Morphology and neurochemical characteristics of the vulnerable neurons in brain aging and Alzheimer's disease.

Since the classical descriptions of the symptomatology and neuropathologic characteristics of Alzheimer's disease (AD), we have witnessed a considerable increase in the knowledge of the morphologic and molecular features not only of AD, but also of normal brain aging. In spite of this progress, the pathogenetic events that differentiate normal brain aging from the early stages of dementia and may lead eventually to AD are not completely understood. This article reviews the possible relationships between the localization of cellular pathologic changes in AD and the distribution of neuronal components of the hippocampal and neocortical circuitry that are affected by these alterations. The relative vulnerability or resistance of a given neuronal type to the degenerative process is discussed in order to provide correlates of the distribution of cellular pathologic changes, neurochemical phenotype related to vulnerability, and affected hippocampal and neocortical circuits.