Relation between the number of myelin lamellae and axon circumference in fibers of vagus and sciatic nerves of mice

Abstract

Nerve fiber populations of the vagus and sciatic nerves of mice were classified according to the number of myelin lamellae present in the sheaths. This method for classifying fiber populations was superior to others used previously since it provided a more sensitive procedure for the analysis of individual fibers and better control over the technical factors involved in tissue processing. The relationship of the number of myelin lamellae in the sheath to axon circumference was found to be linear. In fresh tissue there was one myelin lamella for every 0.24 μ increase in axon circumference above da value of 2.32 μ (the mean circumference of an average‐sized nonmyelinated fiber). A formula was proposed which may be useful for understanding how axons control myelin development and interpreting developmental stages, as well as for evaluating pathologic conditions affecting the peripheral nervous system. The critical diameter above which fibers were found to be myelinated was about 0.8 μ for fixed nerve and 1.1 μ for fresh nerve. The ratio of axon diameter to fiber diameter ranged between 0.5 and 0.9 and was not related to fiber size.