Jay H. Lefkowitch College of Physicians and Surgeons, Columbia University, New York, NY, USA
Learning points
• The liver is derived from a foregut endodermal bud which develops in the third week of gestation and divides into two parts: hepatic and biliary.
• The Couinaud classification subdivides the liver into eight segments (segments I–IV in the left lobe, segments V–VIII in the right lobe) based on vascular and biliary anatomical landmarks.
• The lobule described by Kiernan is the most widely used unit of liver microanatomy, consisting of a hexagon-like region of liver parenchyma with a central vein as its hub and portal tracts located in the periphery of the hexagon.
• Hepatocytes are functionally heterogeneous within the lobular parenchyma, whereby centrilobular cells subserve different functions (e.g. drug metabolism) from periportal cells (e.g. bile salt-dependent bile formation).
• Uncomplicated regeneration of hepatocytes and/or bile duct epithelium usually occurs by cell division of the indigenous cells; however, when normal regenerative capacity is overwhelmed there may be activation of progenitors cells located in the region of the canals of Hering.
Development of the liver and bile ducts
The liver begins as a hollow endodermal bud from the foregut (duodenum) during the third week of gestation. The bud separates into two parts—hepatic and biliary. The hepatic part contains bipotential progenitor cells that differentiate into hepatocytes or ductal cells, which form the early primitive bile duct structures (bile duct plates). Differentiation is accompanied by changes in cytokeratin type within the cell. Normally, this collection of rapidly proliferating cells penetrates adjacent mesodermal tissue (the septum transversum) and is met by ingrowing capillary plexuses from the vitelline and umbilical veins, which will form the sinusoids. The connection between this proliferating mass of cells and the foregut, the biliary part of the endodermal bud, will form the gallbladder and extrahepatic bile ducts. Bile begins to flow at about the 12th week. Connective tissue cells of portal tracts are derived from the mesoderm of the septum transversum. Kupffer cells derive from circulating monocytes and possibly yolk sac macrophages. Hepatic stellate cells appear to be mesodermal derivatives from submesothelial cells located beneath the surface of the developing liver. The fetal liver is the main site of haemopoiesis by the 12th week; this subsides in the fifth month coincident with the onset of bone marrow haemopoietic activity, so that only a few haemopoietic cells remain at birth.
Anatomy of the liver
The liver, the largest organ in the body, weighs 1200–1500g and comprises one-fiftieth of the total adult body weight. It is relatively larger in infancy, comprising one-eighteenth of the birth weight. This is mainly due to a large left lobe.
Sheltered by the ribs in the right upper quadrant, the upper border lies approximately at the level of the nipples. There are two anatomical lobes, the right being about six times the size of the left (Figs 1.1–1.3). Lesser segments of the right lobe are the caudate lobe on the posterior surface and the quadrate lobe on the inferior surface. The right and left lobes are separated anteriorly by a fold of peritoneum called the falciform ligament, posteriorly by the fissure for the ligamentum venosum and inferiorly by the fissure for the ligamentum teres.
The liver has a double blood supply. The portal vein brings venous blood from the intestines and spleen and the hepatic artery, coming from the coeliac axis, supplies the liver with arterial blood. These vessels enter the liver through a fissure, the porta hepatis, which lies far back on the inferior surface of the right lobe. Inside the porta, the portal vein and hepatic artery divide into branches to the right and left lobes, and the right and left hepatic bile ducts join to form the common hepatic duct. The hepatic nerve plexus contains fibres from the sympathetic ganglia T7–T10, which synapse in the coeliac plexus, the right and left vagi and the right phrenic nerve. It accompanies the hepatic artery and bile ducts into their finest ramifications, even to the portal tracts and hepatic parenchyma.
The ligamentum venosum, a slender remnant of the ductus venosus of the fetus, arises from the left branch of the portal vein and fuses with the inferior vena cava at the entrance of the left hepatic vein. The ligamentum teres, a remnant of the umbilical vein of the fetus, runs in the free edge of the falciform ligament from the umbilicus to the inferior border of the liver and joins the left branch of the portal vein. Small veins accompanying it connect the portal vein with veins around the umbilicus. These become prominent when the portal venous system is obstructed inside the liver.
The venous drainage from the liver is into the right and left hepatic veins which emerge from the back of the liver and at once enter the inferior vena cava very near its point of entry into the right atrium.
