An investigation has been made of the relation between axon diameter, fibre diameter, and myelin sheath thickness in myelinated nerve fibres from the peroneal nerve of the rabbit. Fibres were measured: (
a
) in the normal nerve, and (
b
) during regeneration following nerve crushing. In the normal nerve, fibres ranging in diameter from 1 to 20
μ
, were present. These had axons with diameters from 0·5
μ
in the smallest, to about 15
μ
in the largest fibres. The 0·5
μ
axons had myelin sheaths about 0·5
μ
thick, while the largest axons had sheaths about 2·5
μ
thick. Between these extremes the curve relating sheath thickness to axon diameter was at first steep, and then more gradual in slope. In regenerating nerve fibres 15 mm. proximal to the site of lesion the
axons
decline steadily in diameter during the first period of regeneration (100 days). Thereafter they increase in size, although even after 300 days the largest axons have not regained their normal diameter. The axons, 10 mm. below the lesion, extremely thin at their first appearance, increase steadily in diameter, until, 300 days after injury they equal in diameter the axons of the proximal stump. The decrease in diameter of axons proximally is accompanied by an absolute increase in their myelin sheath thickness. This takes place first of all without any alteration in the total diameter of the fibres concerned, the increase in thickness of the myelin compensating the decrease in axon diameter. Between 60 and 100 days after injury, however, continuing axon decrease is accompanied by a decrease in the total diameter of the fibres. Between 100 and 200 days an increase in total fibre diameter parallels the increase in axon diameter, the myelin sheaths remaining abnormally thick. Between 200 and 300 days after injury the continued increase in axon diameter causes a diminution in myelin sheath thickness. Even at 300 days after injury, however, the sheaths are still thicker than in normal nerve. In the distal stump the myelin sheaths, when first developed, are thicker than those of fibres of corresponding diameter in normal nerve. Subsequently they grow still thicker, reaching their maximum thickness about 200 days after nerve injury. Between 200 and 300 days a slight diminution in sheath thickness occurs since the axons continue to increase while the total diameter remains the same. However, even at 300 days the sheaths are still thicker than normal. The bearing of these results on the nature of the forces maintaining the structure of nerve fibres is discussed.
Akt Regulates Axon Wrapping and Myelin Sheath Thickness in the PNS
