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DEVELOPMENT

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The hepatobiliary system originates as a cluster of endodermal cells that forms a diverticulum from the duodenal region of the primitive embryonic gut. The diverticulum divides into 2 components between gestation weeks 18 and 22. (1) A solid cranial component (the pars hepatica) gives rise to the liver and intrahepatic portion of the biliary system, and (2) a hollow caudal component (the pars cystica) gives rise to the gallbladder (GB) and extrahepatic portion of the biliary system.

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Cords of cells from the pars hepatica grow into the vascular mesenchyme and the adjoining septum transversum. The vasculature, lymphatics, and connective tissue of the liver develop from the vascular mesenchyme of this septum. Embryonic capillaries dilate to form the hepatic sinusoids. Cells in the walls of these capillaries differentiate into the reticuloendothelial cells of the liver. The hepatic parenchyma and lining cells of the intrahepatic bile ducts develop from endodermal cells of the pars hepatica.

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During the eighth week of gestation, hepatic precursor cells in the hilar region form a sleeve-like layer of cells around portal veins (PVs), with subsequent extension into the periphery of the liver. This sleeve of tissue is termed the ductal plate. Intrahepatic bile ducts arise by way of remodeling and partial involution of these cylindrical ductal plates. The initial single-layer ductal plate soon becomes double-layered, thereby defining a slit-like primitive bile duct lumen between the 2 layers. There is subsequent extensive resorption of the ductal plate tissue leading to the formation of a network of fine bile ducts surrounding the PV. Bile duct formation by this process of ductal plate remodeling progresses from larger ducts to segmental and lobar branches and finally to small peripheral ductules.

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The pars cystica gives rise to the GB, cystic duct, and extrahepatic bile ducts. In the early stages of development, endodermal cells fill the lumina of these structures. Canalization begins at approximately gestation week 7. The independently developing intrahepatic and extrahepatic components of the biliary system unite to establish continuity of the biliary drainage system by gestation month 3. Hepatic secretion of bile begins during gestation week 12. Secretion of bile into the duodenum occurs between weeks 13 and 16. Hematopoiesis occurs in the fetal liver and is still active at birth; this extramedullary hematopoietic activity ceases by approximately 6 weeks of age in normal infants.

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Anomalies related to abnormal development of the ductal plates (i.e., ductal plate malformations) constitute a spectrum of hepatobiliary lesions, termed fibropolycystic liver disease. Congenital hepatic fibrosis and bile duct hamartomas are due to ductal plate malformations of the small interlobular bile ducts. Autosomal dominant polycystic disease results in ductal plate malformations of the medium-sized intrahepatic ducts. Ductal plate malformations of the large intrahepatic bile ducts occur in patients with Caroli disease. Some choledochal cysts (CCs) are apparently related to ductal plate malformations of the main bile duct. In some patients, manifestations of more than ...

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