Journal of Neuroscience, Vol 5, 1616-1625, Copyright © 1985 by Society for Neuroscience
Injured neurons in the olfactory bulb of the adult rat grow axons along grafts of peripheral nerve
B Friedman and AJ Aguayo
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.