MICROGLIAL CELL ACTIVATION IN AGING AND ALZHEIMER-DISEASE - PARTIAL LINKAGE WITH NEUROFIBRILLARY TANGLE BURDEN IN THE HIPPOCAMPUS

Microglial cells are the main component of the brain's resident immune system and are activated in Alzheimer disease (AD). We quantified the density of activated microglial cells (AMG) in 8 sectors of human hip pocampus to determine if their density is correlated with senile plaqu e (SP) and neurofibrillary tangle (NFT) formation. Ferritin-stained mi croglia, Bielschowsky-stained neuritic plaques, and perikarya containi ng NFTs were counted in 8 young adults, 9 nondemented elderly adults, and 9 demented patients with AD. Microglial cell activation was modera tely higher in elderly nondemented subjects. In AD there was a more st riking activation in all sectors of the hippocampus. Most AMGs were di stributed diffusely in neuropil and were not delimited to SPs or NFTs. Senile plaque counts were not linked with AMG counts within any secto r. Neurofibrillary tangle counts were correlated significantly with AM G counts within one sector, the subiculum. When variations within and between sectors were factored out statistically, the burden of AMGs wa s correlated significantly with the burden of NFTs (r = 0.34; p < 0.00 5), but not SPs. Neuropathologic changes at the origin of the perforan t pathway were correlated significantly with orthograde microglial cel l activation in the termination field. These observations show that co rrelations between microglial cell activation and pathologic features of AD are only rarely significant. When significant linkage was presen t, it involved NFTs and not SPs, and depended on which sector of hippo campus was examined.