The regulation of axonal growth in the mature mammalian nervous system.

Studies of a specific neuronal rearrangement that occurs in the mature rat nervous system have led to conclusions regarding the regulation of axonal growth in maturity and aging. In this brief review, the results of recent experiments are summarized and the general conclusions presented. One hypothesis that has emerged from these studies is that the extent of growth of one portion of a neuron's axonal arbor may affect the potential for growth at another portion of the arbor. In addition, evidence has been obtained in favor of the hypothesis that the decline in neuronal plasticity that occurs in aging is primarily due to a decrease in the support of axonal growth by the CNS environment. Finally, evidence is reviewed for the presence of growth-inhibiting activity of CNS white matter. Each of these findings has potential implications for understanding the regulation of axonal growth in the mature and aging brain.