The nervous system and aging.

1. In the human and some animal species, there is neuronal cell loss with aging. 2. This decrease is area-specific, does not occur equally throughout the central nervous system, and occurs at different rates within different time periods. Therefore, the decrease is area-specific, time-specific and rate-specific. 3. Occurring along with neuronal decrease, although the time sequences have not been examined, is a retraction or loss of dendritic processes involving especially the horizontal dendrites of pyramidal cells. 4. The cell loss in a central nervous system area appears to be related to the embryologic development of that area. This is demonstrated in cerebral cortex, where the greatest amount of cell loss occurs in layers 2 and 4 of the cortex, areas which develop later in the organization of the cerebral cortex. 5. Cateholamine-containing cells within the brain stem (locus coeruleus) decrease in cell number, a feature not demonstrated by other cells in the brain stem. 6. Lipofuscin increases in neurons with age, though not in agreement with accounts in the neuropathology literature. 7. The consequence of lipofuscin accumulation is not understood at this time, but, based on comparison with cell population, this factor does not appear to be responsible for "death of the cell."