What are the staining methods for identifying amyloid in tissue sections?

The three main staining methods for identifying amyloid in tissue sections include Direct Dyes, Fluorescent Dye Staining, and Amyloid Metachromasia. 

Direct Dyes

Staining with Congo red is the most common method used for identifying amyloid tissue sections. This dark reddish-brown, water-soluble, acid dye is the sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid. It belongs to the class of anionic dyes known as ‘direct dyes’ and stains cytoplasm pink. 

Common alternatives to Congo red include Sirius red F3B, Sirius Scarlet GG, and Benzo Scarlet 4BNS.

Fluorescent Dye Staining 

Fluorescent dye staining is a highly sensitive method for visualizing various materials. Its higher sensitivity is partly the result of the stained material glowing more clearly against a darker background, causing it to stand out more vividly.  

While fluorescent stains may not stain amyloid as brightly as it stains some other materials, they still enhance visibility significantly, so it can be seen more clearly. Thioflavine T is typically the preferred dye for this purpose, though Thioflavine S may also be used. Both dyes bind specifically to amyloid fibrils and emit fluorescence on exposure to certain wavelengths of light, enabling visualization of amyloid deposits in tissues.

Amyloid Metachromasia

In amyloid metachromasia, the tissue component stains in a color that’s different from the dye used. Toluidine blue and azure dyes are the most effective dyes for amyloid metachromasia. Crystal violet also produces significant metachromasia, resulting in a distinctive deep red-purple color on a blue background. The metachromatic effect in amyloid staining with crystal violet appears to result from hydrogen bonding and the regular alignment of dye molecules.