Migration of phagocytotic cells and development of the murine intraretinal microglial network: an in vivo study using fluorescent dyes.

This work was undertaken to study whether retinal ganglion cell (RGC) death, which occurs during postnatal development of the mouse retina could aid in assessing the topological and chronological pattern of microglial cell migration. The study was conducted from postnatal day 0 (P0) to adulthood. The fluorescent dyes Fluorogold (FG) or (4-[4-didecylaminostyryl]-N-methylpyridinium iodide (4Di-10ASP) used in this study, were transported retrogradely to the RGC soma when either dye was injected into the superior colliculus (SC) at P0. Some of these labeled RGCs die due to natural apoptosis during this stage of development and are phagocytosed by microglial cells, which move to the site of RGC death, to become labeled with the same dye. The retinas were examined to quantify the microglial cells from P5 to adulthood. In addition, the reaction of microglia to optic nerve crush was studied in adult animals. Both dyes labeled RGCs in the contralateral retina and a few RGCs in the retina ipsilateral to the injected SC. The density of labeled RGCs decreased by 22% between P5 and P7. During this phase, microglial cells become visible as they ingested the fluorescent detritus of the dying RGCs. Microglial cells were evenly distributed across the entire retinal surface and migrated to the outer plexiform layer. Migrating microglia consecutively altered their morphology from the amoeboid to the ramified form. In terms of intracellular storage of the dyes, resident microglial cells retained the fluorescent dye 4Di-10ASP over a period of 12 months. In contrast, FG was completely transferred from the RGCs and microglial cells to intramural cells (pericytes) of the retinal capillaries after 10 months. This resulted in delineation of the entire intraretinal vascular network. Finally, resident retinal microglial cells were also activated by injury to the adult optic nerve and phagocytosed degenerating neurons. Retinal microglial cells can be monitored with vital fluorescent dyes while they migrate across the retina and establish their intra-retinal network. It is possible to label microglia with lipophilic dyes and they remain labeled for a long time. In addition, intramural pericytes can be labeled by slow release of FG from RGCs and microglial cells. The findings suggest that ingested fluorescent dyes having different properties can be used to study different populations of retinal cells in vivo. Copyright 2000 Wiley-Liss, Inc.