Unequivocal demonstration of the hematogenous origin of brain macrophages in a stab wound by a double-label technique

We studied the origin of the phagocytic cells observed in the Neurol parenchyma in response to injury. Suckling albino rats were given repeated intraperitoneal injections of india ink to label circulating macrophages. Following this treatment, a discrete track lesion was produced in the cerebellar vermis by inserting into it a hypodermic needle which would deliver a minute amount of a second tracer, iron dextran. Electron-microscopic observation showed that 2 days later phagocytes within the lesion contained both iron dextran and carbon particles in their cytoplasm, indicating their hematogenous origin and the persistence of phagocytic activity.