[Cryobiology of complex tissues].

Cooling biomaterials to liquid-nitrogen temperature is probably the most effective procedure for the preservation of their biochemical and morphological parameters, in so far as adverse effects of cooling are controlled. Potential damage has not the same explanation under different conditions of cold exposure. Even a moderate decrease of temperature leads to irreversible damage when applied to untreated organisms. Deep freezing injuries are due to crystallization of ice, and cellular consequences are quite different according to the temperature kinetics. The fraction survival, as a function of hypothermic kinetics, has a bimodal distribution, reflecting two distinct mechanisms: a first process, concomitant of a slow cooling rate, with extracellular ice formation, and a second one, during fast cooling rates, with intracellular ice formation. Thereby, cryopreservation protocols have to focus on many variable factors, including choices of a cryoprotectant, which delays ice formation, and a cooling rate leading to an optimal cell survival. Under these conditions, osmotic effects are almost neutralized. Nevertheless, these methods can be applied only to isolated cells. For tissues or organs, ice formation should be avoided at low temperatures. Vitrification is the corresponding process by which tissue water is undercooled to the glass transition temperature and solidified into the amorphous or glassy state. Very high cooling and warming rates are needed. The use of an appropriate cryoprotective agent mixture, called a "vitrification solution" should be useful: however, high-concentration vitrification solutions have still undesired toxic effects. Finally, an optimal cryoprotectant, in terms of cooling/warming rates and toxicity, has to be found.