Journal of Neuroscience, Vol 6, 1814-1821, Copyright © 1986 by Society for Neuroscience
Determinants of directional specificity in the regeneration of lamprey spinal axons
SA Mackler, HS Yin and ME Selzer
The projection patterns of regenerating spinal axons in the larval sea
lamprey (Petromyzon marinus) were determined by intracellular injection of
HRP. Four hundred and eighty-six of 562 stained axons and axon-like
neurites (87%) arising from Muller and Mauthner axons, giant interneurons,
and dorsal cells terminated in an orientation similar to that of their
counterpart control cells. Therefore, lamprey spinal axons regenerate
selectively along their normal projection paths. During the first 4 weeks
of recovery, i.e., before any had regenerated beyond the transection site,
91 of 114 axons and long neurites (80%) projected in the proper direction.
Thus, the correctness of the final projection patterns did not result from
selective retraction of randomly directed long neurites. When the cords
were doubly transected 1 cm apart, orientation of regenerating neurites
remained normal both within the 1 cm island and in the adjacent spinal
cord. This suggests that the directional specificity of axonal regeneration
was determined neither by the location of the scar nor by the availability
of channels formed by the degenerating fibers. Finally, removing 1 cm of
spinal cord eliminated potential synaptic targets for regenerating axons on
either side of the lesion, but did not affect the direction of axonal
growth. These findings are consistent with the hypothesis that the
regeneration of lamprey spinal axons is guided by local chemical cues that
persist long after the pathways are formed early in development.