Amyloid b Protein (Ab) Starts to Deposit As Plasma Membrane-Bound Form in Diffuse Plaques of Brains from Hereditary Cerebral Hemorrhage with Amyloidosis-Dutch Type, Alzheimer Disease and Nondemented Aged Subjects

To clarify where and how b-amyloid begins to deposit in senile plaques, we examined the ultrastructural localization of amyloid b protein (Ab) in diffuse plaques of brains with hereditary cerebral hemorrhage with amyloidosis-Dutch type, Alzheimer disease (AD), and from nondemented aged subjects. Serial ultrathin sections of osmium-plastic blocks were immunogold-labeled for Abx-42 (Ab42), and sections on grids were observed under the electron microscope (EM) after observing the exact localization of the diffuse plaques in sections on glass slides by the reflection contrast microscope. Ab42 deposition, which was decollated with gold particles, appeared in 3 forms in all subjects under the EM: 1) Scattered small bundles of amyloid fibrils between cell processes, frequently seen in the densely stained area of diffuse plaques. 2) Scattered small foci of nonfibrillar materials between cell processes as a relatively minor form. 3) Ab42 on a part of the cell surface plasma membrane of normal appearing cell processes, a major form in weakly immunostained areas. The last form was not associated with degenerative neurites or reactive glia. Ab42 deposition on the cell surface plasma membrane appears to be an initial event in diffuse plaques, and then it develops into amorphous/fibrillar amyloid between cell processes.