Long distance axonal regeneration of identified lamprey reticulospinal neurons.

Retrograde labeling with horseradish peroxidase was used to examine the time course and extent of axonal regeneration of 12 pairs of individually identifiable reticulospinal Müller cells and 2 pairs of Mauthner cells in larval lamprey that received transections of the rostral spinal cord in the gill region. With increasing recovery times (3-32 weeks post-transection) the descending axons of many of these neurons regenerated to progressively more caudal levels of the spinal cord. These results confirm that some reticulospinal neurons are capable of true regeneration. However, the regenerative capacity of these neurons was not uniform, even for neurons in the same brain stem nucleus in close proximity. For example, at 32 weeks post-transection some identifiable reticulospinal neurons could regenerate their axons to 60% body length or as much as 57 mm below the transection site. In contrast, previous studies indicated regeneration distances of 5-6 mm. Other neurons showed modest axonal regeneration, while one cell type showed very limited regeneration. The factors which may be responsible for the variable extent of regeneration among these neurons are considered.