Morphological and Functional Heterogeneity of the Mouse Intrahepatic Biliary Epithelium

Rat and human biliary epithelium is morphologically and functionally heterogeneous. Since no information exists on the heterogeneity of the murine intrahepatic biliary epithelium, and with increased usage of transgenic mouse models to study liver disease pathogenesis, we sought to evaluate the morphological, secretory and proliferative phenotypes of small and large bile ducts and purified cholangiocytes in normal and cholestatic mouse models. METHODS: For morphometry, normal and BDL mouse livers (C57/BL6) were dissected into blocks of 2-4 μm(2), embedded in paraffin, sectioned, and stained with H&E. Sizes of bile ducts and cholangiocytes were evaluated by using SigmaScan to measure the diameters of bile ducts and cholangiocytes. In small and large normal and BDL cholangiocytes, we evaluated the expression of cholangiocyte specific markers, keratin-19 (KRT19), secretin receptor (SR), cystic fibrosis transmembrane conductance regulator (CFTR), and chloride bicarbonate anion exchanger 2 (Cl(-)/HCO(-)(3) AE2) by immunofluorescence and western blot; and intracellular cAMP levels and chloride efflux in response to secretin (100 nM). To evaluate cholangiocyte proliferative responses after bile duct ligation (BDL), small and large cholangiocytes were isolated from BDL mice. The proliferation status was determined by analysis of the cell cycle by FACS and bile duct mass was determined by the number of KRT19-positive bile ducts in liver sections. RESULTS: In situ morphometry established that the biliary epithelium of mice is morphologically heterogeneous, which smaller cholangiocyte lining smaller bile ducts and larger cholangiocytes lining larger ducts. Both small and large cholangiocytes express KRT19 and only large cholangiocytes from normal and BDL mice express SR, CFTR, and Cl(-)/HCO(-)(3) exchanger and respond to secretin with increased cAMP levels and chloride efflux. Following BDL, only large mouse cholangiocytes proliferate. CONCLUSION: Similar to rats, mouse intrahepatic biliary epithelium is morphologically, and functionally heterogeneous. The mouse is a suitable model for defining the heterogeneity of the biliary tree.