Figure 4. Delayed maturation of myelinating Schwann cells in spinal roots of pmp22−/− mice.

A. Spinal roots were dissected from a 4-week-old mouse and processed for semithin section studies. A dorsal root was visualized under oil lens and showed numerous myelinated nerve fibers with variable diameters. B. In contrast, diameters of myelinated nerve fibers are relatively uniform in the ventral roots. Thus, Schwann cells are well differentiated to myelinate the axons by this age. However, when pmp22−/− ventral roots were examined at 4-month-old age, numerous myelinating Schwann cells still remained at an immature stage after establishing 1:1 relationship with axons (arrows in D). Since most myelinating Schwann cells failed to form myelin, tomacula were few in the ventral root (arrowhead in D). Overall axonal density appeared comparable to the wild-type ventral root. C. In contrast, most myelinating Schwann cells have formed myelin in the dorsal roots by the age of 4 months, and tomacula were common (arrowheads). Axonal density of myelinated nerve fibers appeared reduced. Some of the axons were swollen (arrows), suggesting axonal degeneration.