Paper I
Question
Staining characteristics of amyloid
Answer
Amyloid is an extracellular proteinaceous deposit composed of misfolded protein fibrils in an antiparallel beta-pleated sheet configuration, which gives it distinctive staining properties.
Congo red stain
- The classic and most important diagnostic stain
- On light microscopy, amyloid stains a pink-red colour
- Under polarized light, Congo red-stained amyloid shows characteristic apple-green birefringence — the gold-standard confirmatory finding, directly attributable to the regular beta-pleated sheet arrangement of the fibrils
Metachromatic stains
- Crystal violet and methyl violet stain amyloid a pink-violet/reddish colour, different from the blue-violet colour of the surrounding tissue (metachromasia)
Fluorescent stain
- Thioflavin T/S binds amyloid and produces intense yellow-green fluorescence under ultraviolet light — a highly sensitive screening method, though less specific than Congo red
Immunohistochemistry
- Used to type the specific amyloid protein present (e.g., anti-kappa/lambda light chain for AL amyloid, anti-serum amyloid A for AA amyloid, anti-transthyretin for familial/senile amyloid) — essential for determining the underlying cause and guiding treatment
Electron microscopy
- Demonstrates the characteristic non-branching fibrillar ultrastructure of amyloid, approximately 7.5–10 nm in diameter