Cytoarchitectonic alterations in the supramarginal gyrus of late onset Alzheimer's disease

The intellectual status of 28 women of over 75 years of age had been prospectively assessed by the Blessed test score. It ranged from nearly normal to deeply altered by dementia. After autopsy, the supramarginal gyrus was marked at the surface of the brain. Sections, 1 cm thick, were cut with a specially devised macrotome. The volume of the parietal lobe was measured by a point counting method, using Cavalieri principle. A sample from the supramarginal gyrus was taken from the previously marked area and the shrinkage due to the histological procedures was measured (it averaged 12%). More than 500 nucleolated neuronal profiles per case were mapped with a semi-automatic system. Density maps of the neuronal profiles were drawn and mean density was calculated using Dirichlet tessellation. The thickness of the cortical ribbon was standardized on the maps. The density of the neurons per unit volume was calculated, taking into account the section thickness measured for each sample with a length gauge fastened to the Z axis of the microscope. Statistical correlations were sought between the mean and laminar densities of the neurons on one hand, and Blessed test score, the densities of neurofibrillary tangles (NFT) and of senile plaques profiles, on the other hand. Finally, the total number of neurons present in the parietal lobe was estimated in each case. Neuronal loss appeared to be linked with the density of the NFT (r = -0.52; P < 0.004). The correlation was mainly due to a severe drop in neuronal number observed in the cases with more than 5 NFT/mm2. An average difference of 98 x 10(6) neurons per parietal lobe was found between the cases with less than 5 NFT/mm2 and those with more. The neuronal loss predominated in layers II and III (upper part). A multivariate analysis showed that the intellectual status was better correlated with the density of the tangles than with the neuronal loss.