Lymphatic vessels terminate in small groups of glands around the porta hepatis. Efferent vessels drain into glands around the coeliac axis. Some superficial hepatic lymphatics pass through the diaphragm in the falciform ligament and finally reach the mediastinal glands. Another group accompanies the inferior vena cava into the thorax and ends in a few small glands around the intrathoracic portion of the inferior vena cava.
The inferior vena cava makes a deep groove to the right of the caudate lobe about 2cm from the midline.
The gallbladder lies in a fossa extending from the inferior border of the liver to the right end of the porta hepatis.
The liver is completely covered with peritoneum, except in three places. It comes into direct contact with the diaphragm through the bare area which lies to the right of the fossa for the inferior vena cava. The other areas without peritoneal covering are the fossae for the inferior vena cava and gallbladder.
The liver is kept in position by peritoneal ligaments and by the intra-abdominal pressure transmitted by the tone of the muscles of the abdominal wall.
Functional liver anatomy: sectors and segments
Based on the external appearances described above, the liver has a right and left lobe separated along the line of insertion of the falciform ligament. This separation, however, does not correlate with blood supply or biliary drainage. A functional anatomy is now recognized based upon vascular and biliary anatomy. The Couinaud classification defines eight segments (segments I-IV in the left lobe, V-VIII in the right lobe), while the Bismuth classification divides the liver into four sectors. These can be correlated with results seen with imaging techniques.
The main portal vein divides into right and left branches and each of these supplies two further subunits (variously called sectors). The sectors on the right side are anterior and posterior and, in the left lobe, medial and lateral—giving a total of four sectors (Fig. 1.4). Using this definition, the right and left side of the liver are divided not along the line of the falciform ligament, but along a slightly oblique line to the right of this, drawn from the inferior vena cava above to the gallbladder bed below. The right and left side are independent with regard to portal and arterial blood supply, and bile drainage. Three planes separate the four sectors and contain the three major hepatic vein branches.
Closer analysis of these four hepatic sectors produces a further subdivision into segments (Fig. 1.5). The right anterior sector contains segments V and VIII; right posterior sector, VI and VII; left medial sector, IV; left lateral sector, II and III. There is no vascular anastomosis between the macroscopic vessels of the segments but communications exist at the sinusoidal level. Segment I, the equivalent of the caudate lobe, is separate from the other segments and does not derive blood directly from the major portal branches or drain by any of the three major hepatic veins.
This functional anatomical classification allows interpretation of radiological data and is of importance to the surgeon planning a liver resection. There are wide variations in portal and hepatic vessel anatomy which can be demonstrated by spiral computed tomography (CT) and magnetic resonance imaging (MRI) reconstruction.
Anatomical abnormalities of the liver
These are being increasingly diagnosed with more widespread use of CT and ultrasound scanning.
Accessory lobes. The livers of the pig, dog and camel are divided into distinct and separate lobes by strands of connective tissue. Occasionally, the human liver may show this reversion and up to 16 lobes have been reported. This abnormality is rare and without clinical significance. The lobes are small and usually on the undersurface of the liver so that they are not detected clinically but are noted incidentally at scanning, operation or necropsy. Rarely they are intrathoracic. An accessory lobe may have its own mesentery containing hepatic artery, portal vein, bile duct and hepatic vein. This may twist and demand surgical intervention.
Ectopic liver. Small nodules of normal liver derived from the embryologic hepatic bud may be found in less than 1% of laparoscopies and autopsies near the gallbladder, hepatic ligaments, gastrorenal ligament, omentum, retroperitorneum and thorax. These may give rise to hepatocellular carcinoma.
Riedel's lobe. This is fairly common and is a downward tongue-like projection of the right lobe of the liver. It is a simple anatomical variation; it is not a true accessory lobe. The condition is more frequent in women. It is detected as a mobile tumour on the right side of the abdomen which descends with the diaphragm on inspiration. It may come down as low as the right iliac region. It is easily mistaken for other tumours in this area, especially a visceroptotic right kidney. It does not cause symptoms and treatment is not required. Rarely, it is a site for metastasis or primary hepatocellular carcinoma. Scanning may be used to identify Riedel's lobe and other anatomical abnormalities.
Cough furrows on the liver. These are vertical grooves on the convexity of the right lobe. They are one to six in number and run anteroposteriorly, being deeper posteriorly. These represent diaphragmatic sulci and fissures produced by pressure exerted by diaphragmatic muscle on peripheral structurally weak liver parenchymal zones associated with watershed vascular distribution. Chronic cough produces such pressure.
Corset liver. This is a horizontal fibrotic furrow or pedicle on the anterior surface of one or both lobes of the liver just below the costal margin. The mechanism is unknown, but it affects elderly women who have worn corsets for many years. It presents as an abdominal mass in front of and below the liver and is isodense with the liver. It may be confused with a hepatic tumour.
