Biochemistry of natural freeze tolerance in animals: molecular adaptations and applications to cryopreservation.

For a wide variety of animals, winter survival in cold climates includes the ability to tolerate ice formation in extracellular body fluids. Among terrestrially hibernating vertebrates, freeze tolerance has been documented for five amphibian and two reptile species. These species may survive for days or weeks in a frozen state with no breathing and no heart beat, and with up to 65% of total body water as extracellular ice. The biochemical mechanisms involved in natural freeze tolerance include (i) the regulation of extracellular ice formation by proteinaceous ice nucleators in body fluids, (ii) the accumulation of high concentrations of low molecular weight carbohydrates as cryoprotectants to regulate cell volume reduction during freezing and stabilize macromolecular structure, and (iii) a well-developed ischemia tolerance that supports the survival of individual organs while frozen. The present article focuses on recent advances in our understanding of the biochemistry of natural freeze tolerance in lower vertebrates and the application of these studies to the improvement of cryopreservation technology for transplantable mammalian organs.