Cryopreservation of Mammalian Oocytes: Slow Cooling and Vitrification Approaches.

Two basic methods for the laboratory-focused cryopreservation of mammalian oocytes are discussed, based initially on work with murine oocytes and subsequently transferred to human infertility services. One method uses a relatively low concentration of the cryoprotectant propanediol plus sucrose and requires controlled-rate cooling equipment to achieve a slow cooling rate. This method has also produced live births from cryopreserved human oocytes. The second method, which is described here, employs a high concentration of the cryoprotectant dimethyl sulfoxide plus a low concentration of polyethylene glycol. This is a vitrification method, which involves ultra-rapid cooling by plunging standard straws into liquid nitrogen vapor, hence avoiding the need for specialized equipment, but requires technical ability to manipulate the oocytes quickly in the highly concentrated cryoprotectant solutions. Murine oocytes that have been vitrified using this technique have resulted in live births. Vitrification using other cryoprotectant mixtures is now a clinically accepted method of choice for cryobanking of human oocytes.