Proposal of a new disease concept "biliary diseases with pancreatic counterparts". Anatomical and pathological bases.

The biliary tract and pancreas are located closely anatomically, and both develop from the endoderm foregut almost at the same time. Interestingly, the lining epithelia of the bile duct and main pancreatic duct show similar morphologies and phenotypes, and both are accompanied by periductal glands. Furthermore, the exocrine pancreatic acini are remnantly found in the peribiliary glands. Based on these findings, it seems plausible that the biliary tract has features of pancreatic elements in addition to the duct system, which is specialized for the drainage of bile secreted by hepatic parenchyma, particularly, hepatocytes. Interestingly, some pancreatic and biliary diseases show similar pathological features and even biological behaviors. For example, extrahepatic cholangiocarcinoma and ductal adenocarcinoma of the pancreas share many clinicopathological features. Both of them are hypothesized to arise from similar preneoplastic and early neoplastic intraepithelial lesions. Intraductal papillary tumors, with frequent mucin hyperproduction, develop in the pancreas (intraductal papillary mucinous neoplasm) and also in the biliary tract (intraductal papillary neoplasm of the bile duct). IgG4-related disease affects the biliary tract (IgG4-related sclerosing cholangitis) and the pancreas (autoimmune pancreatitis) in the same patients, with both showing similar morphologies. Herein, we propose that these non-neoplastic and neoplastic biliary diseases showing similarities to corresponding pancreatic diseases could be included in a new disease concept "biliary diseases with pancreatic counterparts". Based on this new concept, information obtained in biliary tract diseases could be applied to the analysis of pancreatic disease and vice versa, and also novel therapeutical strategies and molecular and genetic studies on pancreatic and biliary diseases may be developed with a unified approach.