Vulnerability of select neuronal types to Alzheimer's disease.

From a morphological perspective, Alzheimer's disease (AD) is primarily a degenerative disorder of the neuronal cytoskeleton involving lipofuscin-laden cortical projection neurons with long, thin, and sparsely myelinated axons. The neocortical primary fields, relatively small in extent but functionally sophisticated, exhibit an early and brief myelination cycle, whereas the much more expansive but relatively simply organized association areas undergo a late and prolonged myelination process. The greater the degree of myelination and the less intense the pigmentation, the more resistant a given projection neuron may be to oxidative stress as well as to the development of AD-related neurofibrillary changes and vice versa. The neurofibrillary pathology commences from those cortical areas that are less completely myelinated and gradually progresses to the most functionally developed cortical fields that display the highest degree of myelination, thereby reflecting a hierarchy in the susceptibility of diverse cortical areas to the evolution of the AD-associated cytoskeletal pathology.