Neurons that maintain extensive axon collaterals proximal to the site
of axotomy may be better able to survive injury. Early lesions of the
rubrospinal tract lead to retrograde cell death of the majority of axo
tomized immature neurons. Transplants of fetal spinal cord tissue resc
ue axotomized rubrospinal neurons and promote their axonal regeneratio
n. Rubrospinal neurons develop many of their axon collaterals postnata
lly. The present study tests the hypothesis that the axotomized rubros
pinal neurons that are rescued by transplants and regenerate their axo
ns are those neurons that have established axon collaterals to targets
rostral to the lesion. Neonatal rats received a transplant of fetal s
pinal cord tissue placed into a midthoracic spinal cord hemisection. O
ne month after transplantation, the retrogradely transported fluoresce
nt tracers fast blue (FB) and diamidino yellow (DY) were used to ident
ify rubrospinal neurons with collaterals to particular targets. FB was
injected either into the interpositus nucleus of the cerebellum or in
to the gray matter of the cervical enlargement to identify collaterals
to these targets, and DY was injected into the spinal cord approximat
ely 5 mm caudal to the transplant and lesion site to label retrogradel
y the neurons that regenerated their axons. Double labeling was observ
ed in the axotomized neurons of the red nucleus after tracer injection
s into the cervical spinal cord but not after injections into the cere
bellum. This labeling pattern indicates that axotomized rubrospinal ne
urons that are rescued and regenerate axons caudal to the transplant m
aintain axon collaterals at cervical spinal cord levels. Cerebellar co
llaterals do not appear to play a role in the survival and regrowth of
axotomized rubro-spinal neurons. (C) 1997 Academic Press.