TSPO expression as a marker for microgliosis in degenerating and aging retinas. In MacGreen mice, representative photomicrographs show low TSPO expression in retinal microglia (green GFP signal and red TSPO immunofluorescence) and constitutive expression in astrocytes (red TSPO immunofluorescence) (A-C) but strong up-regulation in retinal microglia from MacGreen/Rs1h-/Y mice (D-F). The overlap of TSPO and Iba1 immunostaining also indicates co-expression in human retinal microglia (G-I). TSPO immunostaining staining does not co-localize with the Müller cell marker GFAP (J, K) or the neuronal microtubule marker MAP2 (L, M). ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer, GFP, green fluorescent protein; TSPO, translocator protein (18 kDa); GFAP, glial fibrillary acid protein; MAP2, microtubule-associated protein 2. Scale bar, 50 μm.