Cholangiocytes and blood supply

Abstract

The microvascular supply of the biliary tree, the peribiliary plexus (PBP), stems from the hepatic artery branches and flows into the hepatic sinusoids. A detailed three-dimensional study of the PBP has been performed by using the Scanning Electron Microscopy vascular corrosion casts (SEMvcc) technique. Considering that the PBP plays a fundamental role in supporting the secretory and absorptive functions of the biliary epithelium, their organization in either normalcy and pathology is explored. The normal liver shows the PBP arranged around extra- and intrahepatic biliary tree. In the small portal tract PBP was characterized by a single layer of capillaries which progressively continued with the extrahepatic PBP where it showed a more complex vascular network. After common duct ligation (BDL), progressive modifications of bile duct and PBP proliferation are observed. The PBP presents a three-dimensional network arranged around many bile ducts and appears as bundles of vessels, composed by capillaries of homogeneous diameter with a typical round mesh structure. The PBP network is easily distinguishable from the sinusoidal network which appears normal. Considering the enormous extension of the PBP during BDL, the possible role played by the Vascular Endothelial Growth Factor (VEGF) is evaluated. VEGF-A, VEGF-C and their related receptors appeared highly immunopositive in proliferating cholangiocytes of BDL rats. The administration of anti-VEGF-A or anti-VEGF-C antibodies to BDL rats as well as hepatic artery ligation induced a reduced bile duct mass. The administration of rVEGF-A to BDL hepatic artery ligated rats prevented the decrease of cholangiocyte proliferation and VEGF-A expression as compared to BDL control rats. These data suggest the role of arterial blood supply of the biliary tree in conditions of cholangiocyte proliferation, such as it occurs during chronic cholestasis. On the other hand, the role played by VEGF as a tool of cross-talk between cholangiocytes and PBP endothelial cells suggests that manipulation of VEGF release and function could represent a therapeutic strategy for human pathological conditions characterized by damage of hepatic artery or the biliary tree.

Figure 1

SEMvcc of normal rat liver (orig. magn., 20×). A dense vascular network is arranged around the circumference of the common bile duct. A: transverse section of Extrahepatic peribiliary plexus. Observe the arterial and venous vessels on the outer layer (B) and a dense capillary network in the inner layer of the plexus (C).

Figure 2

SEMvcc of normal rat liver (orig. magn., A: 40×; B: 110×). (A) small portal tract: PBP is characterized by a single layer of capillaries. (B) * = terminal hepatic artery, V = vena porta, arrows = peribiliary plexus, S = sinusoids.

Figure 3

SEMvcc of BDL rat liver (orig. magn., A: 40×; B: 60×). (A) proliferated PBP run at the periphery of the lobule separated from the sinusoidal network (S) by an empty space (*) which corresponds to proliferating connective tissue. (B) not completely developed PBP with typical neovascular organization with dead end vessels (arrows).

Figure 4

Immunohistochemistry for VEGF-A (A) and VEGF-C (B). Proliferating bile ducts in BDL rat show an intense positivity for both VEGF-A and VEGF-C (Orig. magn., 20×).

Figure 5

Immunohistochemistry for VEGF-A. Normal rat liver. The hepatocyte of pericentral zone shows positivity for VEGF-A (Orig. magn., 20×).

Figure 6

Effect of hepatic artery ligation and chronic administration of VEGF on bile duct volume.Bile Duct volume is measured as a volume fraction of the entire liver tissue specimen. HAL induced in BDL rats a disappearance of PBP and an impaired cholangiocyte proliferation. The effects of HAL on PBP and cholangiocyte functions were prevented by r-VEGF by maintaining the integrity of the peribiliary plexus and cholangiocyte proliferation.