SEGMENTAL AND SECTORAL DUCTS

The segmental ducts of the left liver have a relatively constant pattern, although several segmental ducts may drain each particular segment. The left hepatic duct is formed by the union of segment II and III ducts behind the umbilical portion of the left portal vein (Fig. 69.1). The segment IV duct is more variable, but usually drains into the left hepatic duct. The right hepatic duct is formed by the union of the right medial (anterior) and lateral (posterior) sectoral ducts. These sectoral ducts in turn are formed by the segmental ducts: VII and VI from the lateral, and VIII and V from the medial duct. The right lateral sectoral duct is often identified on a cholangiogram as curving around the right medial duct before joining the medial side of the medial sectoral duct: this is often described as Hjortsjo’s crook, and has surgical importance for liver resections. The right hepatic duct and its branches are subject to more variations than the left ductal system, and these variations have been classified by Blumgart into six main types (Table 69.1).

Table 69.1 Variations of the right hepatic duct and its branches

CYSTIC DUCT

The cystic duct drains the gallbladder into the common bile duct. It is between 3 and 4 cm long, passes posteriorly to the left from the neck of the gallbladder, and joins the common hepatic duct to form the common bile duct. It almost always runs parallel to, and is adherent to, the common hepatic duct for a short distance before joining it. The junction usually occurs near the porta hepatis but may be lower down in the free edge of the lesser omentum. The cystic duct may have several important variations in its anatomy (Fig. 69.3). Rarely, the cystic duct lies along the right edge of the lesser omentum all the way down to the level of the duodenum before the junction is formed, but in these cases the cystic and common bile ducts are usually closely adherent. The cystic duct occasionally drains into the right hepatic duct, in which case it may be elongated, lie anterior or posterior to the common hepatic duct, and join the right hepatic duct on its left border. Rarely, the duct is double or even absent, in which case the gallbladder drains directly into the common bile duct. One or more accessory hepatic ducts occasionally emerge from segment V of the liver and join either the right hepatic duct, the common hepatic duct, the common bile duct, the cystic duct, or the gallbladder directly. These variations in cystic duct anatomy are of considerable importance during surgical excision of the gallbladder. Ligation or clip occlusion of the cystic duct must be performed at an adequate distance from the common bile duct to prevent angulation or damage to it. Accessory ducts must not be confused with the right hepatic or common hepatic ducts.

Fig. 69.3 Variations in the anatomy of the cystic and intrahepatic bile ducts. The cystic duct variations are labelled A1–A3 and the letters B–F are used for the variants of the intrahepatic ducts to ensure consistency with the Blumgart nomenclature.

The mucosa of the cystic duct bears 5–12 crescentic folds, continuous with those in the neck of the gallbladder. They project obliquely in regular succession, appearing to form a spiral valve when the duct is cut in longitudinal section. When the duct is distended, the spaces between the folds dilate and externally it appears twisted like the neck of the gallbladder.

HEPATIC BILE DUCTS

The main right and left hepatic ducts emerge from the liver and unite near the right end of the porta hepatis as the common hepatic duct. The extrahepatic right duct is short and nearly vertical while the left is more horizontal and lies along the base of segment IV. The common hepatic duct descends approximately 3 cm before being joined on its right at an acute angle by the cystic duct to form the common bile duct. The common hepatic duct lies to the right of the hepatic artery and anterior to the portal vein in the free edge of the lesser omentum.

COMMON BILE DUCT

The common bile duct is formed near the porta hepatis, by the junction of the cystic and common hepatic ducts (Figs 69.4 and 69.5), and is usually between 6 and 8 cm long. Its diameter tends to increase somewhat with age but is usually around 6 mm in adults. It descends posteriorly and slightly to the left, anterior to the epiploic foramen, in the right border of the lesser omentum, where it lies anterior and to the right of the portal vein and to the right of the hepatic artery. It passes behind the first part of the duodenum with the gastroduodenal artery on its left, and then runs in a groove on the superolateral part of the posterior surface of the head of the pancreas. The duct lies anterior to the inferior vena cava and is sometimes embedded in the pancreatic tissue. It may lie close to the medial wall of the second part of the duodenum or as much as 2 cm from it: even when it is embedded in the pancreas, a groove in the gland marking its position can be palpated behind the second part of the duodenum.

Fig. 69.4 Endoscopic retrograde cholangiopancreatogram.

Fig. 69.5 Magnetic resonance cholangiopancreatogram.

