Rod degeneration and microglial cell responses. Microphotographs of representative retinal cross sections from a naïve SD rat (A) showing a normal retina, a retina from a P23H-1 rat (B) and a retina from an RCS rat (C) in the early stages of retinal degeneration. Rod outer segments appear in red and microglial cells in green. In the P23H-1 rat retina, rhodopsin expression was clearly affected; however, in the RCS rat, rhodopsin expression is not altered since the retinal pigment epithelium (RPE) cells are not phagocytising the rod outer segment debris. In both experimental retinas, microglial cells were activated and migrated from the inner to the outer retinal layers.