Basal migration of POS phagosomes and lysosome localization. (A) Apical-basal distribution of ingested POSs in primary cultures of WT or Klc1^−/− RPE cells, after 10-min incubation with purified POSs. (B) Diagrammatic representation of results from A, showing that phagosomes in Klc1^−/− RPE cells are located more apically than those in WT. (C) Semithin sections of albino retinas of WT (^+/+) or Klc1^−/− mice, within 0.5 mm of the optic nerve head. Images oriented as per orientation of the cultured RPE cells; distal tips of the POSs project down from the top. Arrowheads indicate phagosomes. (D) Proportion of phagosomes in the apical region of the RPE, in retinas fixed 3 h after light onset (***, P = 0.0002). (E) Vertical section of the RPE from albino mouse, with neuroretina detached, showing immunolabeling for CTSD (green) and RHO (red), plus DAPI labeling of nuclei (blue). The more apical (upper) POS phagosomes, which are newer, appear red, whereas the more basal phagosomes have mostly become phagolysosomes and appear yellow (merge of red and green). Ch, choroid. (F) EM showing a lysosome labeled with immunogold, representing CTSD localization. (G) Localization of CTSD-labeled endosomes and lysosomes in different regions of the RPE. Data are from immunoEM images of retinas fixed 3 h after light onset. Bars: (C and E) 10 µm; (F) 1 µm. Error bars are means ± SEM.