Injured neurons in the olfactory bulb of the adult rat grow axons along grafts of peripheral nerve.

Certain neurons in the central nervous system (CNS) of adult mammals extend axons for several cm along peripheral nerve grafts inserted into the brain or spinal cord. It is not clear, however, if these nerve cells constitute a special population or are examples of a general capacity of the injured mammalian CNS to regrow processes under these experimental conditions. Furthermore, because the new axons could originate by collateral sprouting from uninjured neurons, it is important to prove that the interruption of a central axonal projection can be followed by extensive fiber regrowth from the damaged neurons. In this anatomical study, we examined whether: (1) nerve cell type; and (2) axotomy, influence CNS axon regrowth along peripheral nerve grafts. For this purpose, we grafted segments of sciatic nerve into the olfactory bulb (OB) of adult rats and used combinations of neuroanatomical tracers (horseradish peroxidase and the fluorescent dyes True Blue and Nuclear Yellow) to investigate axonal regrowth from the different neurons that normally populate the OB. We demonstrate that OB axons extending along peripheral nerve grafts originate from mitral and tufted cells near the graft tip, rather than from the smaller OB neurons (periglomerular, short axon, and granule cells). Most of the mitral and tufted cells that extend new axons in grafted peripheral nerve segments lose their normal projections through the lateral olfactory tract because of axotomy at the time of grafting. Neuronal type, damage, and proximity to the graft appear to be prerequisites of this regenerative response from the OB.