Extensive elongation of axons from rat brain into peripheral nerve grafts

Abstract

The failure of axons to elongate in the injured central nervous system (CNS) of adult mammals restricts drastically the establishment of connections with target tissues situated more than a few millimetres away. Mechanisms that include a primary inability of some nerve cells to support renewed axonal growth, a premature formation of synapses on nearby neurones¹, an obstruction caused by the formation of a glial scar^2,3 and other influences of the microenvironment^4–7 are presumed to contribute to the failure of nerve fibres to regenerate as effectively in the CNS as in the peripheral nervous system (PNS). Support for the hypothesis that conditions in the glial environment of injured fibres have a decisive role in successful axonal elongation has recently come from studies using transplants containing either central glia or peripheral nerve segments as conduits of axon growth^7,8. While CNS glial grafts have been shown to prevent growth of PNS fibres^7–9, experiments which used labelling techniques to trace the source of axons growing into PNS grafts provided evidence that processes from nerve cells in the spinal cord and medulla oblongata of adult rats may increase in length by 1 or more centimetres when the CNS glial environment is replaced by that of peripheral nerves^10,11. Here we report for the first time the extensive elongation of axons from neurones in the brain of adult rats through PNS grafts introduced into the cerebral hemispheres.