The structure of neuritic plaque in the cerebral cortex of aged rats.

Discrete patches of spongiform degeneration have been found in the cerebral cortex of three rats, 28 and 30 months of age. Many of these patches have in their midst a central nonvacuolated region containing a star-shaped homogeneous core. Using Congo red stain, there is evidence for the presence of amyloid in this core. In the electron microscope, this central region is seen to be composed of degenerating and abnormal neuronal processes, reactive neuroglia, and extracellular filamentous material with the fine structural characteristics of amyloid. Microglial cells are uniquely modified to accommodate bundles of these extracellular filaments, which are often contained in furrows that deeply invaginate the surfaces of these cells. On the basis of its three main constituents, the central region is considered to be a form of neuritic plaque. The large complex clearings in the vacuolar region of each lesion appear to be formed by the coalescence of smaller vacuoles in the surrounding neuropil.