Nonresident macrophages in peripheral nerve of rat: Effect of silica on migration, myelin phagocytosis, and apolipoprotein E expression during wallerian degeneration

Abstract

The selective toxicity of silica quartz dust to macrophages was used to assess the role of these cells in Wallerian degeneration and nerve repair. Left sciatic nerves of adult Wistar rats were crushed and one group of animals received repetitive intraperitoneal injections of silica (200 mg two times per week starting 1 day prior to injury), whereas the control group received saline. Unexpectedly, silica treatmen did not impair the initial invasion of (hematogenous) macrophages into the degenerating dital nerve stump as revealed by histological and immunocytochemical methods. However, 4 weeks after the lesion three specific events in Wallerian degeneration were significantly inhibited in silica‐treated animals: (1) inhibition of phagocytosis and degradation of myelin, (2) delay in disappearance of nonresient macrophages from regenerating nerve, (3) reduction of synthesis and/or secretion of apolipoprotein E in resting macrophages. On the other hand, axonal regrowth and remyelination were not affected by silica. These in situ experiments support and extend previous studies suggesting specific function for nonresident macrophages in wallerin degeneration of peripheral nerve.