Changes of the ratio between myelin thickness and axon diameter in human developing sural, femoral, ulnar, facial, and trochlear nerves

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Abstract
Previous studies on sural nerves were extended to human femoral, ulnar, facial and trochlear nerves. An asynchronous development of axon diameter and myelin sheath thickness was noted in all nerves studied. Whereas axons reach their maximal diameter by or before 5 years of age, maximal myelin sheath thickness is not attained before 16–17 years of age, i.e., more than 10 years later. The slope of the regression lines for the ratio between axon diameter and myelin thickness is significantly steeper in older than in younger individuals; it also differs if small and large fibers with more or less than 50 myelin lamellae are evaluated separately. The number of Schmidt-Lanterman incisures during later stages of development is related to myelin thickness, but the length of the spiral of the myelin lamella, thought to unrolled, in relation to its width, i.e., internodal length, varies considerably during development. The changes of the relationship between axons and myelin sheath thickness during normal human development have to be taken into account if hypomyelination is considered as a significant pathological phenomenon in peripheral neuropathies, especially in children. The implications of the present findings concerning conduction velocity of peripheral nerve fibers and other electrophysiologic parameters are discussed.