DSC measurement of cell suspensions during successive freezing runs: implications for the mechanisms of intracellular ice formation.

The formation of intracellular ice in biological cells during freezing is considered almost universally lethal and is the major contributor to cell damage at high cooling rates. Despite its importance, our understanding of the mechanisms of intracellular ice formation (IIF) is still incomplete. In this paper differential scanning calorimetry is used to study IIF in human lymphocytes in the presence of dimethyl sulfoxide (Me2SO). Under conditions where damage due to IIF on the initial cooling run is 40-60%, the samples are studied as a function of multiple successive cooling runs. This enables the study not only of the cell fraction which undergoes IIF, but also of the fraction which survives. The temperature at which IIF occurs and the fraction of cell volume which undergoes IIF are analyzed as functions of successive cooling runs. Taking advantage of the large number of cells present in the samples (ca. 10(6)), the effect of successive cooling runs on susceptibility to IIF is examined.