Laminar and regional distributions of neurofibrillary tangles and neuritic plaques in Alzheimer's disease: a quantitative study of visual and auditory cortices

The number of Thioflavine S-positive neurofibrillary tangles (NFT) and neuritic plaques (NP) was determined in visual and auditory cortical regions of 8 patients with Alzheimer's disease. On both a regional and laminar basis, NFT exhibited very distinctive and consistent distribution patterns. The mean (+/- SEM) number of NFT in a 250-micron- wide cortical traverse was very low in area 17, primary visual cortex (0.9 +/- 1.0), increased 20-fold in the immediately adjacent visual association cortex of area 18 (19.7 +/- 3.6), and showed a further doubling in area 20, the higher-order visual association cortex of the inferior temporal gyrus (35.5 +/- 8.8). Similar differences in NFT number were present between primary auditory (1.6 +/- 0.5) and auditory association (18.9 +/- 5.4) regions. On a laminar basis, NFT were predominantly present in layers III and V, although there were striking regional differences in the proportion of NFT in these 2 layers. Layer III contained 79% of the NFT in layers III and V in area 18, 41% in area 20, and only 27% in area 22. In contrast, NP showed different, and less specific, regional and laminar distribution patterns. Total NP number was similar in the 3 visual areas, although there were marked regional differences in the type of NP present. Nearly 80% of the NP in area 17 was of the NPc type (i.e., contained a dense, brightly fluorescent core), whereas over 70% of the NP in both areas 18 and 21 was of the NPnc type (i.e., lacked a dense, brightly fluorescent core). NP were present in every cortical layer but were most numerous in layers III and IV. The distinctive distribution patterns of NFT are very similar to the regional and laminar locations of long corticocortical projection neurons in homologous regions of monkey neocortex. This association suggests that NFT reside in the cell bodies of a subpopulation of pyramidal neurons, namely, those that furnish long corticocortical projections. In contrast, the distribution patterns of NP suggest that multiple neuronal systems contribute to their formation.