Neocortical cell counts in normal human adult aging

Fifty‐one brains from clinically and neuropathologically normal individuals ranging in age from 24 to 100 years were studied to determine what changes occur in neocortical neuroectodermal cell populations as a function of normal aging. Twenty‐m̈‐thick sections from the midfrontal, superior temporal, and inferior parietal areas were examined with an image‐analysis apparatus with combined manual and automatic editing capacity. Neuroectodermal cells were counted, measured, and assigned to one of ten categories, which were later summarized in three: large neurons (<90 m̈2), small neurons (41 to 90 m̈2), and glia (5 to 40 m̈2). Determinations were also made of brain weight, cortical thickness, neuronal density, neuron‐‐glia ratio, and percentage of cell area. The results showed statistically significant age‐related decrements in the following values: brain weight, cortical thickness in the midfrontal and superior temporal areas, large neurons in all three areas, and the neuron‐‐glia ratio in the midfrontal and inferior parietal areas. The total number of neurons, neuronal density, and percentage of cell area were all unchanged. Increasing with age were the number of small neurons in the midfrontal cortex and glia in the midfrontal and superior temporal areas. The following conclusions were drawn: (1) Aging affects the frontal and temporal lobes more than the parietal; the salient change is shrinkage of large neurons with consequently increasing numbers of small neurons; (2) constant neuronal density coupled with diminished cortical volume (decreased brain weight and cortical thinning) indicate that there is some neuronal loss with age, but it is of much lesser magnitude than previously supposed; and (3) the number of glia increases with age.