Lobar atrophy. Interference with the portal supply or biliary drainage of a lobe may cause atrophy. There is usually hypertrophy of the opposite lobe. Left lobe atrophy found at post-mortem or during scanning is not uncommon and is probably related to reduced blood supply via the left branch of the portal vein. The lobe is decreased in size with thickening of the capsule, fibrosis and prominent biliary and vascular markings. The vascular problem may date from the time of birth. Loss of left lobe parenchyma in this instance develops by the process of ischaemic extinction due to impaired flow from the affected large portal vein branch. Replacement fibrosis ensues. This large vessel extinction process should be distinguished from cirrhosis in which the entire liver is affected by numerous intrahepatic and discrete extinction lesions, which affect small hepatic veins and portal vein branches during the course of inflammation and fibrosis. Hence, in cirrhosis the entire liver surface is diffusely converted to regenerative parenchymal nodules surrounded by fibrosis.
Obstruction to the right or left hepatic bile duct by benign stricture or cholangiocarcinoma is now the most common cause of lobar atrophy. The alkaline phosphatase is usually elevated. The bile duct may not be dilated within the atrophied lobe. Relief of obstruction may reverse the changes if cirrhosis has not developed. Distinction between a biliary and portal venous aetiology may be made using technetium-labelled iminodiacetic acid (IDA) and colloid scintiscans. A small lobe with normal uptake of IDA and colloid is compatible with a portal aetiology. Reduced or absent uptake of both isotopes favours biliary disease.
Agenesis of the right lobe. This rare lesion may be an incidental finding associated, probably coincidentally, with biliary tract disease and also with other congenital abnormalities. It can cause presinusoidal portal hypertension. The other liver segments undergo compensatory hypertrophy. It must be distinguished from lobar atrophy due to cirrhosis or hilar cholangiocarcinoma.
Situs inversus (SI). In the exceedingly rare SI totalis or abdominalis the liver is located in the left hypochondrium and may be associated with other anomalies including biliary atresia, polysplenia syndrome, aberrant hepatic artery anatomy and absent portal vein. Hepatic surgery (partial hepatectomy, liver transplantation) is feasible, but complex. Other conditions associated with displacement of the liver from its location in the right upper quadrant include congenital diaphragmatic hernias, diaphragmatic eventration and omphalocoele.
Anatomical abnormalities of the gallbladder and biliary tract are discussed in Chapter 12.
Anatomy of the biliary tract (Fig. 1.6 )
The right and left hepatic ducts emerge from the liver and unite in the porta hepatis to form the common hepatic duct. This is soon joined by the cystic duct from the gallbladder to form the common bile duct.
The common bile duct runs between the layers of the lesser omentum, lying anterior to the portal vein and to the right of the hepatic artery. Passing behind the first part of the duodenum in a groove on the back of the head of the pancreas, it enters the second part of the duodenum. The duct runs obliquely through the posteromedial wall, usually joining the main pancreatic duct to form the ampulla of Vater (c. 1720). The ampulla makes the mucous membrane bulge inwards to form an eminence, the duodenal papilla. In about 10–15% of subjects the bile and pancreatic ducts open separately into the duodenum.
The dimensions of the common bile duct depend on the technique used. At operation it is about 0.5–1.5 cm in diameter. Using ultrasound the values are less, the common bile duct being 2–7 mm, with values greater than 7 mm being regarded as abnormal. Using endoscopic cholangiography, the duct diameter is usually less than 11mm, although after cholecystectomy it may be more in the absence of obstruction.
The duodenal portion of the common bile duct is surrounded by a thickening of both longitudinal and circular muscle fibres derived from the intestine. This is called the sphincter of Oddi (c. 1887).
The gallbladder is a pear-shaped bag 9cm long with a capacity of about 50mL. It always lies above the transverse colon, and is usually next to the duodenal cap overlying, but well anterior to, the right renal shadow. The fundus is the wider end and is directed anteriorly; this is the part palpated when the abdomen is examined. The body extends into a narrow neck which continues into the cystic duct. The valves of Heister are spiral folds of mucous membrane in the wall of the cystic duct and neck of the gallbladder. Hartmann's pouch is a sacculation at the neck of the gallbladder; this is a common site for a gallstone to lodge.
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Excerpted from Sherlock's Diseases of the Liver and Biliary System Copyright © 2011 by Blackwell Publishing Ltd.. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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