Aging and environmental influences on two types of dendritic spines in the rat occipital cortex

The density of dendritic spines was determined by counting spines on 34-μm segments from basal branches of pyramidal cells in layers II and III of the rat occipital cortex. Counts began at the first bifurcation site from the soma, and one segment from each side of the neuron was studied. The results of this investigation indicated a marked decrease in spines with a lollipop (type L) configuration from 90 to 414 days of age. After this decrease, the density of type L spines increased to 630 days to the same density as they were at 90 days. The presence or absence of type L spines was not affected by housing the animals in an enriched environment. In contrast to the type L spines, another type of spine, those with a nubbin configuration (type N) increased in density at each age. The density of these type N spines also appeared subject to environmental influences in the oldest group. We conclude that type L spines, after decreasing to the adult level, increase as the animal approaches senescence perhaps as a compensatory mechanism. On the other hand, type N spines increase with age and are responsive to the environment in the old animals. Type N spines possibly represent degenerating spines.