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cryopreservation
Cryopreservation or cryoconservation is a process where cells, whole tissues, or any other substances susceptible to damage caused by chemical reactivity or time are preserved by cooling to sub-zero temperatures. At low enough temperatures, any enzymatic or chemical activity which might cause damage to the material in question is effectively stopped. Cryopreservation methods seek to reach low temperatures without causing additional damage caused by the formation of ice during freezing. Traditional cryopreservation has relied on coating the material to be frozen with a class of molecules termed cryoprotectants. New methods are constantly being investigated due to the inherent toxicity of many cryoprotectants.
By default it should be considered that cryopreservation alters or compromises the structure and function of cells unless it is proven otherwise for a particular cell population.
== Natural cryopreservation ==
Water-bears (Tardigrada), microscopic multicellular organisms, can survive freezing by replacing most of their internal water with the sugar trehalose, preventing it from crystallization that otherwise damages cell membranes. Mixtures of solutes can achieve similar effects. Some solutes, including salts, have the disadvantage that they may be toxic at intense concentrations. In addition to the water-bear, wood frogs can tolerate the freezing of their blood and other tissues. Urea is accumulated in tissues in preparation for overwintering, and liver glycogen is converted in large quantities to glucose in response to internal ice formation. Both urea and glucose act as "cryoprotectants" to limit the amount of ice that forms and to reduce osmotic shrinkage of cells. Frogs can survive many freeze/thaw events during winter if not more than about 65% of the total body water freezes. Research exploring the phenomenon of "Freezing frogs" has been performed primarily by the Canadian researcher, Dr. Kenneth B. Storey.
Freeze tolerance, in which organisms survive the winter by freezing solid and ceasing life functions, is known in a few vertebrates: five species of frogs (''Rana sylvatica'', ''Pseudacris triseriata'', ''Hyla crucifer'', ''Hyla versicolor'', ''Hyla chrysoscelis''), one of salamanders (''Hynobius keyserlingi''), one of snakes (''Thamnophis sirtalis'') and three of turtles (''Chrysemys picta'', ''Terrapene carolina'', ''Terrapene ornata'').〔 Snapping turtles ''Chelydra serpentina'' and wall lizards ''Podarcis muralis'' also survive nominal freezing but it has not been established to be adaptive for overwintering. In the case of ''Rana sylvatica'' one cryopreservant is ordinary glucose, which increases in concentration by approximately 19 mmol/l when the frogs are cooled slowly.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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