Re-Evaluation of the Structural Organization of Neuritic Plaques in Alzheimerʼs Disease

We re-examined the relationship among synaptic pathology, subcellular abnormalities within neurites in the plaques and β-amyloid deposits of Alzheimer's disease (AD) using laser confocal imaging and computer-aided serial section reconstruction techniques. Analysis of serial optical sections of neuritic plaques double-immunolabeled for anti-β-amyloid/anti-tau-2 revealed that 35% of this type of plaque contained a dense amyloid core with clusters of peripheral abnormal neurites. The other 65% were without a dense core and were mainly composed of abundant abnormal neuritic clusters with bundles of amyloid distributed throughout the neuritic plaque. While two-dimensional (2-D) analysis of the plaques has suggested that neurites are distributed in the plaque periphery with β-amyloid localized in its center, serial section analysis showed the opposite arrangement can also be true. Three-dimensional (3-D) reconstructions of serial optical sections showed that the tau-positive tortuous axons clustered in the neuritic plaques were often continuous with synaptophysin-positive distended terminals. Analysis of electron micrographs from serial sections showed continuity among the different segments of the neurites. Further analysis of the computer generated 3-D reconstructed neuritic plaques (both from serial electron micrographs and serial optical sections), viewed as continuous rotating loops, confirmed that a great majority of the plaque volume was occupied by the clustered and continuous abnormal neurites, while the amyloid fibrils were compressed and displaced to the periphery of the plaque. The 3-D imaging of the neuritic plaques in AD suggests a more widespread and active neuritic damage than that predicted from 2-D observations. This more complete view supports a theory in which synapto-axonal damage plays an important role in the pathogenesis of the plaque.