Transversal apical membrane structures form during BC lumen morphogenesis in embryonic liver and persist in adulthood. (a) EM images of two serial sections (I and II) of a forming BC in a E15.5 liver. The black arrows point to the TJs within the bulkhead-like membrane connections B1 and B2. Scale bar: 500 nm. (b) Serial sections (I–III) focusing on bulkhead B1 show that the bulkhead does not separate the lumen completely. The lumen is continuous in section I, and the bulkhead appears above in sectionsII and III. Scale bar: 500 nm. IV, 3D reconstruction of the bulkhead B1 from the serial sections. (c) Fluorescent microscopy images of cultured mature hepatocytes isolated from adult livers. F-actin staining by phalloidin shows a striped pattern similar to that in Fig. 1 b. Scale bar: 10 µm. (d) EM image of BC in an adult liver section. Black arrows point to the TJs in the bulkhead. Scale bar: 500 nm. (e) Serial sections of the bulkhead in d. The bulkhead protrudes into the lumen but does not separate it, as the lumen in II is continuous. Scale bar: 500 nm. IV, 3D reconstruction of the bulkhead in d. White arrows point to the parts of bulkhead contributed by opposing cells connected via TJs.