Increase in descending brain–spinal cord projections with age in larval lamprey: Implications for spinal cord injury

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Abstract
The purpose of this study was to determine whether new descending brain‐spinal cord projections are added with age in larval lamprey and might contribute substantially to restoration of these projections following spinal cord injury. Retrograde horseradish peroxidase (HRP) labeling of descending brain neurons was performed in “young” and “old” larval lamprey that differed in age by at least one year. In old larval lamprey, significantly more descending brain neurons projected to specific rostral levels of the spinal cord than in young animals. Furthermore, in young and old lamprey, the main morphological change in Müller and Mauthner cells was an increase in soma size. The major conclusion from the present study is that in larval lamprey, some new brain‐spinal cord projections are added with age that could be due to axonal elongation by preexisting brain neurons and/or descending projections from new neurons (i.e., neurogenesis or maturation of incompletely differentiated neurons). Following spinal cord transections, the numbers of descending projections were not significantly different than those in normal, unlesioned animals. Thus, some new descending projections are added with age, but at a relatively slow rate, and the rate does not appear to be affected significantly by spinal cord transections. Together, the present results and those from our recent double‐labeling study suggest that following spinal cord transection in larval lamprey, axonal regeneration by descending brain neurons, rather than the relatively slow addition of new brain‐spinal cord projections with age, probably accounts for the majority of restored projections and recovery of locomotor function. J. Comp. Neurol. 447:128–137, 2002.