Plasticity of the aging cerebral cortex.

The results we have presented indicate that during aging the changes occurring in the different layers are similar in some instances and not in others. The environment does affect the different layers in separate ways. By using only our standard colony environment we learned that there were greater decrease in the cell number per unit area in the lower half of the cortex compared with the upper half during the first 100 days of life. Thereafter, there were no such differences at least until 650 days of age. Our results also showed that the environment can affect the dendrites in the cortical layers differently. Different dendritic patterns were noted between layers II and III and layer Vb, for example. In layer II and III there were more and longer oblique dendrites in the old standard colony animals than in the old isolated animals. Whereas, in layer Vb there were longer oblique dendrites in the isolated animals than in the standard colony animals. There were more nubbin type spines in the isolated rats than in the standard colony rats in both layers II and III and in Vb, but not in Va. The lollipop spines were always more frequent than the nubbin spines and in layer Vb there were more L spines in IC than in SC. These results continue to add information to the general pool indicating the importance of the role of the external environment in studying the aging nervous system and also the importance of considering more than one region of the brain at a time when mapping aging patterns.