Change of intracellular fluidity during keratinocyte differentiation measured by fluorescence polarization

The fluorescence polarization method was applied to measure the intracellular fluidity of fractionated guinea pig keratinocytes. Guinea pig epidermal cell suspension was obtained by treatment with EDTA and trypsin, and was separated into high, intermediate, and low density fractions using Percoll density gradient centrifugation. Morphological observation and cytofluorometric analysis of DNA content in the fractionated epidermal cells showed that the high, intermediate, and low density fractions were basal, spinous, and granular cell-rich fractions, respectively. Intracellular fluorescence polarization of each fraction was determined by a polarization spectrofluorometer (Hitachi MPF-4, prototype) with fluorescein diacetate. The P-values were calculated for high, intermediate, and low density fractions as 0.192 +/- 0.021, 0.172 +/- 0.019, and 0.147 +/- 0.012, respectively. Since low P-values indicate a high degree of fluidity, the results indicate that intracellular fluidity of keratinocytes is lower in basal cells and higher in granular cells. Dye-binding experiments showed that fluorescein-binding proteins were not detected in the soluble fraction of the epidermal cells. The present findings suggest that intracellular fluidity of the guinea pig keratinocyte increases during the process of its differentiation.