82. The Foregut: The Liver and Biliary Apparatus (gallbladder and Ducts)

  1. The liver and biliary apparatus: their primordium appears at about week 3 as an anterior thickening and outgrowth or bud from the entodermal epithelium of the most distal end of the foregut (future duodenum). This hepatic diverticulum or bud, consisting of rapidly proliferating cell strands, penetrates the septum transversum (the mesodermal plate between the pericardial cavity and the stalk of the yolk sac) where it enlarges and divides into 2 parts
    1. THE LARGER CRANIAL PART is the liver primordium
      1. The proliferating entodermal cells form interlacing cords of liver cells or hepatic parenchyma and the epithelial lining of the intrahepatic portion of the biliary apparatus
      2. As the cords penetrate the septum transversum, they cause fragmentation of the umbilical and vitelline veins supplying the liver, resulting in the formation of the hepatic sinusoid network
      3. The fibrous and hematopoietic tissue and Kupffer cells of the liver are derived from the splanchnic mesenchyme of the septum transversum
      4. The liver grows rapidly and soon fills most of the abdominal cavity. At first, the right and left lobes of the liver are equal in size, but the right lobe becomes larger, and the caudate and quadrate lobes develop as subdivisions of the left lobe
    2. THE SMALLER CAUDAL PORTION of the liver diverticulum expands to form the gallbladder and its stalk, the cystic duct
      1. Bile pigment begins to form during weeks 13 to 16 and enters the duodenum, making its contents (meconium) a greenish color
      2. At first, the extrahepatic biliary apparatus is occluded with entodermal cells, but it later recanalizes
      3. The stalk connecting the hepatic and cystic ducts to the duodenum becomes the common bile duct
        1. The common bile duct is initially attached to the ventral side of the duodenal loop, but with duodenal growth and rotation, the duct finally enters the duodenum on its dorsal side
  2. Hematopoiesis begins during week 6 and the liver becomes a bright red. This function is responsible for the relatively large size of the liver during month 2. The liver weighs about 10% of the total fetal body weight by week 9
    1. liver HEMATOPOIESIS subsides gradually during the last 2 months of intrauterine life, and only small hematopoietic islands remain at birth, at which time, the liver is only about 5% of the total body weight
  3. The ventral mesentery is a thin, double-layer mesodermal membrane which gives rise to 3 structures
    1. THE LESSER OMENTUM: lying between the liver and the ventral borders of the stomach (gastrohepatic ligament), and the liver and duodenum (duodenohepatic ligament)
      1. In the free margin of this omentum are the bile duct, the portal vein, and the hepatic artery
    2. THE FALCIFORM LIGAMENT: between the liver and the ventral abdominal wall
      1. Contains the umbilical vein (from umbilical cord to liver) in its inferior border
    3. THE VISCERAL PERITONEUM OF THE liver covers the liver except for the area in contact with the diaphragm, the bare area of the liver

the foregut: the liver and biliary apparatus (gallbladder and ducts): image #1