Heterogeneity of the intrahepatic biliary epithelium

Abstract

The objectives of this review are to outline the recent findings related to the morphological heterogeneity of the biliary epithelium and the heterogeneous pathophysiological responses of different sized bile ducts to liver gastrointestinal hormones and peptides and liver injury/toxins with changes in apoptotic, proliferative and secretory activities. The knowledge of biliary function is rapidly increasing because of the recognition that biliary epithelial cells (cholangiocytes) are the targets of human cholangiopathies, which are characterized by proliferation/damage of bile ducts within a small range of sizes. The unique anatomy, morphology, innervation and vascularization of the biliary epithelium are consistent with function of cholangiocytes within different regions of the biliary tree. The in vivo models [e.g., bile duct ligation (BDL), partial hepatectomy, feeding of bile acids, carbon tetrachloride (CCl4) or alpha-naphthylisothiocyanate (ANIT)] and the in vivo experimental tools [e.g., freshly isolated small and large cholangiocytes or intrahepatic bile duct units (IBDU) and primary cultures of small and large murine cholangiocytes] have allowed us to demonstrate the morphological and functional heterogeneity of the intrahepatic biliary epithelium. These models demonstrated the differential secretory activities and the heterogeneous apoptotic and proliferative responses of different sized ducts. Similar to animal models of cholangiocyte proliferation/injury restricted to specific sized ducts, in human liver diseases bile duct damage predominates specific sized bile ducts. Future studies related to the functional heterogeneity of the intrahepatic biliary epithelium may disclose new pathophysiological treatments for patients with cholangiopathies.

Figure 1

[Top] Isolation of small (A), approximately 8 μm diameter] and large (B), approximately 14 μm diameter] cholangiocytes from small and large ducts, respectively, from normal rats. Small and large cholangiocytes were purified by counterflow elutriation followed by immunoaffinity purification. Original magn., × 625. Reproduced with permission from Ref[12]. [Bottom] Isolation of small (C) and large (D) IBDU from normal rat liver. Small (< 15 μm in diameter) and large (> 15 μm in diameter) IBDU were pruned off from large ducts by a nitrogen pulsed dye laser and subsequently separated (D) by picking up IBDU with a micromanipulator micropipet. Original magnification × 2000. Reproduced with permission from Ref 13.

Figure 2

Measurement of H₃ histone gene expression in small and large cholangiocytes from 1-wk BDL rats and 1-wk BDL rats treated with CCl₄ or mineral oil. H₃ histone gene expression in large cholangiocytes decreased on d 2 before returning to control values on d 7 after CCl₄ treatment. H₃ histone gene expression (which was absent in small cholangiocytes from BDL rats) was expressed by small cholangiocytes on d 1 and 2 before returning to control undetectable values on d 7 after CCl₄ treatment. Administration of mineral oil to 1-wk BDL rats did not alter H₃ histone gene expression in large cholangiocytes. The message for H₃ histone gene was absent in small cholangiocytes from oil-treated rats. Comparability of RNA used was assessed by hybridization for GAPDH (housekeeping gene). Autoradiograms were quantified by densitometry. Densitometric values are means of 2 experiments. Reproduced with permission from Ref 50.

Figure 3

Working model for the heterogeneity of the intrahepatic biliary epithelium. The model proposes that: (1) bile ducts are morphologically heterogeneous with small ducts lined by small cholangiocytes and large ducts lined by large cholangiocytes; (2) small and large ducts similarly express both γ-GT and cytokeratin-19; and (3) large (but not small) ducts express the secretin and somatostatin receptor, CFTR and Cl^-/HCO₃^- and respond physiologically to these two hormones. The model also shows that following BDL, only large cholangiocytes proliferate and that a single dose of CCl₄ induces damage and loss of large duct function, whereas small cholangiocytes (resistant to CCl₄) de novo proliferate and secrete to compensate for the loss of large duct function. Reproduced with permission from Ref. 73.