Surgical Anatomy of the Liver

Embryology

The hepatic diverticulum, a ventral outpouching of the distal foregut observed early in the fourth week of gestation (3-mm embryo), is the origin of the hepatobiliary system. This outgrowth of proliferating endodermal cells infiltrates the embryonal ventral mesentery and extends into the septum transversum to form the early liver primordium. The rapidly proliferating primordium expands into the left and right vitelline veins (omphalomesenteric veins) to stimulate extensive remodeling into separate liver chords and portal sinusoids from the primordium mass, creating separate right and left intrahepatic portal circulations. The vitelline veins undergo further differentiation into an intrahepatic component containing hepatic chords and portal sinusoids, a cranial component delivering blood from the embryonic liver to the heart, and a caudal component carrying blood from the yolk sac to the liver. Later endodermal cell proliferation yields hepatic cords and biliary epithelia that coalesce to create sinusoids, whereas hemopoietic tissue, Kupffer cells, and interstitial connective tissue originate from the splanchnic mesenchyme of the septum transversum.

The hepatic veins originate from the vitelline venous system. The cranial component of the left vitelline vein initially involutes, shunting all returning blood to the heart through the cranial component of the right vitelline vein, known as the embryonic common hepatic vein . The common hepatic vein functions as an early single outflow source from the liver to the heart and persists as the later right hepatic vein. The vitelline venous system of the left side of the liver later reconstitutes channels that mature into left and middle hepatic veins to augment venous return from the liver to the heart and define the permanent anatomical arrangement. Vitelline venous system development is manifested by the surgical findings of a distinct right hepatic vein emptying directly into the vena cava as compared with the middle and left hepatic veins that typically empty via a common channel.

The development of the extrahepatic main portal vein is one of the most complex processes observed in embryology. Origination of the extrahepatic main portal vein begins with fusion of left and right vitelline venous elements returning blood from the gut–yolk sac complex. The objectives are to create a single inflow source to the liver from the bilateral vitelline veins while preserving the anatomical relationship of the main portal vein to the developing duodenum. As the yolk sac regresses, the omphalic portions of the vitelline veins disappear while the mesenteric branches proliferate, increasing in length and complexity to serve the intestinal tract. Between the fourth and sixth weeks (4.5- to 9-mm embryo), caudal elements of both vitelline veins unite through intravenous channels and undergo segmental involution to form a composite, S-shaped vessel, located posterior to the first portion of the duodenum, that drains both vitelline venous beds as a single vessel to the liver.

The intrahepatic left portal vein is also a composite vessel originating from a communication between the vitelline veins and a segment of the left umbilical vein. The umbilical veins are originally paired; however, the left umbilical vein is invaded by hepatic tissue and hypertrophies, whereas the right atrophies before contact with the liver. Umbilical blood initially flows through a meshwork of intrahepatic sinusoids, but as volume increases, these sinusoids coalesce to receive the proximal portion of the developing left portal vein and form a single vessel shunting blood through the liver, the ductus venosus. The ductus venosus receives branches from the liver before joining the hepatic veins to drain into the inferior vena cava. After birth the ductus venosus closes to form the ligamentum venosum.

The biliary and arterial systems develop later, along the latticework provided by the established portal venous system. The right biliary and arterial branches follow the portal system exactly, whereas the left biliary and arterial systems divide into equal-size branches on either side of the intrahepatic portion of the umbilical vein.

The embryonal liver develops rapidly to occupy most of the abdominal cavity. By 9 weeks’ gestation, the liver accounts for approximately 10% of the embryo’s total weight with relatively equal hepatic mass on each side of the falciform ligament. The initial equality in volume between topographical lobes is lost by 12 weeks as the topographical right lobe hypertrophies to spawn the caudate lobe (initially recognizable at 6 weeks) and become the dominant hepatic mass.

The ventral mesentery forms the gastrohepatic ligament and the fibrous visceral peritoneum of the liver. This was first described by Glisson in 1659, as a peritoneal sheath that envelops the organ, except for a “bare area” on the superoposterior surface of the right lobe where the organ is in direct contact with the inferior vena cava, diaphragm, and superior aspect of the right adrenal gland. Glisson’s capsule involutes into the parenchyma as intrahepatic septa or trabeculae that support vascular structures and serve as surgical landmarks.

Functional milestones in embryonic development include intrahepatic hematopoiesis during the sixth week, hepatocyte bile formation at the twelfth week, and excretion of bile into the duodenum by the sixteenth week. The third trimester marks the cessation of hematopoiesis with a concomitant decrease in liver growth to account for approximately 5% of the newborn’s body weight.

