To understand the hilar region, we must first of all understand that although the extrahepatic portal triad consisting of the hepatic artery, the portal vein and the bile duct is enclosed by connective tissues and peritoneum up to the hepatic hilum, the term Glisson’s sheath is reserved for the Glisson’s capsule which extends into the intrahepatic portion of the liver beyond the hilum. Also, because the hepatic hilar plate system is formed by the fusion of Glisson’s capsule with the connective tissue sheaths surrounding the biliary and vascular elements at the inferior aspect of the liver, branches of the biliary and vascular elements can ramify in the hepatic hilar plate system before they go into the Glisson’s sheath of a particular sector or segment of the liver. Alternatively, the elements can go straight into a particular Glisson’s sheath after branching. Sometimes, for a particular liver sector or segment, branches of the artery, and bile duct can act differently from branches of the portal vein before they go into the Glisson’s sheath.

5.1 Lymphatics in the Plate System

The lymphatics of the hilar area are distributed in a very complex manner, consisting of superficial and deep lymphatic systems, coursing through the plate system of the hilar area. The deep system is also called the lymphatics of Glisson’s sheath which drains into this system. The superficial lymph vessels travel through the junction between the upper edge of the hilar plate and the capsule of segment 4a. Histology showed a thick layer of connective tissues with numerous lymph vessels between segment 4a and the ventral aspect of the portal vein. In contrast, the connective tissue between the dorsal aspect of the portal vein and the caudate lobe is thin and contains very few lymph vessels.

5.2 Anatomy of the Hilar Region

The confluence of the right and left hepatic ducts is located in the hilar plate. In the treatment of carcinoma of the hilar bile ducts, it is important to resect the hilar duct confluence en bloc with the hilar plate because tumour cells can easily invade into the adjacent plate tissues.

On the right side of the hilar plate, the right hepatic artery courses between the cystic plate and the hilar plate, and the branches of the right portal vein run dorsally to the artery before they enter into the Glisson’s sheaths.

On the left side, the middle hepatic artery, after branching off from the left hepatic artery, courses through the umbilical plate, and the left branch of the portal vein lies dorsal to the artery. At these sites, the hilar ducts can be separated easily from the portal vein by dividing the common bile duct caudally and lifting it cranially (Fig. 5.1).

Fig. 5.1
figure 1

Components of the plate system. GB Gallbladder, G Glisson’s Sheath

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5.3 Vascular and Biliary Branches to the Caudate Lobe and to Segment 4

The hepatic plate system is continuous with the Glisson’s sheath. The bile ducts and blood vessels usually course through the plate system to enter into the Glisson’s sheath intrahepatically. The arrangement of the bile ducts and blood vessels going into the Glisson’s sheaths is different in the caudate lobe and segment 4 of the liver.

For the caudate lobe, the bile ducts and arteries of the caudate lobe ramify in the hilar plate, but the branches of the portal vein run directly to the caudate lobe. Thus, while the caudate lobe artery and bile duct branches do not form a Glisson’s triad immediately after branching within the liver, the portal vein branches do. Each portal vein branch is enclosed within a Glisson’s sheath together with the arterial and ductal elements immediately after branching to form a Glisson’s triad (Fig. 5.2).

Fig. 5.2
figure 2

Vascular and biliary branches to the Caudate Lobe. LHD left hepatic duct, PV portal vein, HA hepatic artery, P portal vein branch of S1 (left side), A hepatic artery branch S1 (left side), B bile duct branch of S1 (left side), G Glisson’s sheath of S1 (left side)

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The intrahepatic bile duct branches of segment 4 of the liver make a J-shape turn. Each branch then runs out of the Glisson’s sheath, pieces the umbilical plate and ultimately joins the left hepatic duct. Branches of the middle hepatic artery enter into the Glisson’s sheaths of segment 4 soon after branching. By contrast, the portal vein branches to segment 4 arises from the right side of the umbilical portion of the portal vein in a straight line, and each branch enters into its corresponding Glisson’s sheath to form a Glisson’s triad together with its hepatic arterial and bile ductal elements after travelling for a distance intrahepatically. The vessels and ducts of segment 4 do not form a Glisson’s sheath immediately to the right of the umbilical portion of the portal vein, but at some distance away from it (Fig. 5.3). These anatomical differences in that ducts and blood vessels of segment 4 are attributable to the fact that this segment develops later than the other segments of the liver, and that the portal vein branches of segment 4 develop from the tertiary level of the portal system in the embryo. Such a view, however, is still controversial and not universally accepted (see Chap. 2).

Fig. 5.3
figure 3

Vascular and biliary branches to segment 4. PV portal vein, LPV left portal vein, UP umbilical portion of the left portal vein, LHD left hepatic duct, G2 Glisson’s sheath to segment 2, G3 Glisson’s sheath to segment 3, G4b Glisson’s sheath to segment 4b

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