Intralysosomal formation of amyloid fibrils.

Unusual inclusions, which occurred in the reticuloendothelial cells intimately associated with fresh amyloid deposits, were analyzed by electron microscopy. The inclusions were located in the areas rich in the primary lysosome type of dense bodies and the cytoplasmic invaginations containing well-oriented amyloid fibrils. They were single-membrane-bounded, measured 0.3 to 0.8 mu in width and 0.5 to several microns in length, and showed considerable variation in the electron density of their contents. The latter consisted of two different ultrastructural elements: fibrillar profiles and a homogeneous or finely granular electron-dense substance. The fibrillar profiles were virtually identical in ulstrastructure to the amyloid fibrils and were well-oriented parallel to the long axis of the inclusion. The homogeneous or finely granular electron-dense substance appeared to be comparable to that composing the dense body matrix. The inclusions were usually acid phosphatase positive, but did not take up intravenously injected Thorotrast particles. These data led us to conclude that these inclusions were transitional forms from the usual dense bodies to the deep cytoplasmic invaginations containing well-oriented amyloid fibrils (which are accepted by most investigators as the sites of amyloid formation) and thus constitute direct evidence for the involvement of lysosomes in amyloid fibril formation.