A QUANTITATIVE ULTRASTRUCTURAL STUDY OF REGENERATION FROM ISOLATED PROXIMAL STUMPS OF TRANSECTED UNMYELINATED NERVES1, 2, 3

Abstract

Following injury to unmyelinated nerve fibers, differentiation between degenerative and regenerative changes is difficult. Therefore, in order to study unmyelinated nerve fiber regeneration more precisely, siliconized rubber tubes with central dividing walls were inserted between the proximal and distal stumps of transected rabbit anterior mesenterie nerves. Nerve fibers which grew into the tube from the proximal nerve stumps were examined by phase contrast and electron microscopy at intervals from 2 days to 13 weeks after anterior mesenteric nerve transection. Axons and Schwann cells were evaluated quantitatively. By 2 days after injury, small axons approximately 0.4 microns in diameter were present. Initially as many as 200 of these axons were observed in relation to individual Schwann cells. Such axonal bundles were progressively partitioned into smaller groups by Schwann cells and their processes. However, axon-Schwann cell ratios remained higher than normal throughout the period of study. Thus, regeneration of transected unmyelinated nerve fibers simulates fetal patterns of neurogenesis. Regenerating axons remained smaller than normal, however, suggesting that axonal maturation cannot occur when longitudinal growth is obstructed.