Development of vitrification solutions

The development of the science and technology of vitrification of embryos and oocytes is described elsewhere in this manual. The essential requirements include a high concentration of cryoprotectant, non-toxicity at this concentration, and a rapid rate of cooling. The obvious practical way to obtain a rapid rate of cooling is by plunging into liquid nitrogen and this has been adopted almost universally as the cooling method in vitrification. As researchers began to realize that vitrification may offer a simplified technique of embryo and oocyte cryopreservation they utilized the cryoprotectants that had proved successful for conventional preservation by slow cooling. It is clear now that the requirements of the cryoprotectant for successful vitrification of embryos are more demanding than they are for slow cooling. Practitioners of conventional cryopreservation, in the main, choose the cryoprotectant arbitrarily and mainly use glycerol and dimethylsulfoxide. The work of various groups1-5 indicated that the common cryoprotectants vary in the rate of permeation of embryos, and in toxicity6-8to embryos. Rate of permeation is also dependent on temperature.3,9 There are also species differences in the rate of permeation of cryoprotectants.3-5 With the necessity to use high concentrations of cryoprotectants for vitrification it became obvious that toxicity was now an important consideration in the selection of cryoprotectants for this purpose.