Analysis of neuritic outgrowth from severed giant axons in Lumbricus terrestris

Abstract This study analyzes the detailed morphometric pattern at various postoperative times of neuritic outgrowths from the proximal and distal stumps of two uniquely identifiable axons. Morphological patterns of neuritic outgrowths from stumps of severed axons were compared for medial and lateral giant axons in the central nervous system of the earthworm Lumbricus terrestris. Outgrowths from proximal and distal stumps were labeled by injection of fluorescent dye into axonal stumps and assessed according to morphometric parameters. Outgrowths from axonal stumps of severed giant axons were statistically indistinguishable for most morphometric measures of neuritic quantity, shape, direction, and location. There were two exceptions to this general rule: 1) proximal stumps of medial giant axons produced significantly more neurites than distal stumps of medial giant axons, and 2) proximal stumps of lateral giant axons produced significantly longer neurites than proximal stumps of medial giant axons. No measure of neuritic outgrowth showed a significant change from the second through seventh postoperative week, suggesting that most outgrowth occurred in the first two postoperative weeks and that neuritic morphology remained stable through the seventh postoperative week. Neurites grew across the lesion site in relatively straight trajectories parallel to the longitudinal axis of the ventral nerve cord and often grew alongside the appropriate axonal stump across the lesion site. The length of neurites growing in close apposition to appropriate axonal stumps or giant axons was much greater than expected, had outgrowth been randomly directed. These data provide a basis for future investigations of the mechanisms that regulate neuritic outgrowth.