The origin of the neural macrophage: a quantitative ultrastructural study of cell population changes during Wallerian degeneration.

The changes that follow a localised crush injury to the rat sural nerve have been used to study endoneurial populations during Wallerian degeneration. The removal of products of degeneration, and in particular myelin debris, is accomplished by globule-laden cells which appear in the endoneurium during the first few days of repair. The origin of these cells has been investigated using a quantitative ultrastructural technique. Serial planimetric measurements of all populations, identifiable in terms of criteria that did not pre-judge their true nature, were made at intervals over a period of 15 days. Cell counts obtained immediately below the site of injury and 1 and 3 cm distally were compared, and graphs of endoneurial population changes constructed from these measurements. Additional descriptive evidence was invoked to assist in establishing the actual identity of the extratubal vacuolated cells which had been classified and measured empirically. Comparing the changes in the number of these cells with that of the intratubal vascuolated cell population, and taking account of the presence of immature macrophages in both proximal and distal situations, lead to the conclusion that the extratubal vacuolated cells are mostly derived from the bloodstream, the rest being of local intratubal origin. There was no evidence to support the notion that Schwann cells transform into macrophages.