopreservation by slow cooling

Preservation at low temperatures is a practice that has found several applications in medicine and biotechnology. Several types of cells and tissues can be stored virtually indefinitely in liquid nitrogen (-196 degrees C) and returned to physiological temperatures. However, phenomena occurring during cooling and warming may affect the structure and function of the stored material. To some extent, the use of substances with special protective properties, referred to as cryoprotective agents, may avoid this effect, but normally a fraction of the cryopreserved material suffers a partial or total loss of viability. In the context of human in vitro fertilization (IVF), for decades cryopreservation has been used to store different materials of reproductive function, mainly embryos at various developmental stages and spermatozoa. In recent times, the realm of cryopreservation has been expanded to also include fully grown unfertilized oocytes. The storage of oocytes offers obvious advantages, eluding legal and ethical problems that afflict, and sometimes make inapplicable, embryo cryopreservation. Until recently, technical and applicative advances of oocyte cryopreservation were inhibited by the preconception that the unique nature of this cell was an insurmountable obstacle to safe and efficient preservation at low-temperature storage. In fact, through the two alternative methodological approaches of controlled rate slow cooling and vitrification, oocyte cryopreservation has been developed both technically and clinically to a stage that is beginning to challenge the supremacy of embryo freezing as the preferred form of fertility preservation for the treatment of infertile couples. Although several questions remain to be answered as to whether oocyte cryopreservation can ensure adequate standards of efficacy and safety, there appears to be little doubt that in the near future oocyte cryopreservation will be recognized as an established form of IVF treatment.