Age-related changes in peripheral nerves of the dog. II. A morphologic and morphometric study of cross-sectional nerve.

Qualitative histologic studies and quantitative measurements were made on cross-sectional preparations of common peroneal and ulnar nerves of 32 neuromuscular disease-free dogs from birth to 15 years of age, to provide normative data not available previously. Minimal lesions were seen in nerves of dogs from birth to seven years; however, in older dogs, the incidence of axonal degeneration and segmental demyelination and remyelination increased. Total fiber density of both nerves was over 40,000 fibers/mm2 at birth and declined rapidly during the first six to nine months to level off at about 10,000 fibers/mm2 by one year of age. Density of small (less than 5 micrometers) and large (greater than or equal to 5 micrometers) diameter fibers attained adult values by one year of age. The frequency distribution of the myelinated fibers was unimodal at birth and became bimodal between three and six months of age. The peak of the small and large fiber groups occurred at 3 micrometers and 6 micrometers, respectively. Larger diameter fibers (10 micrometers to 12 micrometers) reached adult values between nine months and one year of age. A 2.5 fold increase in mean fiber diameter occurred during the first year of life. There was no statistically significant change in any histometric parameter after maturity (approximately one year of age).