The term amyloidosis refers to a fascinating group of disorders that share a common pathogenesis of extracellular deposition of amyloid material. Fundamentally, it is a disorder of the secondary structure of select proteins whereby the amyloidogenic proteins are misfolded into a β-pleated sheet configuration, resulting in the formation of insoluble extracellular amyloid fibrils. The amyloid fibrils appear as amorphous eosinophilic material when hematoxylin and eosin–stained tissue is examined under light microscope. Electron microscopy reveals remarkable similarity between the amyloid fibrils derived from different precursor proteins in that they range from 7.5 to 10 nm in diameter. This ultrastructural similarity is the underlying basis for the characteristic red-green birefringence with Congo red staining observed under polarized microscopy, the pathological hallmark of the disease.
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