Tangles and plaques in healthy aging and Alzheimer's disease: Independence or interaction

Although amyloid plaques and neurofibrillary tangles are the hallmark of Alzheimer's disease (AD), they also occur in healthy, nondemented aging. Tangles are present in all brains over the age of 55, concentrated in specific structures around the hippocampus. The number of tangles increases with age, and with dementia severity in AD, but their relative distribution remains the same. Plaques are not ubiquitous in nondemented cases, but are prominent in all demented cares. While the incidence of plaques increases with age, their density is not related to age or dementia severity. Unlike tangles, plaques develop in the neocortex, with fewer in the hippo-campus. The differential distribution of plaques and tangles indicates independent initial formation during non-demented aging. In AD amyloid may accelerate tangle formation.