The role of endoneurial fibroblasts in myelin degradation

Counts of nuclei in electron micrographs of rat sciatic nerve show that approximately one third of the nuclear population is fibroblasts. Comparison of these figures with counts in teased fibers indicates that the majority of fibroblasts adheres tightly to myelinated nerve fibers. The early reactions and cellular transformations of the endoneurial fibroblast population upon intraneurial injections of india ink or of a purified preparation of rat sciatic myelin were studied for intervals of from 6 hours to 3 days after injection. There was phagocytosis of carbon or myelin by endoneurial fibroblasts as early as 6 hours after injection. A subsequent massive increase in phagocytic endoneurial cells appeared to develop from, and at the expense of, the normal endoneurial fibroblast population. The extent to which hematogenous monocytes also contributed to the macrophage population could not be assessed accurately by morphological criteria. Schwann cells never ingested carbon or myelin preparation during the period of investigation. Early invasion of the Schwann tube by macrophages from the endoneurial spaces could be demonstrated however. These macrophages were labeled by either carbon or myelin preparation which they had taken up from the endoneurial spaces before they entered the fibers through small gaps of the basal lamina, preferentially at the nodes of Ranvier. Carbon- or myelin-labeled hematogenous granulocytes entered nerve fibers in the same way, but the invasion of fibers by granulocytes was a transient early response limited to the first two days after injection. The data demonstrate the phagocytic capacities of endoneurial fibroblasts, their capacity for myelin degradation, and an invasion of Schwann tubes by elements of the endoneurial macrophage population during very early phases of fiber injury.