Application of thiazole dyes to amyloid under conditions of direct cotton dyeing: correlation of histochemical and chemical data.

The fluorescent brightening agent Phorwhite (Blankophor) BBU imparts intense selective fluorescence to amyloid, but this modern reagent is no longer readily available on the biological dye market. Conventional Thioflavine S and T stains require differentiation and are not specific. To improve selectivity, direct and cationic thiazole dyes were substituted in the alkaline Congo Red and the Phorwhite BBU procedure. With the former technic Diphenyl Brilliant Yellow 8G, Clayton Yellow, Thiazol Yellow, Thioflavine T and Seto Flavine T imparted strong to intense selective fluorescence to amyloid. Under the conditions of the Phorwhite BBU reaction these dyes were suitable only for formalin-fixed amyloid. Several thiazole dyes did not fluoresce. Fluorescence is a function of the molecular orbital system, the thiazole rings per se cannot induce fluorescence. Paper chromatograms indicated two or more fractions in the dyes studied. Different samples of the same dye can vary significantly in their staining and fluorescence properties. This heterogeneity is inherent in the mode of synthesis. In some cases the cationic thiazole dyes rendered certain amyloid deposits, e.g. in vessel walls, intensely fluorescent; other amyloid deposits in the same sections showed only weak fluorescence. Further studies are required to correlate these peculiar patterns with immunological data on amyloid types.