tailieunhanh - Ebook Walker’s pediatric gastrointestinal disease (6/E): Part 2

Part 2 book “Walker’s pediatric gastrointestinal disease” has contents: Intestinal failure, intestinal tumors, intestinal motility, motility disorders, bowel injury, the liver, physiology, disorders of the biliary tract, infections, genetic and metabolic disorders, fatty liver disease in children, and other contents. | 2867 IV THE LIVER 2868 26 The Liver: Anatomy and Embryology Mark Davenport, ChM, FRCS (Paeds), FRCS (Eng)m, FRCPS (Glas) EMBRYOLOGY The liver first appears in the embryo as a hollow endodermal bud derived from the foregut. This grows within the ventral mesogastrium to reach the septum transversum (Figure 26-1). Hepatoblasts (expressing keratin [K] 8, 18, 19) are derived entirely from the budding endoderm and are identifiable from about 28 days. Actual hepatocytes appear by 20 weeks of gestation, initially expressing weak immunoreactivity for K 8, 18, particularly in areas away from the ingrowing portal veins. The process is believed to be triggered by hematopoietic cells within the fetal liver secreting the cytokine Oncostatin M (OSM) that promotes hepatocyte differentiation together with glucocorticoids, hepatocyte growth factor (HGF), and the Wnt signaling pathway. They are arranged in plates, initially 3–4 cells thick, which line the vascular sinusoidal network. Mesodermal cells from this loose arrangement of tissue go on to form the mesenchymal framework of the liver, including its perisinusoidal Ito cell population (vide infra). Kupffer cells appear at about five weeks and appear to originate outside of the liver. 2869 Figure 26-1 Timeline for the development of the liver. Bile ducts are derived from two distinct elements. Intrahepatic bile ducts appear from about seven weeks while the extrahepatic bile ducts are derived from the budding foregut Each branch of what will become the portal vein is surrounded by a layer of mesenchyme and then a cylindrical double cell layer of darkly staining cells derived from hepatoblasts termed the ductal (or limiting) plate. Remodeling of this layer occurs at about 12 weeks to form a network of interconnected bile ducts enveloped into the mesenchyme but retaining connections with the intrahepatic bile This process begins at the porta hepatis and related to the arborization of the portal