EMBRYOLOGY AND STRUCTURE OF THE LIVER
At the 18th day of gestation (2.5-mm stage), a thickening of the ventral floor of the distal foregut, corresponding to the future duodenum, heralds the appearance of the hepatic diverticulum. This diverticulum is formed from the proliferation of endodermal cells at the cranioventral junction of the yolk sac and foregut. Subsequently, the hepatic diverticulum penetrates the adjacent mesoderm and capillary plexus, known as the septum transversum. Cellular interactions between the endoderm and mesoderm result in rapid cell proliferation and the formation of hepatocytes, angioblasts, and sinusoids.
By the third and fourth weeks of gestation (3- to 4-mm stage), the growing diverticulum enlarges to form a double diverticulum that projects into the septum transversum, then divides into a solid cranial portion and hollow caudal portion, which is evident by the 5-mm stage. The cranial portion differentiates into proliferating cords of hepatocytes and intrahepatic bile ducts, while the smaller caudal portion (pars cystica) forms the primordium of the gallbladder, common bile duct, and cystic duct.
The budding liver sequentially invades the vitelline veins and then the umbilical (placental) veins. The vitelline veins run from the gut–yolk sac complex to the heart. As the liver invades the vitelline veins, the midsection of the veins becomes capillarized. The caudal ends persist as the primitive portal veins, and the cranial ends as the primitive hepatic veins. During the 6- to 7-mm stage, part of the left umbilical vein becomes the ductus venosus, which shunts placenta derived arterial blood from the umbilical vein to the inferior vena cava. After birth, the obliterated prehepatic segments of the umbilical veins atrophy to become the round ligament, and the ductus venosus becomes the ligamentum venosum.
The hepatocytes of the hepatic portion grow as thick epithelial sheets intermingling between branching channels of the vitelline veins within the septum transversum to form a system of connecting liver cell plates, and the capillaries become the hepatic sinusoids. The sinusoids, present by 5 weeks of gestation, act as templates for the 3-dimensional growth of the hepatic cords. The liver cell plates are initially 3 to 5 cells thick. However, over time, they gradually transform to 1-cell-thick plates, a process that is not complete until 5 years of age. Intrahepatic bile ducts begin to form at 6 weeks of gestation within the hilum of the liver and gradually spread to the periphery until complete at 3 months.
The pars cystica is initially hollow, but epithelial proliferation obliterates the lumen early in its development. Therefore, both the primitive gallbladder and common bile duct consist of solid chords of epithelial cells directly beneath the developing liver in the 6- to 7-mm embryo. Recanalization of the common bile duct and hepatic duct occurs in the 7- to 8-mm and 10-mm embryo, respectively. At the 16-mm stage, the proximal gallbladder and cystic duct are hollow. At the third ...