The perirhinal cortex (Brodmann's area 35): Cytoarchitecture, neurochemical and neurofibrillary tangle phenotype observations in the normal and Alzheimer's disease human brain

Perirhinal cortex was described by Brodmann almost a century ago and designated area 35 in his numerical scheme. Perirhinal cortex receives projections from all sensory modalites and has strong reciprocal connections with entorhinal cortex and hippocampus. A variable sulcal topography and the concept of "transentorhinal cortex" have impeded the study of pedrhinal cortex. In this study, ventromedial temporal lobe blocks from cognitively normal subjects were sectioned and stained for Nissl substance and other neurochemical markers. Mediolateral and rostrocaudal boundaries of perirhinal cortex were studied. It was found that area 35 is divisible into a periallocortical area 35a medially and a proisocortical area 35b laterally. Our findings reveal that area 35 is located in the dorsal temporal lobe at the level of the temporal incisura. Ventrally, it lies lateral to the rhinal sulcus and on the medial bank of the collateral sulcus. Posteriorly, it shifts from the depths of the collateral sulcus to occupy the medial portion of the posterior parahippocampal gyrus adjacent to the parasubiculum. Thus, perirhinal cortex surrounds entorhinal cortex fully and is an extensive cortical entity.; It has been demonstrated in Alzheimer's disease that perirhinal cortex is the first area of cortex where neurofibrillary tangles are observed. The neurofibrillary tangle and the Alz-50 phenotype confirm our cytoarchitectural observations and, moreover, suggest that there is a continuity of the neurofibrillary tangle phenotype between the entorhinal layer 11 and isocortical layer V.; Finally, the morphology of the neurofibrillary tangle phenotype was assessed in perirhinal cortex and it was found that the pyramidal neurofibrillary phenotype dominates over the stellate neurofibrillary tangle phenotype in Alzheimer's disease. Further, in normal brains, pyramidal neurons outnumbered stellate neurons in this area. Thus, our data suggests that the pyramidal neurolibrillary tangle phenotype is the most vulnerable to neurofibrillary tangle formation in perirhinal cortex. Its topography and continuity with a distinct neurofibrillary tangle phenotype in layer V of the adjoining isocortex has implications for the evolution of pyramidal neurons in the cerebral cortex.