The relationship between freezing rate, ultrastructure and recovery in a human diploid cell line has been studied by freezing cells at rates that are known to give high and low recoveries and examining them immediately after thawing. Some correlation was found between structural damage and recovery. The main types of damage observed were loss of cytoplasm and nucleoplasm, indicating disruption of cellular membranes, and swelling of subcellular organelles due to osmotic changes during the freeze-thaw cycle. No simple correlation was found between freezing rate and structural damage. In the absence of a cryoprotectant both rapid and slow freezing produced similar types and amounts of damage resulting in low recovery. In the presence of 10% dimethylsulphoxide, however, slowly frozen cells showed few signs of damage and recovery was high. DMSO had no such protective effect on rapidly frozen cells.

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