The ileal mucus of the CftrΔF508 mouse is attached to the epithelium and cannot be easily aspirated. (A) Carnoy-fixed ileal sections from WT (n = 7) and CF (n = 7) mice were immunostained for Muc2 (green) and nuclei (blue). Arrows point to mucus attached to goblet cells. Bars, 100 µm. (B) Transmission electron micrographs of CCh- and PGE₂-stimulated ileal explants of WT (n = 2) and CF (n = 2) mice. Empty denotes space empty from mucus between the epithelial cell microvilli and mucus layer. Note the distended crypt with condensed material in CF mice. Bars, 2 µm. (C) Bright field images of WT mouse ileal tissue mounted in the horizontal chamber with charcoal to visualize the upper surface of the transparent mucus layer before and after aspiration. See Video 1 to watch removal of charcoal labeled mucus (arrow points at easily distinguished crypt opening). (D) Same experiment as in C, but on ileal explant from the CF mouse. See Video 2 to watch the difficulty in removing the thick and opaque mucus covering the villi (arrows). (E) Representative confocal images of a single villus from ileal tissue (red) from WT (n = 3) or CF (n = 4) mice mounted in the horizontal chamber overlaid with 2 µm fluorescent beads (green) for 40 min. Experiment illustrates mucus penetrability. Bars, 50 µm.