Identification and detection of in situ cellular and regional differences of lipid composition and class in lipid-rich tissue using hot stage polarizing light microscopy.

To determine whether in situ tissue lipid characterization is possible, we examined carefully prepared frozen sections from a variety of lipid-rich tissues of the cholesterol-fed rabbit by hot stage polarizing light microscopy and conventional histologic staining. Heating of frozen sections to less than 60 degrees C did not affect tissue architecture or staining characteristics making pathologic and physical chemical correlations possible. The melting temperatures of cholesterol ester inclusions in individual foam cells in rabbit atherosclerotic lesions and adrenal gland could be determined as well as the melting characteristics of crystals and triglyceride in these and other tissues. Differential scanning calorimetry and polarized light transmittance were used to confirm melting temperatures determined by microscopy. Combining data from histologic staining, polarizing light microscopy, and the thermal characteristics of lipid enables the various lipid classes to be identified within individual cells. Differences in melting temperatures between lipids of the same class give indications of the degree of saturation of the lipids. Regional differences of cholesterol ester-melting temperatures in the chow-fed rabbit adrenal cortex were detected which implied differences in chemical composition. Cholesterol feeding raised the melting temperature and tended to abolish the marked regional differences in melting temperature of the cholesterol esters in the adrenal cortex. Rabbit atherosclerotic lesions, induced by balloon deendothelialization and cholesterol feeding, revealed differences in foam cell-melting temperatures within the same lesion. Melting temperatures of cholesterol ester deposited in the liver were more uniform. Each tissue studied revealed distinctly different cholesterol ester-melting characteristics.