Kinetics of water loss and the likelihood of intracellular freezing in mouse ova

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Abstract
To avoid intracellular freezing and its usually lethal consequences, cells must lose their freezable water before reaching their ice-nucleation temperature. One major factor determining the rate of water loss in the temperature dependence of the water permeability,L p (hydraulic conductivity). Because of the paucity of water permeability measurements at subzero temperatures, that temperature dependence has usually been extrapolated from above-zero measurements. The extrapolation has often been based on an exponential dependence ofL p on temperature. This paper compares the kinetics of water loss based on that extrapolation with that based on an Arrhenius relation betweenL p and temperature, and finds substantial differences below −20 to −25°C. Since the ice-nucleation temperature of mouse ova in the cryoprotectants DMSO and glycerol is usually below −30°C, the Arrhenius form of the water-loss equation was used to compute the extent of supercooling in ova cooled at rates between 1 and 8°C/min and the consequent likelihood of intracellular freezing. The predicted likelihood agrees well with that previously observed. The water-loss equation was also used to compute the volumes of ova as a function of cooling rate and temperature. The computed cell volumes agree qualitatively with previously observed volumes, but differ quantitatively.