To study the fate of Müller’s glia following experimental retinal detachment, using a “pulse/chase” paradigm of bromodeoxyuridine (BrdU) labeling for the purpose of understanding the role of Müller cell division in subretinal scar formation.
Experimental retinal detachments were created in pigmented rabbit eyes, and 3 days later 10 µg of BrdU was injected intravitreally. The retinas were harvested 4 h after the BrdU was administered (i.e., day 3) or on days 4, 7, and 21 post detachment. The tissue was fixed, embedded in agarose, and sectioned at 100 µm. The sections were labeled with various combinations of probes, including anti-vimentin and anti-S100 (as markers for Müller cells), anti-BrdU, anti-phosphohistone H3 (to identify mitotic cells), and the isolectin B4 (to identify macrophages and microglia). Images were captured using an Olympus Fluoview 500 confocal microscope. To aid in our understanding of how Müller cell nuclei undergo cell division, two additional procedures were used: 1) electron microscopy of normal cat and rabbit retinas and 2) a new method using 5-fluorouracil and subsequent anti-BrdU labeling to detect all Müller cell nuclei, using confocal imaging.
Three days after detachment, anti-vimentin labeled all Müller cells, some of which were also labeled with anti-BrdU. On day 4, many of the anti-BrdU-labeled Müller cell nuclei appeared in columns with one labeled nucleus in the inner nuclear layer and another directly sclerad to it in the outer nuclear layer. By day 7, most anti-BrdU-labeled nuclei were observed in subretinal scars. At 3 weeks, some anti-BrdU-labeled nuclei that remained within the retina did not express vimentin or S100. Anti-phosphohistone H3-labeled (i.e., mitotic) cells, some of which were also labeled with anti-BrdU, were only observed in the outer nuclear layer on day 4, and these nuclei were surrounded by an accumulation of vimentin filaments. Isolectin B4-labeled microglia and macrophages also incorporated BrdU and were observed throughout the retina and in subretinal scars during all times of detachment. Electron microscopy and immunofluorescence labeling of the 5-fluorouracil-injected eyes revealed the presence of a unique structural relationship between Müller cell nuclei and intermediate filament proteins.
Following retinal detachment, many Müller cell nuclei initially migrate to the outer retina, undergo mitosis, and eventually reside in subretinal glial scars, suggesting a possible link between the early division of Müller cells and the process of subretinal gliosis. In addition, a subpopulation of anti-BrdU-labeled cells, presumably once Müller cells, appears to stop expressing well accepted Müller cell marker proteins, suggesting a potential dedifferentiation of some of these cells over time. Additionally, Müller cell nuclei may use intermediate filaments as a “track” for migration into the outer retina and later as an important component of cell division by the accumulation of vimentin filaments around the mitotic nuclei.