Subepithelial argyrophilic spicular structures in renal amyloidosis--an aid in diagnosis. Pathogenic considerations

Eighteen cases of amyloidosis with renal involvement were classified utilizing clinical and laboratory data as to the most likely major amyloid fibril protein type and studied as to their histological, tinctorial, immunofluorescence, and electron microscopic features. No differences could be appreciated between the AA and AL types of amyloidosis. Immunofluorescence did not aid in the diagnosis of amyloidosis and was confusing in some cases owing to apparent absorption of serum proteins. Subepithelial spicular structures were noted in the glomerular capillary loops in 14 of 18 cases (78 per cent), and similar structures were found related to tubular epithelial cells in six of these cases and were related to the parietal epithelium of Bowman's membrane in one case. These spicular structures were a valuable aid in the diagnosis of early amyloidosis by light microscopy, but electron microscopy was essential for confirmation. We postulate that because of the intense staining quality of spicular structures using argyrophilic techniques, these spicules result from a unique interaction between amyloid fibrils and locally produced substances, most probably renal epithelial basement membrane glycoprotein.