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 combinatio ns of dehydration, freezing point depression, supercooling and intracellula r vitrification. Artificial cryopreservation achieves intracellular vitrifi cation with rapid cooling, modest osmotic contraction and,, often, added cr yoprotectants. High warming rates are required to avoid crystallization dur ing warming. Environmental cooling is much slower and temperatures less col d, but environmental freezing damage is important ecologically and agronomi cally. For modest sub-freezing temperatures, supercooling sometimes allows survival. At lower temperatures, extracellular water usually freezes and ce lls may suffer large osmotic contractions. This contraction concentrates so lutes and thus assists vitrification, but is not necessarily reversible: th e rapid osmotic expansion during thawing may rupture membranes, Further, me mbranes and other ultrastructural elements may be damaged by the large, ani sotropic mechanical stresses produced when their surfaces interact via hydr ation forces. Salutes reduce these stresses by osmotic, volumetric and othe r effects. (C) 2001 Elsevier Science Ltd and IIR. All rights reserved.