Normal and Abnormal Liver Development

Two primordia contribute to the development of the liver (Fig. 9.1). The parenchyma is entirely derived from the hepatic diverticulum, while the stroma is derived from the septum transversum. The hepatic diverticulum appears as an outgrowth of the endodermal epithelium at the distal end of the foregut (future duodenum) at 29 days (stage 11) (O’Rahilly and Muller, Human embryology and teratology, 3rd ed. Wiley-Liss, New York, 2001). This outgrowth penetrates the septum transversum, that is, the mesodermal plate that separates the embryonic thoracic and abdominal cavities (Fig. 9.1a). The endodermal cells in the cranial part of the diverticulum that penetrate the septum transversum differentiate into hepatocytes and epithelial cells of the bile ducts. The mesenchymal cells of the septum transversum form the connective tissue elements of the hepatic stroma and capsule. As the liver rapidly grows during the weeks following the initial budding of the hepatic diverticulum, it separates from the residual mesenchyme of the septum transversum, which becomes the diaphragm. The caudal part of the hepatic diverticulum, the portion that does not invade the septum transversum, forms the gallbladder, cystic duct, common bile duct, and ventral pancreatic anlage.