Long-term status of pyramidal cell axon collaterals and apical dendritic spines in denervated cortex

Abstract

Cortices of animals in 3 groups were denervated by undercutting at 4 or 40 days of age or at adulthood. Months later electrocortical seizures were studied in terminal experiments and cortical tissues prepared for light microscopic study. Four measures, assumed to be descriptive of or related to axon collateral proliferation, were made: number of axon collaterals, length of collaterals, number of collateral branches and number of apical dendritic spines. Data from adult intact cortex served as controls. Increases in number of collaterals and collateral length were observed, the best evidence being in the 40-day animals. However, since collateral branching decreased, the data do not support the concept of proliferation as described in young brains. If dendritic spines are valid indicators of synaptic contacts then the observed significant loss of spines argues against the maintenance of functional connections, if indeed they were established, following denervation at any age. There were no obvious relationships between morphological changes and duration of afterdischarges. A persistent alteration was frequently seen in cortex which had been undercut at 4 days of age. Lower lying pyramidal cell axons bifurcated with nonrecurrent spread parallel to the glia infiltrated site of the old fiber paths. This glial barrier appeared to have prevented centripetal axon growth.