An electron microscopic study of the relationship between axon diameter and the initiation of myelin production in the peripheral nervous system

Mid‐thoracic dorsal and ventral roots from adult rats, cats and cows were prepared for electron microscopy using standard techniques. Axon diameters were measured on photographs of known magnification. Minimal diameters of the unmyelinated fibers in the spinal cords of these animals were measured also. With the exception of a few overlapping diameters unmyelinated fibers were smaller than the smallest myelinated fibers in all the material examined. In the dorsal roots of the cow, the unmyelinated fibers, as a group, were larger than those of the rat and cat, and very few were enclosed by any one Schwann cell. The largest unmyelinated fibers were singly enclosed. In addition, the dorsal root of the cow contains many more small myelinated fibers than those of the other two species. In the ventral roots, a progressive increase in the size and relative numbers of myelinated preganglionic fibers from rat through cat to cow was noted. These facts indicated that myelination is directly dependent on fiber diameter irrespective of age, species or function although less precisely than was indicated by the earlier studies of Duncan ('34). Although the critical diameter for myelination in the peripheral nervous system is about one micron, in the central nervous system it is 0.3 μ or less.