Fundamental cryobiology of mammalian oocytes and ovarian tissue.

Embryo cryopreservation is a widely used and relatively well-established procedure. By contrast, ovarian tissue and unfertilized oocytes are only rarely cryopreserved, even though for germ line storage these often would be preferable to embryo cryopreservation. There are many reasons for this discrepancy. Unfertilized mature (MII) stage oocytes are more difficult to cryopreserve than cleavage stage embryos of the same species. Many factors contribute to this including the oocyte's surface to volume ratio, single membrane, temperature-sensitive metaphase spindle and zona, and its susceptibility to parthenogenetic activation and chill-injury. A completely different set of problems applies to primordial follicles. Oocytes in primordial follicles are very small and tolerate cryopreservation by slow cooling very well. The problem lies in the difficulty in producing mature oocytes from these primordial follicles. Better and/or more convenient cryopreservation procedures for both oocytes and ovarian tissue are being developed. This paper describes some of the advances in this area and outlines the relative merits and limitations of several currently available egg and ovarian tissue cryopreservation procedures.