Transplanted glial scar impedes olfactory bulb reinnervation

The olfactory system is the only region of the mammalian central nervous system in which degeneration of the primary sensory neurons results in the development of new neurons and reinnervation of the secondary sensory neurons. Axotomy of the olfactory nerve at the cribriform plate does not cause the formation of a glial scar which blocks nerve regeneration. The purpose of this study was to determine whether a glial scar that formed in the optic nerve would suppress axonal regeneration when transplanted to the site of olfactory nerve axotomy. Primary olfactory neurons were axotomized along the cribriform plate in adult rats. A compact glial scar formed by transection of an adult rat optic nerve 50 to 60 days prior to removal was transplanted into the olfactory nerve axotomy site. The rats were allowed to survive for 1, 2, 3, or 4 weeks. Transport of horseradish peroxidase (HRP) from the nasal cavity by the olfactory neurons was used to examine the temporal and spatial pattern of regeneration of the olfactory nerve after axotomy and axotomy followed by glial scar transplantation. Twenty-one days after olfactory nerve axotomy, HRP was found in the glomerular layer where the primary olfactory axons synapse on the apical dendrites of the secondary olfactory neurons. In the presence of the transplanted glial scar, HRP labeling was not found in certain glomeruli even at 4 weeks postaxotomy. Glial scars formed within the optic nerve impede reinnervation of the olfactory bulb by neurons which have an exceptional regenerative capacity due in part to the ensheathing glia.