Altered ratio between axon diameter and myelin sheath thickness in regenerated nerve fibers.

Summary The fine structure of regenerated nerve fibers, 6–24 months after a lesion, differed from that of normal nerve fibers in several respects: (1) The thickest regenerated nerve fibers were thinner than the thickest normal ones. (2) Although some axons became rather large, the myelin sheats of the thickest regenerated fibers remained relatively thin. (3) A considerable overlap of myelinated and unmyelinated fibers was found to be due to equal diameters of a proportion of aorophic, myelinated axons with extraordinarily thick sheaths, and of a number of unmyelinated, thick axons. Accordingly, the regression line for the relation between the axon diameter and the myelin sheath thickness of regenerated nerve fibers increased more gradually in slope than did that for normal nerves. These observations were made on regenerated nerve fibers both distal to nerve grafts in dogs and, under more favorable conditions for regeneration, distal to crush lesions in rats. The thickest regenerated axons, and their myelin sheaths, finally reached greater dimensions, relatively and absolutely, in rats than in dogs. It is suggested that the observed reduction of the myelin sheath thickness of the thickest regenerated nerve fibers constitutes tha main cause for the known decrease of the conduction velocity of regenerated nerve fibers.