The anlage of the biliary ducts and liver consists of a diverticulum that appears on the ventral aspect of the foregut in 3 mm embryos. The cranial portion becomes the liver; a caudal bud forms the ventral pancreas; and an intermediate bud develops into the gallbladder. Originally hollow, the hepatic diverticulum becomes a solid mass of cells that later recanalizes to form the ducts. The smallest ducts—the bile canaliculi—first appear as a basal network between the primitive hepatocytes that eventually expands throughout the liver (Figure 25–1). Numerous microvilli increase the canalicular surface area. Bile secreted here passes through the interlobular ductules (canals of Hering) and the lobar ducts and then into the hepatic duct in the hilum. In most cases, the common hepatic duct is formed by the union of a single right and left duct, but in 25% of individuals, the anterior and posterior divisions of the right duct join the left duct separately. The origin of the common hepatic duct is close to the liver but always outside its substance. It runs about 4 cm before joining the cystic duct to form the common bile duct. The common duct begins in the hepatoduodenal ligament, passes behind the first portion of the duodenum, and runs in a groove on the posterior surface of the pancreas before entering the duodenum. Its terminal 1 cm is intimately adherent to the duodenal wall. The total length of the common duct is about 9 cm.
Scanning electron photomicrograph of a hepatic plate with adjacent sinusoids and sinusoidal microvilli and a bile canaliculus running in the center of the liver cells. Although their boundaries are indistinct, about four hepatocytes constitute the section of the plate in the middle of the photograph. Occasional red cells are present within the sinusoids. (Reduced from × 2000.) (Courtesy of Dr James Boyer.)
In 80%-90% of individuals, the main pancreatic duct joins the common duct to form a common channel about 1-cm long. The intraduodenal segment of the duct is called the hepatopancreatic ampulla, or ampulla of Vater.
The gallbladder is a pear-shaped organ adherent to the undersurface of the liver in a groove separating the right and left lobes. The fundus projects 1-2 cm below the hepatic edge and can often be felt when the cystic or common duct is obstructed. It rarely has a complete peritoneal covering, but when this variation does occur, it predisposes to infarction by torsion. The gallbladder holds about 50 mL of bile when fully distended. The neck of the gallbladder tapers into the narrow cystic duct, which connects with the common duct. The lumen of the cystic duct contains a thin mucosal septum, the spiral valve of Heister that offers mild resistance to bile flow. In 75% of persons, the cystic duct enters the common duct ...