Topographical Anatomy

Topographical anatomy of the liver dates to early Babylon (3000-2000 bc ), where the liver was described according to external landmarks. This anatomical system dominated through the late nineteenth century but is currently only of historical interest. The principal landmarks defining topographical anatomy include the falciform ligament, umbilical fissure, gallbladder fossa, and transverse hilar fissure. These landmarks delineate four lobes ( Fig. 2-1 ): left (medial to falciform), right (lateral to falciform), quadrate, and caudate (spigelian).

The liver is supported in position through peritoneal reflections continuous with Glisson’s capsule that attach to the duodenum, stomach, diaphragm, and anterior abdominal wall. These peritoneal reflections include the falciform ligament, right and left triangular ligaments, and right and left coronary ligaments, as well as the lesser omentum. The falciform ligament extends from the ligamentum teres superiorly along the anterior liver surface in continuity with both the diaphragm and anterior abdominal wall above the umbilicus. The ligamentum teres is a remnant of the vestigial umbilical vein. Normally obliterated, it may recanalize in disease conditions like cirrhosis, decompressing the portal circulation through collaterals of the periumbilical superficial venous plexus. This shunts portal blood to the systemic circulation through superficial venous plexuses, producing the characteristic “caput medusa.” As the falciform ligament continues toward the diaphragm, the peritoneal sheets composing the ligament separate to adopt a triangular shape that broadly covers the entry of the hepatic veins into the suprahepatic vena cava.

At the level of the suprahepatic vena cava, the peritoneal reflections progress laterally to become the anterior layers of the left and right coronary ligaments. The coronary ligaments anchor the superior surface of the liver through anterior and posterior reflections to the diaphragm. As the right and left coronary ligaments extend laterally, each unites with the posterior reflections to form the respective right and left triangular ligaments. The right coronary ligament may continue and fuse to the superior pole of the right kidney to form the hepatorenal ligament.

The lesser omentum is a continuous fold of peritoneum arising from the posterior reflection of the left triangular ligament. The lesser omentum extends from the liver onto the lesser curvature of the stomach and first 2 cm of the duodenum to form the gastrohepatic and hepatoduodenal ligaments, respectively. The hepatoduodenal ligament forms the anterior border of the epiploic foramen of Winslow and contains the porta hepatis.

Lobar Anatomy

Galen (130-201 ad ) postulated the hepatic arterial and portal venous systems terminated as minute connections that reconstituted into hepatic veins draining to the inferior vena cava. Galen’s concept of separate arterial and portal venous systems reconstituting into hepatic veins resurfaced in 1888, when Hugo Rex studied hepatic corrosion casts from mammals. Rex concluded that the right and left branches of the portal vein functioned as unique vascular systems, dividing the liver into separate halves. In 1897 James Cantlie extended these findings to humans, proposing a functional division of the liver into two lobes (“Cantlie’s line”) of relatively equal size based on the branching of the portal vein (and followed by the hepatic ducts). Cantlie’s line has no visible surface topography but rather is a virtual plane that bisects the gallbladder fossa and the suprahepatic vena cava. This plane roughly overlies the course of the middle hepatic vein and can be demonstrated in clinical practice by devascularization of the hemiliver (right or left).

Cantlie’s description of functional anatomy shifted the entire quadrate lobe (topographical term), as well as a large component of the caudate lobe (topographical term), into the anatomical boundaries of the left lobe rather than the right. This classification system, founded on intrahepatic functional anatomy rather than surface topographical landmarks, was the underpinning of a modern surgical revolution in anatomically based hepatic resections. Tiffany reported the first liver resection performed in the United States in 1890 (although the accuracy of this publication is widely disputed), and Professor William Keen of Jefferson Medical College confidently and somewhat prematurely proclaimed to members of the Pennsylvania State Medical Society on May 17, 1899, “after my experience with these three cases [liver resections], I should hardly hesitate to attack almost any hepatic tumor without regard to its size.”

Cantlie’s reference to hepatic lobes created two definitions for the same term and was the source of continuing confusion. Europeans continued to describe hepatic lobes based on topographical anatomy, whereas North American surgeons adopted lobectomy as the hemiliver defined by Cantlie. One must be certain as to the reference system in use (topographical anatomy or Cantlie’s anatomical classification) when applying the term lobe or lobectomy . A more appropriate scheme is to refer to Cantlie’s anatomical lobes as hemilivers, thus describing a right or left hepatectomy.

