Phagocytosis in the rat optic nerve following Wallerian degeneration

Summary Unilateral enucleation of the eye in adult male rats was performed in an attempt to resolve the long-standing controversy as to the nature of the phagocytic cells during Wallerian degeneration in the central nervous system. Previously both resident microglia and circulating monocytes, as well as oligodendrocytes, have all been considered to be the phagocytic cells. In these present experiments macrophages and microglia were studied using lectin histochemistry for Griffonia simplicifolia agglutinin and the monoclonal antibody ED1 at light microscopic level. Oligodendrocytes were demonstrated ultrastructurally using immunohistochemistry with monoclonal antibodies against myelin oligodendrocyte glycoprotein (MOG). Ultrastructural examination of the degeneration optic nerves confirmed longstanding reports of the slow nature of breakdown in the adult central nervous system. During the early periods of breakdown, starting at 1 week and continuing to 1 or 2 months, it was difficult to type, on ultrastructural examination alone, the nature of all the cells undergoing phagocytosis, but many of them resembled microglia/macrophages. Myelin debris cleared very slowly and could still be recognised prominently in the nerve up to 22 months post-enucleation. Lectin and immunochemical examination showed that the early major phagocytic component of phagocytosis was carried out by macrophages, probably both circulating and resident. In addition, however, myelin and axonal debris was taken up or retracted into oligodendrocyte processes, which were stained with antibodies to MOG. This oligodendrocyte component appeared to be small in relationship to the overall degree of debris. Phagocytosis in the optic nerve, therefore, is in many respects similar to that in the peripheral nervous system, which has been shown to be carried out mainly by circulating macrophages, with only a minor role played by the Schwann cell.Supported by a grant MA 5818 from the Medical Research Council of Canada