Whether an axon will regenerate after it is cut depends on the balance between the intrinisic ability of the axon to regrow and the permissiveness of the environment surrounding it. The permissiveness of the environment is determined by the glial cells at the site of injury, and in the CNS both oligodendrocytes and astrocytes produce inhibitory molecules. Neurones differ greatly in the ability of their axons to regrow following axotomy. This intrinsic growth ability is greater in embryonic than adult neurones, varies from one neuronal type to another, depends on whether the axon is cut close to or far from the cell body, and can be increased by appropriate neurotrophic molecules.