Fig. 2. Visualization of the apical endomembrane system, following internalization of a fluorescent marker by apical endocytosis. (A) Overlap of a marker for apical endocytosis (Texas-Red-labeled dextran), endolyn-78 (green) and Rab11 (blue). Note the overlap of endolyn-78 and Rab11 (cyan) and the overlap of dextran and Rab11 (magenta). Colocalization of all three probes (white) is restricted to the pericanalicular region. Small compartments (<300 nm) are distinguishable close to the apical membrane that contain both endolyn-78 and Rab11, probably reflecting their colocalization in the SAC, as endolyn-78 accumulates in the SAC prior to its delivery to lysosomes. Note that Rab11 is thought to be particularly enriched in the distal part of the SAC or ARE. (B) Overlap of a marker for apical endocytosis with endolyn-78 and basolateral-to-apical transcytosing polymeric IgA receptor (pIgAR). Note the overlap of dextran and endolyn-78 (yellow), of endolyn-78 and pIgAR (cyan), some of which overlaps dextran (white), and of dextran with pIgR-dIgA (magenta). Bars, 5 µm. The apical endocytic pathway (i.e. the route from the AEE to the SAC to lysosomes taken by dextran) intersects the transcytotic route taken by pIgR-dIgA in a small (<300 nm) endolyn-positive SAC. (C-F) Electron micrographs of liver infused retrogradely with HRP for various time intervals. HRP initially localizes to 60-100 nm tubulovesicular structures close to the apical surface (C, 5 minutes). After 10 minutes, multivesicular bodies (MVBs; D) and cup-shaped vesicles (150-200 nm; E) became labeled with HRP, prior to delivery to the lysosomes (F; 15 minutes). Bar, 500 nm. For further details, see text and Rahner et al. (Rahner et al., 2000).