Changes in cell density within the human hippocampal formation as a function of age.

In this study the entire hippocampal formation, both grey and white matter, was analyzed as a unit and the number of neurons and glia per unit weight was determined. Tissues were obtained from the right cerebral hemispheres of 25 human brains. We found that the neuron population density increases slightly between 20 and 87 years of age. Previous studies using classical stereological techniques or electron-optical image analysis of histological sections have shown no increase of neuron density of the grey matter of the hippocampus with increasing age. However, our data, based upon a technique of cell dispersion and hemacytometer counting, are not strictly comparable. We attribute our findings of an apparent increase in neuron density with age to the fact that we analyzed the entire hippocampal formation, both with white and grey matter, which had undergone senescent shrinkage--particularly within the white matter. The findings of the present study are to be contrasted with an earlier study of the aging visual cortex using 20 of the same brains and employing the same techniques. Taken together, the two reports suggest that age-dependent neuron loss takes place at very different rates in different areas of the human brain and that the mean life span of neurons in the hippocampus is considerably longer than that of the neurons in the visual cortex.