Figure 4.
Axonal constrictions in tomacula. All data in this figure were derived from noncompressed nerves. pmp22+/− mice were cross-bred with YFPtg+/+ mice, so that all axons were labeled by YFP expressed from the YFP transgene. Sciatic nerves were dissected, fixed, and teased into individual fibers, which were then stained with antibodies against Caspr. A1–A3, Three nodes from three myelinated nerve fibers were flanked by the Caspr-stained paranodes. Some Caspr spirals were also visible (arrowheads in A1). In the nodal/paranodal regions, there was a natural decrease of axonal diameter (between arrows in A2). There were no tomacula in these regions under phase-contrast imaging (overlay in A3). B1–B3, Another myelinated nerve fiber was examined and showed tomacula in both paranodes that flanked the node of Ranvier (arrowheads in B3). Axonal segments in the tomacula were constricted and the narrowed axonal segments extended far beyond the paranodal regions (between arrows in B3). C1–C3, A pmp22+/− myelinated nerve fiber had an asymmetry in its paranodes. The left paranode was affected by a tomacula with constricted axon (between arrows in C3). In contrast, the right paranode with no tomacula showed a normal axonal diameter. D, In some tomacula, axons may become enlarged (between arrows in D). E, The appearance of the enlarged axon in D is consistent with the EM finding in E. Axonal membranes occasionally became convoluted along the folding of myelin in tomacula (arrowhead array). This membrane would appear enlarged when it is labeled by YFP and viewed under light microscopy. F, A constricted axon on transverse section. G, The lengths of normal paranodal axons and constricted axonal segments were measured in randomly selected fibers (diameters of these selected fibers were comparable between the groups), and showed in this histogram (4.2 ± 0.2 μm for normal paranodal axons vs 19.4 ± 4.9 μm for constricted axonal segments; p < 0.0001).