Cellular cryobiology: thermodynamic and mechanical effects

Several physical stresses kill cells at low temperatures. Intracellular ice is usually fatal, so survival of freezing temperatures involves combinations of dehydration, freezing point depression, supercooling and intracellular vitrification. Artificial cryopreservation achieves intracellular vitrification with rapid cooling, modest osmotic contraction and, often, added cryoprotectants. High warming rates are required to avoid crystallization during warming. Environmental cooling is much slower and temperatures less cold, but environmental freezing damage is important ecologically and agronomically. For modest sub-freezing temperatures, supercooling sometimes allows survival. At lower temperatures, extracellular water usually freezes and cells may suffer large osmotic contractions. This contraction concentrates solutes and thus assists vitrification, but is not necessarily reversible: the rapid osmotic expansion during thawing may rupture membranes. Further, membranes and other ultrastructural elements may be damaged by the large, anisotropic mechanical stresses produced when their surfaces interact via hydration forces. Solutes reduce these stresses by osmotic, volumetric and other effects.