ABSTRACT
The amyloidoses are a heterogeneous group of protein misfolding disorders that share common attributes. They all have extracellular eosinophilic deposits that stain with Congo red imparting a unique green birefringence and dichroism in such stained preparations when viewed in a polarizing microscope (160). This is in contrast to elastic tissue and some other components of connective tissue that also have an affinity for Congo red. Amyloid also stains metachromaticallywith certain triphenylmethane (crystal violet) and thiazine dyes (toluidine blue), most consistently when fresh unfixed tissue is examined. Amyloid is often autofluorescent or exhibits fluorescence after staining with thioflavin-T or thioflavin-S. In tissue, all types of amyloid appear by transmission electron microscopy (TEM) as fine nonbranching fibrils (approximately 10 nm in diameter) organized in random array. The diameter of isolated amyloid fibrils (7-7.5 nm) is thinner than those in tissue sections and a subunit of about 3.5 nm has also been identified.