BILIARY STONES

Gallstones usually form in the gallbladder. As the gallbladder empties, gallstones move towards the cystic duct. When small stones enter the cystic duct they may irritate the columnar mucosa, this leads to spasm of the smooth muscle in the cystic duct wall, and the spasm generates pain known as biliary colic, which is often very severe. The mucosal folds in the neck of the gallbladder and the cystic duct provide a common site of entrapment of gallstones. Stones occluding the neck of the gallbladder may cause a sterile distension of the gallbladder: providing the gallbladder has not undergone acute inflammation previously it remains non-fibrotic and readily distensible, and an enlarged fundus often becomes palpable below the costal margin. Stones lodged in the distal cystic duct may cause swelling in the tissues around the duct. The close relationship of the distal cystic duct to the common hepatic duct means that this swelling may cause secondary compression of the common hepatic duct, resulting in partial obstruction to the flow of bile and the appearance of mild jaundice, the so called ‘Mirizzi syndrome’. Once stones have passed through the cystic duct they often become impacted at the junction of the common bile duct and pancreatic duct just proximal to the hepatopancreatic ampulla, producing obstructive jaundice and/or acute pancreatitis.

ENDOSCOPIC CHOLANGIOGRAPHY

The common bile duct may be accessed endoscopically from the duodenum for diagnostic cholangiography and therapeutic interventions. The angled relationship of the distal common bile duct and pancreatic duct may make direct cannulation of the bile duct difficult. When the duct lies in a more vertical position, embedded within the wall of the duodenum, a direct incision in the base of the hepatopancreatic ampulla and the adjacent wall of the duodenum (pre-cut sphincterotomy) may expose the duct to allow cannulation. This is occasionally associated with haemorrhage from the duodenal wall vessels. Division of the smooth muscle fibres of the common bile duct sphincter may be necessary for endoscopic access to the bile duct, but it often results in uncontrolled reflux of duodenal contents into the distal common bile duct, and this may result in recurrent biliary infections.

BILIARY DRAINAGE

The proximity of the fundus of the gallbladder to the anterior abdominal wall provides a useful route of access for percutaneous drainage of a distended, obstructed gallbladder. It is rarely obscured by liver tissue, can often be accessed below the costal margin, and is most often performed under ultrasound guidance. Because of the small calibre and spiral conformation of the cystic duct, drainage of the gallbladder is rarely adequate to decompress the biliary tree if it is blocked, and this must be achieved endoscopically, surgically or by a percutaneous, transhepatic approach. This latter technique requires the identification of dilated intrahepatic bile ducts, usually by ultrasound imaging with direct guided puncture via a right subcostal approach. Percutaneous access is obtained via segment III in the left lobe of the liver and via segments V and VI in the right lobe of the liver.

GALLBLADDER

The fundus of the gallbladder is completely covered by a serosa, and the inferior surfaces and sides of the body and neck of the gallbladder are usually covered by a serosa. If the gallbladder possesses a mesentery the serosa extends around the sides of the body and neck onto the superior surface and continues into the serosa of the mesentery, whereas the serosa is limited to the inferior surfaces only if the gallbladder is intrahepatic. Beneath the serosa is subserous loose connective and adipose peritoneal tissue. The gallbladder wall microstructure generally resembles that of the small intestine. The mucosa is yellowish-brown and elevated into minute rugae with a honeycomb appearance (Fig. 69.2). In section, projections of the mucosa into the gallbladder lumen resemble intestinal villi, but they are not fixed structures and the surface flattens as the gallbladder fills with bile. The epithelium is a single layer of columnar cells with apical microvilli; basally, the spaces between epithelial cells are dilated (Fig. 69.8). Many capillaries lie beneath the basement membrane. The epithelial cells actively absorb water and solutes from the bile and concentrate it up to ten-fold. There are no goblet cells in the epithelium. The thin fibromuscular layer is composed of fibrous tissue mixed with smooth muscle cells arranged loosely in longitudinal, circular and oblique bundles.

Fig. 69.8 Low power micrograph showing the gallbladder wall, with a mucosal projection which flattens in the full gallbladder, and the thin muscular layer.

BILE DUCTS

The walls of the large biliary ducts consist of external fibrous and internal mucosal layers (Fig. 69.9). The outer layer is fibrous connective tissue containing a variable amount of longitudinal, oblique and circular smooth muscle cells. The mucosa is continuous with that in the hepatic ducts, gallbladder and duodenum. The epithelium is columnar and there are numerous tubuloalveolar mucous glands in the duct walls.

Fig. 69.9 Low power micrograph showing the human common bile duct structure. Shown are the lumen (L), mucosal lining (m), smooth muscle coat (SM) and outer connective tissue (C), with numerous nerve fibre bundles (N).

(By courtesy of Mr Peter Helliwell and Dr Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals Trust, UK.)

Expulsion of gallbladder contents is under neuroendocrine control. Fat in the duodenum causes the release of cholecystokinin (CCK), which stimulates the gallbladder to contract because muscle cells in its walls bear surface receptors for CCK. When the pressure exceeds 100 mm H₂O of bile, the sphincter of Oddi relaxes and bile enters the duodenum. The termination of the united bile and pancreatic ducts is packed with villous, valvular folds of mucosa that contain muscle cells in their connective tissue cores. Contraction is thought to result in retraction and clumping of these folds, so preventing reflux of duodenal contents and controlling the exit of bile.