Changes of the ratio between myelin thickness and axon diameter in the human developing sural nerve.

Axon caliber and myelin sheath thickness of individual nerve fibers were evaluated in the developing human sural nerve using three different methods of measurement: 1. ocular micrometer evaluation of large fibers, 2. photographic enlargements for evaluating large numbers of nerve fibers of all sizes, and 3. electron microscopic enlargements for more precise measurements in selected nerves. The average axonal diameter doubles from 5 months gestation to about 5 years of age. Large fiber group axons increase, during the same period, by a factor of 3--3.5 with a slight decrease thereafter. The myelin thickness increases more slowly, but continuously, between 5 months gestation until the age of 14. This asynchronous development of axons and myelin sheaths results in a statistically significant change of the ratio between axonal caliber and myelin thickness. The slope of the regression line is steeper in older than in younger individuals, and the correlation coefficient increases during development of the nerve.