CHAPTER 8
Liver: anatomy, microscopic structure, and cell types

Maria Westerhoff and Laura Lamps

University of Michigan, Ann Arbor, MI, USA

Embryology

Embryologically, the primordial liver first appears toward the end of gestational week 3. It derives from an outpouching of the foregut called the “hepatic diverticulum.” This diverticulum has cranial and caudal buds; the cranial portion grows into the septum transversum and becomes the right and left liver lobes as well as the intrahepatic bile duct structures. The caudal aspect becomes the gallbladder and cystic duct. The stalk of the diverticulum connecting the developing liver and foregut narrows, forming the extrahepatic biliary system (Figure 8.1). The endodermal cells of the diverticulum that form the hepatic parenchyma and the intrahepatic duct structures are called “hepatoblasts.”

The vascular network is originally derived from both the vitelline and umbilical veins. The hepatoblast cords enmesh the vessels, forming the hepatic sinusoids. By week 5, most of the major vessels are identifiable, including the right and left umbilical veins, the transverse portal sinus, and the ductus venosus (Figure 8.2). The portal vein develops from the vitelline veins and then subdivides into the right and left branches (Figure 8.3).

The bile canaliculi are first seen as intercellular spaces between hepatoblasts at gestational week 6. They eventually become linked to the extrahepatic biliary system via the intrahepatic bile ducts. Intrahepatic bile ducts develop from the “ductal plate,” primitive duct structures formed by a layer of hepatoblasts that surround the portal tracts (Figure 8.4).

Of note, extramedullary hematopoiesis is prominent in the liver during fetal development; the red blood cell precursors are present mostly in the parenchyma (Figure 8.5) while myeloid and megakaryocytic precursors are particularly more evident within the portal tracts.

Hemosiderin is usually visible in early stages and becomes most marked as intrahepatic hematopoiesis decreases. Hemosiderin then gradually decreases but may still be seen at birth in the periportal hepatocytes. The Kupffer cells appear by the third gestational month.

Gross anatomy

The weight of the adult liver varies from 1200 to 1800 g, depending on the overall body size. The liver has four anatomical lobes: right, left, caudate, and quadrate (Figures 8.6 and 8.7). Functionally, the liver is divided based on the branching pattern of the portal structures (portal vein, hepatic, artery, and bile duct). This creates a total of eight functional segments, each demarcated by their own vascular and biliary drainage (see Figure 8.2).

The portal vein is formed by the convergence of the superior mesenteric and splenic veins. The hepatic vein is composed of three major tributaries (right, middle, and left), while the hepatic artery ascends along the hepatoduodenal ligament and eventually divides into the right and left main branches. The biliary drainage of the right lobe is derived from anterior and posterior segmental duct branches that merge to form the right hepatic duct. Lateral and medial segmental branches merge to form the left hepatic duct, which drains the left lobe. The intrahepatic components of these vessels and ducts follow along the varous hepatic segments (Figure 8.8).

Microanatomy

The classic Rappaport acinus concept can be appreciated under the light microscope. It subdivides the parenchyma into zones 1 (periportal), 2 (between periportal and pericentral/perivenular), and 3 (area around hepatic venule), with decreasing oxygenation and increasing susceptibility to ischemia and toxic/drug‐induced injury toward zone 3. Biliary drainage runs parallel to the vascular sinusoidal circulation (Figure 8.9).

The portal tract (Figure 8.10) includes a portal venule, an interlobular bile duct, and a hepatic arteriole of the same size as the duct. The hepatic artery is responsible for the blood supply of the bile duct. The fibrous tissue surrounding the portal structures varies in amount depending on the distance of the portal tract from the hepatic hilum. This is important in the determination of how much pathological fibrosis is present in staging a liver biopsy.