Ultrastructural localization of argyrophilic substances in diffuse plaques of Alzheimer-type dementia demonstrated by methenamine silver staining.

The ultrastructure of argyrophilic substances in diffuse plaques of an Alzheimer-type dementia brain was examined using methenamine silver (MS) electron microscopy with the pre-embedding method. Electron-dense substances, which were sparse aggregations of bundle-like structures with silver granules, were disseminated in the diffuse plaques. Comparison of serial MS and routine ultrathin sections revealed that diffuse plaques usually show scattered bundles of amyloid fibrils or amorphous materials, or both, between indistinct cell process membranes. Some of these processes were identified as astrocytic or dendritic in origin. A few degenerative neurites were frequently noted in large silver-positive areas but never in small areas. These findings suggest that the appearance of amyloid fibrils and amorphous materials between certain cell processes is a very early morphological change in the process of senile plaque formation.