Preservation of Tissue in vitro 1G3 



of ageing, while the donor animal wends its way to senility. 

 The idea of achieving a state of suspended animation at low 

 temperatures is, of course, very old, and depends in its modern 

 form on the reas()nal)le assumption that all vital j)rocesscs 

 would be suspended at temperatures such as those obtained 

 by the use of dry ice (-79° C.) or liquid air ( — 190° C). Until 

 recently, however, the act of freezing and thawing was fatal 

 to almost all normal cells of vertebrates, so that little pro- 

 gress was possible in the study of experimental biostasis. 

 This situation was altered radically a few years ago by the 

 observation that glycerol has remarkable properties in pro- 

 tecting living cells against the otherwise fatal effects of 

 freezing to very low temperatures (Polge, Smith and Parkes, 

 1949). 



This discovery has made it possible to keep a variety of 

 living cells at —79° C. or —190° C. in an apparently stable 

 state for periods of months or years, and the work has added 

 greatly to our knowledge of the dynamics of the living cell 

 and in particular of the effects of low temperatures. 



Resistance to Freezing and Thawing 



It appears that freezing damage is primarily of two kinds, 

 one due to the hypertonicity of the residual fluid as water 

 freezes out as ice, and the other to thermal shock arising 

 from rapid change of temperature either above or below 

 freezing point. The former can be avoided to a large extent 

 by ultra-rapid freezing, the latter by slow cooling. In other 

 words, it is almost impossible to freeze a normal cell by 

 ordinary methods without subjecting it to one or other type 

 of damage. Glycerol apparently exerts its effect by decreasing 

 the hypertonicity hazard, and permitting cooling to be slow 

 enough to avoid thermal shock (Lovelock, 1953). 



Stability at Low Temperatures 



The loss of cells on freezing and thawing without storage 

 at the low temperature may be negligible, appreciable, sub- 

 stantial or very large according to the conditions and the 



