How is amyloid defined?

Updated: May 09, 2019
  • Author: Robert O Holmes, Jr, DO; Chief Editor: Herbert S Diamond, MD  more...
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Amyloid is defined as in vivo deposited material distinguished by the following:

  • Fibrillar appearance on electron micrography

  • Amorphous eosinophilic appearance on hematoxylin and eosin staining (see first image below)

  • Beta-pleated sheet structure as observed by x-ray diffraction pattern

  • Apple-green birefringence on Congo red histological staining (see second image below)

  • Solubility in water and buffers of low ionic strength

    Amorphous eosinophilic interstitial amyloid observ Amorphous eosinophilic interstitial amyloid observed on a renal biopsy.
    Congo red staining of a cardiac biopsy specimen co Congo red staining of a cardiac biopsy specimen containing amyloid, viewed under polarized light.

All types of amyloid consist of one major fibrillar protein that defines the type of amyloid. Polymorphisms that slightly vary native peptides or inflammatory processes set the stage for abnormal protein folding and amyloid fibril deposition. [2] Many classic eponymic diseases were later found to be related to a diverse array of misfolded polypeptides (amyloid) that contain the common beta-pleated sheet architecture.

Native or wild-type quaternary protein structure is usually born from a single translated protein sequence with one ordered conformation with downstream protein interactions. [3] However, amyloid fibrils can have different protofilament numbers, arrangements, and polypeptide conformations. [4] It is also important to understand that the same polypeptide sequence can produce many different patterns of interresidue or intraresidue interactions. [3]

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