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Congenital hepatic fibrosis (CHF) and Caroli's disease are though to result from ductal plate malformation, and the basal laminar components play important roles in biliary differentiation during development. To clarify the involvement of basal laminar components in the ductal plate malformation, this study examined the immunohistochemical expression of laminin and type IV collagen in the livers of CHF and Caroli's disease. Using the polycystic kidney (PCK) rat, an animal model of Caroli's disease with CHF,in vivoandin vitroexperiments were also performed. Immunostaining showed that basement membrane expression of laminin and type IV collagen around intrahepatic bile ducts was degraded in CHF, Caroli's disease, and the PCK rats. The degradation of laminin and type IV collagen around bile ducts was also observed in foci of cholangiocarcinomain situof Caroli's disease.In vitro, PCK cholangiocytes were found to overexpress plasminogen and a serine proteinase, the tissue-type plasminogen activator (tPA). When PCK cholangiocytes were cultured in Matrigel™, the amounts of laminin and collagen in the gel were significantly reduced, and addition of α2-antiplasmin in the culture medium inhibited the degradation of laminin and collagen in Matrigel™. These results suggest that biliary overexpression of plasminogen and tPA leads to the generation of excessive amounts of plasmin, and subsequent plasmin-dependent lysis of the extracellular matrix molecules may contribute to the biliary dysgenesis in CHF and Caroli's disease, including progressive cystic dilatation of the intrahepatic bile ducts in Caroli's disease. In addition, it is suggested that once cholangiocarcinomain situdevelops in the biliary epithelium of CHF and Caroli's disease, it tends to transform into invasive carcinoma, due to instability of the basement membrane of the bile ducts. Copyright © 2008 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.