S100 is preferentially distributed in myelin-forming Schwann cells.

In order to elucidate the role of S100 protein in vivo we used postembedding electron microscopical immunocytochemistry with commercially available antibodies to S100, and secondary antibodies conjugated to colloidal gold to determine the distribution of S100 protein in the sciatic nerve and the cervical sympathetic trunk of the rat. We found that S100 immunoreactivity in Schwann cells was differentially distributed. The amount of S100 immunoreactivity in myelinated fibres appeared to correlate directly with the thickness of the myelin sheath formed by the Schwann cell. Unmyelinated fibres were identical to the small myelinated fibres in S100 immunoreactivity. Within the Schwann cells, the immunoreactivity was seen in the nucleus and in the perikaryal cytoplasm, as well as in the Schmidt-Lanterman clefts and in the paranodal loops at the node of Ranvier, but not in the myelin itself. Using these antibodies we did not see reproducible or convincing intra-axonal immunoreactivity in normal nerve. These results suggest that in Schwann cells S100 expression may be related to axon diameter and degree of myelination. Future studies of S100 expression in development or in response to injury may clarify the role of S100 in Schwann cell biology.