Defective differentiation of peripheral nerves in the dystrophic mouse.

Abstract

Sections or teased nerve fibers from developing common peroneal (CPN), tibial (TN) and sural nerves (SN) of dy/dy and normal mice were morphometrically examined at light and electron microscope levels. The data show that: (1) in the dy/dy CPN and TN, and to a lesser degree in the dy/dy, SN onset of myelination is delayed compared to normal nerves during the first postnatal week. This correlates with a retarded progression of segregated axon → promyelin axon → myelinated axon, and with a reduced rate of growth of axons in diameter; (2) at the completion of myelination fewer than normal numbers of myelinated axons are produced in the dy/dy CPN and TN, while in the dy/dy SN the numbers are normal; (3) in the dy/dy CPN and TN, and less in the dy/dy SN average internode is shorter than normal from early stages of myelination. It elongates at a slower than normal rate but roughly in parallel with the dy/dy limb, which has normal length and rate of elongation; (4) in the dy/dy nerves shorter than normal average internode is attributable partly to delays in onset of myelination, partly to shorter than normal axonal territory acquired by Schwann cells at the onset, and, particularly in the dy/dy CPN and TN, partly to fewer than normal short internodes eliminated during maturation. We conclude that: (1) in the dy/dy peripheral nerves (PNS) abnormalities of myelination are established during the first 2 weeks of postnatal development; and (2) differentiation of the dy/dy motor PNS is more defective than that of the dy/dy sensory cutaneous PNS.