Light and electron microscopic examination of amyloid-rich primitive plaques: comparison with diffuse plaques.

In Alzheimer-type dementia brains, numerous "amyloid-rich primitive plaques (PPs)" were observed with beta-protein immunostaining and periodic-acid methenamine (PAM) staining. These amyloid-rich primitive plaques were accompanied by various degrees of small argyrophilic rod-like, granular or filamentous structures. Routine and modified-PAM electron microscopy revealed many bundles and flecks consisting of amyloid fibrils scattered widely throughout the plaques. Degenerate neurites, astrocytic processes and bundles of glial fibres also participated in the formation of the plaques. The similarities and differences between these amyloid-rich primitive plaques and diffuse plaques are described.