The role of vitrification techniques of cryopreservation in reproductive medicine.

Traditional cryopreservation methods allow ice to form and solute concentrations to rise during the preservation process: both ice and high solute concentrations can cause damage. Cryoprotectants are highly soluble, permeating compounds of low toxicity; they reduce the amount of ice that crystallises at any given temperature and thereby limit the solute concentration factor. Vitrification methods use cryoprotectant concentrations that are sufficient to prevent the crystallisation of ice altogether: the material solidifies as an amorphous glass and both ice and solute concentration are avoided. However, the concentrations of cryoprotectant required are very high indeed and therefore are potentially, and often actually, harmful to cells. Optimisation of the temperature and the rate of introduction and removal of such high cryoprotectant concentrations are critical. The necessary concentration can be lowered if very rapid cooling, and even more rapid warming, are used. This paper draws on experience in other fields of cryobiology to discuss these basic phenomena and to consider the place of vitrification techniques in the cryopreservation of human gametes, embryos and gonads.