Morphological alterations in neurons forming corticocortical projections in the neocortex of aged Patas monkeys.

Recent studies indicate that the cognitive processes mediated by the prefrontal cortex, such as working memory, are impaired during normal aging. These disturbances in cortical function may be a consequence of abnormalities in neocortical circuits, even though the numbers of cortical neurons are preserved in normal aging. We performed retrograde tract-tracing of cortical projections connecting the temporal cortex to the prefrontal cortex in combination with dye-filling and three-dimensional neuronal reconstructions in aged patas monkeys. Age-related changes affected the apparent complexity of the apical dendrites of projection neurons and caused a significant loss of dendritic spines at all levels of their dendritic trees. These results indicate that normal aging is accompanied by neuronal changes that are quite subtle, and possibly involves discrete cellular components of certain cortical neurons selectively rather than inducing major alterations such as cell death.