Bulkhead-like apical membrane structures between hepatocytes are required for anisotropic lumen expansion and liver tissue morphogenesis

Cell polarity is key to epithelial organization. Whereas polarized epithelial cells have a single apico-basal axis, hepatocytes exhibit a complex multi-axial polarity. During development, the apical surfaces of hepatocytes elongate anisotropically, generating a 3D tubular network of bile canaliculi (BC). Here, to elucidate the mechanisms of hepatocyte polarity and re-engineer it into simple epithelial polarity, we optimised a culture system of primary mouse hepatoblasts that recapitulates hepatocyte differentiation and BC morphogenesis. Remarkably, we discovered a pattern of specific extensions of the apical membrane sealed by tight junctions traversing the lumen between two adjacent hepatocytes that remind the bulkheads of boats. These apical bulkheads were observed also in the developing liver. Screening for molecular factors required for hepatocyte polarity revealed that silencing of Rab35 caused loss of the bulkheads, conversion into simple polarity, formation of cyst-like structures and change in cell fate. By exploiting Rab35 depletion in the developing liver we could re-engineer hepatocyte polarity and trigger formation of epithelial tubes. Our results suggest a new model of BC morphogenesis based on mechanical stabilization of the tubular lumen.