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Development of Liver Pancreas and Spleen. the Liver Is Endodermal in Origin

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Development of Liver Pancreas and Spleen. the Liver Is Endodermal in Origin Development of liver pancreas and spleen. The liver is Endodermal in origin. It starts developing early in the fourth week of intrauterine life. The liver bud or hepatic diverticulum arises from the ventral aspect of the caudal part of foregut. It will form the liver, gallbladder, and biliary duct system. The hepatic diverticulum grows rapidly moves ventrally and cranially and enters into septum transversum from its inferior surface. Septum transversum is a thick mesoderm plate between thoracic and abdominal cavities. The hepatic diverticulum penetrates rapidly into the septum transversum. It enlarges and expands within the septum transversum and forms liver. As a result of the enormous growth of liver, the mesenchyme of septum transversum becomes stretched and membranous thus forming 1.falciform ligament, between anterior abdominal wall and liver 2.lesser omentum between liver and abdominal part of foregut Mesenchyme of septum transversum around the developing Liver differentiates into visceral peritoneum. It forms the false capsule of liver. bare area of liver. The umbilical vein passes in the free border of the falciform ligament on its way from the umbilical cord to the liver, After birth it is obliterated to form ligamentum teres hepatic. The ventral mesentery also forms the visceral peritoneum of the liver. Lesser omentum has two components – hepato-gastric ligament. – hepato-duodenal ligament. The free margin of lesser omentum contains bile duct, portal vein and hepatic artery. In adults the right border of lesser omentum forms the anterior boundary of epiploic foramen. Epiploic foramen is the connection between lesser sac and greater sac. Histogenesis Histologically liver has three components 1. Parenchyma 2. Stroma 3. Blood vessels Before entering into sinus venosus, vitelline veins form a plexus around duodenum and pass through septum transversum. The liver cords growing into septum transversum interrupt the course of vitelline veins, and extensive vascular network called hepatic sinusoids forms. Anterior view of primitive major veins during early fourth week. Initially umbilical veins pass on each side of liver but soon they shake hand with the rapidly growing and expanding liver. The whole right umbilical vein and proximal part of left umbilical vein disappear. The distal part of left umbilical vein is the only vein to carry blood from placenta to liver. Anterior view of primitive major veins of embryonic body during fifth week. With the increase of placental circulation, a direct communication is formed between left umbilical vein and right hepato-cardiac channel. This is called ductus venosus. After birth left umbilical vein becomes ligamentum teres hepatis and ductus venosus becomes ligamentum venosum respectively. Major veins during 7th week of intra-uterine life. Major veins during 10th week of intra-uterine life. The liver grows very rapidly and, from the fourth to tenth weeks, fills a large part of the abdominal cavity. Transverse section of upper part of abdomen by the end of 4th week. Transverse section of upper part of abdomen by the end of 5th week. Initially, the right and left lobes are about the same size, but the right lobe soon becomes larger. Hematopoiesis begins during the sixth week, giving the liver a bright reddish appearance. Large nests of proliferating cells lie between hepatic cells and walls of the vessels. These cells produce red and white blood cells. This hemopoietic activity (formation of various types of blood cells and other formed elements) is mainly responsible for the relatively large size of the liver between the 7th and 10th weeks of development. The hemopoiesis begins in the liver at about the sixth week of intrauterine life (IUL) and continue till birth. The hepatocytes start secreting bile at about twelfth week (3 months) of IUL. By the 10th week, the liver accounts for about 10% of the total body weight of the fetus. This activity gradually subsides during the last two months of intrauterine life, and only small hematopoietic islands remain at birth. The weight of the liver is then only 5% of the total body weight. The proximal part of hepatic diverticulum remains narrow and forms biliary system. A small ventral bud develops from the developing bile duct. It expands distally to form gall bladder while its proximal part remains narrow and form cystic duct. The stalk connecting the hepatic and cystic ducts to the duodenum becomes the bile duct (common bile duct). Initially the extra-hepatic biliary apparatus is occluded with epithelial cells. Later on it is canalized because of vacuolization resulting from degeneration of these cells. Initially, the bile duct attaches to the ventral aspect of the duodenal loop; however, as the duodenum grows and rotates, the entrance of the bile duct is carried to the dorsal aspect of the duodenum. Hepatic cells begin to form bile during twelfth week of intrauterine life. The bile entering the duodenum through the bile duct after the thirteenth week gives the meconium a dark green color. Phrygian cap: It occurs when fundus of the gallbladder folds on itself to form a cap-like structure—the Phrygian cap. Development of pancreas .
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