Microtubules and calibers in normal and regenerating axons of the sural nerve of the rat

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Abstract
The calibers and microtubular content of axons were studied in normal and regenerating fibers of the sural nerve from 17 to 122 days after a lesion of the sciatic nerve of young adult rats. During this period (70–175 days of age), the cross-sectional area of control myelinated axons almost doubled but that of nonmedullated axons did not change. In regenerating nerves, after 122 days of recovery, the cross-sectional area of myelinated fibers was still 38% below that of the normal side. In contrast, the regenerating nonmedullated population was richer in fine (< 0.2 μm2) and in coarse (> 0.9 μm2) fibers than on the control side; the cross-sectional area averages were 0.50 and 0.54–0.70 μm2 for the normal and regenerating populations, respectively. The microtubular density of normal 3-μm myelinated fibers averaged 24.0 microtubules/μm2. In regenerating fibers of the same size the density varied between 19.2 and 23.2 microtubules/μm2. Microtubular density values of normal and regenerating fibers were not statistically different. In nonmedullated fibers, the microtubular content (expressed as microtubular density or number of microtubules per axon) correlated with the caliber of the fiber. In these correlations, only minor differences were observed between regenerating and uninjured fibers. Our results indicate that nonmedullated fibers terminate their radial growth well before myelinated fibers do, and that axonal microtubular content correlates with the local size of the fiber and is largely insensitive to regeneration.