Rudolph Virchow, introduced the term “amyloid” to refer to extracellular deposits in human tissues that exhibited a positive blue-violet staining reaction to iodine and dilute acid. Based on this reaction, Virchow mistakenly identified these aggregates as composed of starch (amylum is Latin for starch). Subsequent microscopic studies have shown that amyloid deposits exhibit an affinity for Congo red dyes, which also yield a property known as “dichroic birefringence” in which crossed polarizing filters produce apple-green birefringence. In the 20th century, X-ray diffraction analysis revealed amyloid to be composed of fibrils ordered in a beta pleated sheet conformation. Using the criteria of “congophilia,” dichroic birefringence, and fibrillar morphology, more than 30 biochemically distinct forms of amyloid have been identified, many associated with unique clinical syndromes. Examples of protein that can form an amyloid configuration include: immunoglobulin light chain, amyloid A, beta-amyloid, transthyretin, apolipoprotein A-I, lysozyme, and fibrinogen A α-chain. Identification of the amyloid protein can have important diagnostic, prognostic, and therapeutic implications.
|Sural nerve in a case of amyloid neuropathy: congophilic amyloid circled|