A HIGH-RESOLUTION ULTRASTRUCTURAL COMPARISON OF ISOLATED AND IN-SITU MURINE AA AMYLOID FIBRILS

There is an inconsistency between the ultrastructural organization of AA amyloid fibrils that have been isolated, which are composed of a sl owly twisting set of two or more protofibrils, and those seen in situ, which are tubular entities with a tight helical substructure. In this study, the ultrastructure of fibrils isolated from experimental murin e AA amyloid were observed at high resolution and compared with those seen in situ in the hope of clarifying the reason for this inconsisten cy. The fibrils in situ were composed of a microfibril-like 8-9 nm wid e core covered by a layer of heparan sulfate proteoglycan (HSPG) to wh ich 1 nm wide filaments, immunohistochemically identified as AA protei n, were externally associated. Following isolation with the standard d istilled water washing procedure, the HSPG layer and AA protein filame nts detached from their core and dispersed into the water. The remaini ng denuded variously loosened cores lost their typical appearance. In distilled water the detached 1 nm wide AA protein filaments became qui re conspicuous and coiled themselves into 3 nm wide right helices whic h in turn assembled into the characteristic slowly twisting sets of tw o parallel protofibrils similar to that previously reported as ''isola ted amyloid fibrils''. The results emphasize that great caution must b e taken in extrapolating amyloid fibril structure from isolated prepar ations to in situ tissue conditions.