Electron micrograph of diffuse plaques. Initial stage of senile plaque formation in the Alzheimer brain.

The ultrastructure of diffuse plaques (devoid of both amyloid core and swollen neurites) in the frontal cortex of an Alzheimer brain was examined, using paired routine electron microscopic ultrathin sections and adjacent 0.4-micron-thick semithin sections. The semithin sections, treated with formic acid and beta protein immunostain, showed abundant diffuse plaques. In the adjacent ultrathin section, diffuse plaques usually showed some scattered bundles of amyloid fibrils between focally blurred membranes of cell process. The diffuse plaques were nearly undetectable without the semithin beta protein immunostained preparations, because the neuropils within these plaques appeared almost normal morphologically. Capillaries, which rarely appeared in the center of the diffuse plaques, demonstrated no amyloid fibrils in their walls. These findings suggest that in the initial stages of senile plaque formation amyloid fibrils are formed sporadically between cell processes, but not around the capillaries.