Amoeboid and ramified microglia: their interrelationship and response to brain injury

Rio-Hortega's hypothesis that transiently appearing amoeboid microglia might become ramified microglia in the adult and that the latter could differentiate into brain macrophages in the event of brain damage could not be proved because of inherent limitations in existing techniques. The present investigation used a novel method of labelling the rat supraventricular amoeboid microglia with an enduring fluorescent marker, rhodamine B isothiocyanate, introduced intraperitoneally. Observation of their subsequent development showed that they became transformed into the ramified microglia. Both the amoeboid and ramified microglia were OX-42 positive, indicating their macrophage/monocyte lineage. Other microglia in the cerebral neocortex, which were also OX-42 positive, were not derived from any of the rhodamine-labelled cells. Rhodamine-labelled microglia did not migrate toward the site of a superficial cerebral injury. Following a deep lesion reaching the corpus callosum, greatly increased numbers of labelled amoeboid microglia were frequently observed at or near the lesion site. Large rhodamine-labelled cells, which were OX-42 positive, appeared at all lesioned sites, and such were most likely blood derived monocytes. The antigenicity of the ramified microglia became elevated when rhodamine B isothiocyanate was present intracellularly and even more so with the presence of a nearby intracerebral stab wound.