Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.