The anatomical system of Cantlie was later expanded by the North American anatomists Healey and Schroy, who based their nomenclature on biliary anatomy, rather than on Cantlie’s description of portal venous anatomy, while retaining the term lobe . The right lobe was divided into anterior and posterior segments by a right segmental fissure, whereas the left lobe was divided into medial and lateral segments by a left segmental fissure. The left segmental fissure corresponds to the falciform ligament, whereas the segmental “fissure” that divides anterior and posterior sectors of the right lobe is not easily discerned by surface landmarks, though one can infer its location based on the plane of insertion of the extrahepatic right portal pedicle. Healey and Schroy’s classification scheme led to the descriptive but imprecise term hepatic trisegmentectomy for extended right hepatectomy, and the often used term left lateral segment for the topographical portion of the liver containing modern segments II and III.

Modern Segmental Anatomy

The most sophisticated classification of intrahepatic anatomy is by Couinaud, who in 1954 founded his anatomical description on the portal venous system. Portal vein distribution within the liver was subdivided into eight “segments.” Individual segments each receive a “portal pedicle” consisting of a portal venous branch, hepatic arterial branch, and a bile duct radicle with segmental drainage through a dedicated hepatic venous branch. The eight functional segments embrace the hepatic veins that provide outflow to the inferior vena cava ( Fig. 2-2 ).

The hepatic veins travel in planes termed fissures or scissurae , dividing the liver into four sectors (see Fig. 2-1 ). The left portal fissure contains the left hepatic vein, the main portal fissure contains the middle hepatic vein (in the plane of Cantlie), and the lateral-most (right) portal fissure contains the right hepatic vein. Three of the four sectors contain smaller fissures that subdivide each into two segments to form a total of seven segments. Only the caudate lobe (segment I) is a functionally autonomous segment supplied by both the left and right branches of the portal vein and hepatic artery with drainage directly into the inferior vena cava. Clinically this relationship is well demonstrated in patients with Budd-Chiari disease who compensate for major hepatic vein outflow obstruction by development of alternative outflow tracts via veins draining directly from segment I into the retrohepatic vena cava.

Biliary drainage of segment I occurs via small anterior radicles draining directly into the posterior surface of the biliary confluence. A well-defined segment 1 duct consistently drains into the proximal left hepatic duct between the hepatic duct bifurcation and the umbilical fissure. It is important to recognize and control this duct during resectional surgery and for partial liver allografts involving the left lobe. Segments II and III correspond to the posterior and anterior segments of the topographical left lobe, respectively. Segment IV, the largest segment and the only one derived from an undivided hepatic sector, extends from the left portal fissure to the main portal fissure (Cantlie’s line) and includes the entire volume of the quadrate lobe.

The right portal fissure divides the right lobe into an anteromedial sector and a posterolateral sector, each of which is subdivided into anterior and posterior segments. The two anterior segments of the right lobe include segment V (inferiorly adjacent to the gallbladder fossa) and segment VIII (superiorly). The two posterior segments of the right lobe include segment VI (inferiorly, adjacent to the right kidney), and segment VII (superiorly). The posterior segments VI/VII are located posterior to the peritoneal reflection and are therefore retroperitoneal structures that are not visible at laparotomy without mobilizing the right lobe of the liver (see Fig. 2-2 ).

The recognition of the segmental anatomy of the liver was a significant advancement for hepatic surgery. In 1982 Bismuth integrated Couinaud’s classification scheme into a formal anatomical approach to hepatectomy that has been widely adopted by hepatobiliary surgeons to standardize techniques and nomenclature. Rather than perform atypical resections based on the size or location of a lesion, hepatic resections could be performed along functional planes that would minimize intraoperative blood loss and postoperative necrosis of devitalized tissue, in addition to potentially improving oncological control of malignancy after resection. This classification has revolutionized hepatic surgery by providing a foundation for the development of highly selective anatomical resections as well as innovations in transplantation using surgically created partial-liver allografts.

Applied Surgical Anatomy

Couinaud’s anatomical classification permitted the theoretical construction of partial-liver allografts based on the known regenerative capacity of the liver ( Fig. 2-3 ), which was realized in clinical practice during the 1980s. The successful application of partial-liver allografts mandates detailed anatomical considerations because these procedures predispose to unique surgical complications. Fundamental to the application of these techniques is an understanding of intrahepatic vascular and biliary anatomy. Although the incidence of vascular complications has declined with the widespread application of microsurgical techniques, a relatively high incidence of biliary complications persists.

Four distinct allografts have been used routinely in partial-liver transplantation (see Fig. 2-3 ). These include the right hemiliver (Couinaud segments V to VIII), the left hemiliver (Couinaud segments II to IV), the topographical left lobe (Couinaud segments II to III), and the topographical right lobe (Couinaud segments IV to VIII).