ORIGIN OF MICROGLIA IN THE QUAIL RETINA - CENTRAL-TO-PERIPHERAL AND VITREAL-TO-SCLERAL MIGRATION OF MICROGLIAL PRECURSORS DURING DEVELOPMENT

The origin, migration, and differentiation of microglial precursors in the avascular quail retina during embryonic and posthatching developm ent were examined in this study. Microglial precursors and developing microglia were immunocytochemically labeled with QH1 antibody in retin al whole mounts and sections. The retina was free of QH1(+) macrophage s at embryonic day 5 (E5). Ameboid QH1(+) macrophages from the pecten entered the retina from E7 on, These macrophages spread from central t o peripheral areas in the retina by migrating on the endfeet of the Mu ller cells and reached the periphery of the retina at E12. While earli er macrophages were migrating along the inner limiting membrane, other macrophages continued to enter the retina from the pecten until hatch ing (E16). From E9 on, macrophages were seen to colonize progressively more scleral retinal layers as development advanced. Macrophages firs t appeared in the ganglion cell layer at E9, in the inner plexiform la yer at E12, and in the outer plexiform layer at E14. Therefore, it see ms that macrophages first migrated tangentially along the inner retina l surface and then migrated from vitreal to scleral levels to gain acc ess to the plexiform layers, where they differentiated into ramified m icroglia. Macrophages appeared to differentiate shortly after arrival in the plexiform layers, as poorly ramified QH1(+) cells were seen as early as E12 in the inner plexiform layer and at E14 in the outer plex iform layer. Radial migration of macrophages toward the outer plexifor m layer continued until posthatching day 3, after which retinal microg lia showed an adult distribution pattern. We also observed numerous vi treal macrophages intimately adhered to the surface of the pecten duri ng embryonic development, when macrophages migrated into the retina. T hese vitreal macrophages were not seen from hatching onwards, when no further macrophages entered the retina. (C) 1995 Wiley-Liss, Inc.