In P21 rd10 retina, ONL nuclei immunopositive for activated caspase-3 (red, arrows), an enzyme activated during early apoptosis, were found separately from phagocytized photoreceptor nuclei in microglial phagosomes (green, arrowheads). Scale bar, 40 μm.
Immunohistochemistry for cleaved poly(ADP-ribose) polymerase (PARP), a substrate of activated caspase-3, shows the absence of immunopositive ONL nuclei in adult wild-type (WT) mouse. In the rd10 retina, immunopositive ONL nuclei were rare at P18, prevalent at P22, and decreased at P30, as expected from the progression of rod degeneration. At P22, ONL nuclei phagocytosed by microglia (arrows in insets) are predominantly immunonegative for cleaved PARP (arrowheads in orthogonal views). Scale bar, 40 μm.
Photoreceptor cones are not phagocytosed by microglia during rod degeneration. (Upper panels) At P21–23, although infiltrating microglia in the ONL (CD11b, green) contain phagocytosed nuclei (DAPI, blue), none of these were found to be associated with cone arrestin immunopositivity (red), despite the close proximity of arrestin-positive cone somata (highlighted by circled area) to infiltrating microglia. (Lower panels) Example of a CD11b-positive microglial cell in the ONL juxtaposed closely to an arrestin-positive soma (highlighted by circled area). Analysis of orthogonal views of the confocal image stack demonstrates the absence of cone phagocytosis by microglia. Scale bar, 10 μm.
