Congo red method for demonstrating amyloid in paraffin sections

A rapid, simple, and immunocytochemically compatible staining method for amyloid applicable to sections of human specimens embedded in paraffin is described. Human necropsy material was fixed in buffered formalin, sectioned at 5-10 mu m, mounted on slides, deparaffinized, and partially hydrated (70% ethanol). After partial hydration, sections were stained for amyloid for 10 min in a solution of 0.2% Congo red in 70% isopropanol. Sections were then washed and counterstained for 10 min in a solution of 0.01% thionin, 0.01% cresyl violet, 0.05% methyl green, 10% formaldehyde, and 0.5% acetic acid. After washing and dehydration, sections were cleared in eucalyptol and mounted in Eukitt. Amyloid deposits appeared pink to red whereas cell nuclei and Nissl substance stained blue. This simple and consistent stain provides excellent nuclear staining that is useful for light microscopy observation and photography. With polarizing microscopy amyloid deposits show the characteristic green dichroism. The present method can be used in combination with pre- or post-immunocytochemical techniques.