What are the histologic features of amyloidosis?
Macroscopically the kidneys are enlarged, firm, and pale, and may be waxy. On light microscopy AA and AL amyloid have the same morphologic features and involve mainly the glomerulus and blood vessels, with less involvement from the tubulointerstitium. The glomerular deposits are seen predominantly in the mesangium as amorphous acidophilic deposits, weakly periodic acid–Schiff positive and negative or weakly positive with Silver stain ( Fig. 39.3 A). There is usually extension of the deposits along the peripheral capillary wall and the deposits form delicate spikes on the outer surfaces. The classical diagnostic test is the Congo red stain, which shows an orange-red color and apple-green birefringence when examined by polarized microscopy. Potassium permanganate bleaches AA amyloid. Specific immunohistochemistry tests identify Ig LCs or amyloid A protein, with the majority of AL amyloidosis caused by lambda LC deposits. Electron microscopy shows the distinctive amyloid fibrils, which are non-branching, are randomly arranged, measure 9 to 12 nm, and have electrolucent cores but cannot distinguish between AA, AL, and AH amyloidosis
