(A) Expression of the homeodomain protein Pdx1 (green) labels multipotent progenitors of the E10.5 pancreatic primordium. At this stage, all Pdx1+ cells co-express claudin 1 (red; arrow). (B) Claudin 1 (red; arrow) localizes to the lateral membrane of the E10.5 pancreatic epithelium. At this stage, expression of the tight junction marker ZO-1 (green; arrowhead) is nearly absent in the pancreas. (C) By E12.5, when apicobasal polarity has been established, claudin 1 co-localizes with ZO-1 (arrow) in primitive pancreatic ducts. (D,E) At P5, claudin 1 co-localizes with ZO-1 at TJ complexes in ducts (arrows; dotted line in E), but not in islets (dashed line in E). (F) Also at this stage, claudin 1 co-localizes with ZO-1 in acinar cells (arrow; dotted lined). (E) Auto-fluorescent red blood cells are marked with asterisks. (F) Non-specific claudin 1 cytoplasmic staining is observed in acinar cells at P5. Images in B–F were taken with a confocal microscope, and cell nuclei were stained with DAPI (blue). Scale bars: 25 μm (A), 10 μm (B,D–F), 5 μm (C).