Accumulation of 14C-5,6-dihydroxytryptamine-melanin in intrathecal and subependymal phagocytes of the rat CNS and possible routes of their elimination from brain.

14C-5,6-DHT-Melanin, a labelled synthetic polymer resembling the naturally occurring melanin formed in brain by autoxidation of dopamine, was injected into the left lateral ventricle of adult rats, and its fate followed by autoradiography and by transmission electron microscopy of structures identified as labelled in preceding light micrographs, and by EM-autoradiography. Shortly after injection, melanin particles (easily identified in the em because of their size, structure and electron opacity) were seen ingested by supraependymal and epiplexus cells, by cells residing in the piaarachnoid, i.e. free subarachnoidal cells and perivascular cells, and by subependymally located microglia-like cells with intraventricular processes. Up to day four, an increase in the number of labelled phagocytes in the CSF was noted which transformed into typical reactive macrophages. Beyond this time, many intraventricular melanin-loaded phagocytes formed rounded clusters; cells of such clusters were subsequently found to invade the brain parenchyma by penetrating the ependymal lining and to accumulate in the perivascular space of brain vessels. 14C-Melanin-storing macrophages were found in the marginal sinus of the deep jugular lymph nodes suggesting emigration of CNS-derived phagocytes via lymphatics or prelymphatics that contact the subarachnoidal space compartment. This does not exclude the possibility that some of the macrophages leave the brain via the systemic circulation by penetrating the vascular endothelium; these may be disposed of in peripheral organs other than the lymph nodes. The ability of supraependymal, epiplexus, free subarachnoidal and perivascular cells in the pia and of subependymal microglia cells to accumulate synthetic melanin by phagocytosis suggests that these cells are local variants of the same type of resting potential phagocytes of the mammalian brain. The CSF appears to be replenished with phagocytes by mitosis and migration of microglia-like cells or their precursors located underneath the ependyma of the lateral ventricle and by mitosis of resting intrathecal cells.