Figure 73-1 A teased unmyelinated fiber cluster (UFC) stained with cresyl violet. The preparation allows linear measurement of internuclear lengths (upper panel). Transverse sections of a teased UFC under light (inset) and electron microscopy (left lower panel). Frequency distribution of internodal lengths (gray bars) of myelinated fibers (MFs) as compared with internuclear lengths (yellow bars) of UFCs (right lower panel).
We used serial semithin sections of rat sural nerve to reconstruct longitudinal ensheathment of UFCs. To do this we focused on UFCs widely separated from other clusters so as to more readily recognize them in serial sections. Following three Schwann cell clusters (SCCs) for distance greater than 50% of their internuclear lengths, it was possible to show that a variable number of axons were ensheathed by contiguous Schwann cells (Fig. 73-2).
Figure 73-2 With skip serial sectioning of a Schwann cell cluster for a distance greater than 50% of its internuclear length, it was possible to show that a variable number of axons were ensheathed by contiguous Schwann cells.
In two seminal studies, Peyronnard and colleagues1,2 used teased unmyelinated nerve fiber clusters to estimate internuclear distance in superior cervical trunk and sural nerve of rats during development. They found that internuclear distance was approximately 100 μm for both nerves, with considerable variability around the mean. In regeneration after crush, the mean internuclear length was considerably shorter.1 In a later study, the same authors studied the internuclear length of rat during development and found that it increased with development.2
Our studies, also of teased UFs of sural nerve of rats approximately 300 g in weight, confirm the studies of Peyronnard and associates, and show the clear difference of ensheathment length of UFs and MFs. The internuclear distance of UFCs average about 100 μm, with a range of 50 to 150 μm, whereas the internodal length of MF of the same nerve varied from approximately 200 to 1000 μm.
Present studies on UFCs and MFs of sural nerve of SD rats confirm the studies of Peyronnard and coworkers. The internuclear length of Schwann cells of UFCs is shorter and not overlapping with that of MFs. Typically their length in the former varies from 50 to 150 μm, whereas for the latter, it varies from approximately 200 to 1000 μm. Typically, one Schwann cell ensheaths many UFs, whereas Schwann cells of MFs ensheath only one fiber. Linear arrays of Schwann cells along the length of UFCs ensheath a variable number of UFs. Sometimes, the cytoplasm is a continuous sheet, and at other levels, it is separated into separate islands. These structural differences between UFs and MFs need to be known in interpreting functional and structural alterations in disease.