Do axons grow during adulthood? A study of caliber and microtubules of sural nerve axons in young, mature, and aging rats

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Abstract
Calibers and microtubules of sural nerve axons were studied in young (6-week-old), mature (14-week-old), and aging (2-year-old) rats. The mean cross-sectional area of nonmedullated fibers was about 0.50 μm2 (range: 0.47–0.52) in the three age groups. Their caliber spectra were also similar. In contrast, myelinated axons grew from 6.6 to 16.7 μm2 between the sixth and 14th week of age. The increase of cross-sectional area was greater, the greater the initial caliber of axon (range 44–154%). No further change of caliber was observed in the aging rat. The cross-sectional area of nerve allotted per myelinated fiber was 42, 66, and 97 μm2 in young, mature, and aging rats, respectively. The fraction of nerve tissue occupied by the axoplasm, though, did not change substantially; it was 20, 28, and 21%, respectively. The microtubules density of 3-μm3 myelinated axons had a general average of 21 microtubules μm2. Differences between groups were not significant. In nonmedullated fibers, the microtubular density decreased as the size of the axon increased. No differences were observed between age groups. We conclude that nonmedullated fibers of the sural nerve stop growing before the sixth week whereas myelinated fibers keep growing until the 14th week of age. The correlation between microtubular content and axonal caliber is a lifelong feature of axons.