Schwann cell properties. II. The identity of phagocytes in the degenerating nerve

The present study was undertaken to identify the cells from which phagocytes originate following traumatic injury to the sciatic nerves in rats. The morphologic evolution of the phagocytes was correlated daily with changes in axon, myelin, Schwann cell and neurilemmal tube at light and electron microscopic levels. Autoradiography with tritiated thymidine was employed to label the proliferative cells immediately before and following the injury. The result indicated that degeneration of Schwann cells occurred with the onset of myelin breakdown and that the degenerated products of myelin, axon and Schwann cells were removed by macrophages. While most of the macrophages were originally blood monocytes, some were derived from vascular pericytes. They penetrated the neurilemmal tubes on the third postoperative day and began engulfing first the Schwann cells and then the myelin and axons. Having filled their cytoplasm with debris, some macrophages moved out of the neurilemmal tube while others remained temporarily inside the envelopes formed by the persisting neurilemmal tubes-thus the macrophages can inherit a basal lamina from degenerated Schwann cells.