Morphogenesis of the Liver and Biliary System

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Chapter 346 Morphogenesis of the Liver and Biliary System

Morphogenesis of the liver and biliary system is a complex process. It follows that altered development has significant consequences, including cholestatic disorders such as Alagille syndrome and biliary atresia.

During the early embryonic process of gastrulation, the 3 embryonic germ layers (endoderm, mesoderm, ectoderm) are formed. The liver and biliary system arise from cells of the ventral foregut endoderm; their development can be divided into 3 distinct processes (Fig. 346-1). First, through unknown mechanisms, the ventral foregut endoderm acquires competence to receive signals arising from the cardiac mesoderm. These mesodermal signals, in the form of various fibroblast growth factors (FGFs) and bone morphogenetic proteins (BMPs), lead to specification of cells that will form the liver and activation of liver-specific genes. During this period of hepatic fate decision, “pioneer” transcription factors, including Foxa and Gata4, bind to specific binding sites in compacted chromatin, open the local chromatin structure, and mark genes as competent. But these will only be expressed if they are correctly induced by additional transcription factors. Newly specified cells then migrate in a cranial ventral direction into the septum transversum in the 4th wk of human gestation to initiate liver morphogenesis.

The growth and development of the newly budded liver require interactions with endothelial cells. Certain proteins are important for liver development in animal models (Table 346-1). In addition to these proteins, microRNA, which consists of small noncoding, single-stranded RNA, have a functional role in the regulation of gene expression and hepatobiliary development in a zebrafish model.

Within the ventral mesentery, proliferation of migrating cells forms anastomosing hepatic cords, with the network of primitive liver cells, sinusoids, and septal mesenchyme establishing the basic architectural pattern of liver lobule (Fig. 346-2). The solid cranial portion of the hepatic diverticulum (pars hepatis) eventually forms the hepatic parenchyma and the intrahepatic bile ducts. The hepatic lobules are identifiable in the 6th week of human gestation. The bile canalicular structures, including microvilli and junctional complexes, are specialized loci of the liver cell membrane; these appear very early in gestation, and large canaliculi bounded by several hepatocytes are seen by 6-7 wk.

Hepatocytes and bile duct cells (cholangiocytes) originate both from hepatoblasts as common precursors. Notch signaling, which is impaired in Alagille syndrome, promotes hepatoblast differentiation into biliary epithelium, whereas hepatocyte growth factor (HGF) antagonizes differentiation. The development of the intrahepatic bile ducts is determined by the development and branching pattern of the portal vein. Around the 8th week of gestation, starting at the hilum of the liver, primitive hepatoblasts adjacent to the mesenchyme around the portal vein branches form a cylindrical sleeve, termed the ductal plate. From 12 weeks of gestation onward, a “remodeling” of the ductal plate occurs, with some segments of the ductal plate undergoing tubular dilatation and excess epithelial tissue gradually disappearing. The ramification of the biliary tree continues throughout fetal life and at the time of birth the most peripheral branches of the portal veins are still surrounded by ductal plates; these require 4 more weeks to develop into definitive portal ducts. Lack of remodeling of the ductal plate results in persistence of primitive ductal plate configurations, an abnormality called ductal plate malformation. This histopathologic lesion has been observed in liver biopsies of a variety of liver conditions, including congenital hepatic fibrosis, Caroli disease, and biliary atresia.

The caudal part (pars cystica) of the hepatic diverticulum becomes the gallbladder, cystic duct, and common bile duct. The distal portions of the right and left hepatic ducts develop from the extrahepatic ducts, whereas the proximal portions develop from the first intrahepatic ductal plates. The extrahepatic bile ducts and the developing intrahepatic biliary tree maintain luminal continuity and patency from the beginning of organogenesis (see Fig. 346-2C).

Fetal hepatic blood flow is derived from the hepatic artery and from the portal and umbilical veins, which form the portal sinus. The portal venous inflow is directed mainly to the right lobe of the liver and umbilical flow primarily to the left. The ductus venosus shunts blood from the portal and umbilical veins to the hepatic vein, bypassing the sinusoidal network. After birth, the ductus venosus becomes obliterated when oral feedings are initiated. The fetal oxygen saturation is lower in portal than in umbilical venous blood; accordingly, the right hepatic lobe has lower oxygenation and greater hematopoietic activity than the left hepatic lobe.

The transport and metabolic activities of the liver are facilitated by the structural arrangement of liver cell cords, which are formed by rows of hepatocytes, separated by sinusoids that converge toward the tributaries of the hepatic vein (the central vein) located in the center of the lobule (see Fig. 346-2D). This establishes the pathways and patterns of flow for substances to and from the liver. In addition to arterial input from the systemic circulation, the liver also receives venous input from the gastrointestinal tract via the portal system. The products of the hepatobiliary system are released by 2 different paths: through the hepatic vein and through the biliary system back into the intestine. Plasma proteins and other plasma components are secreted by the liver. Absorbed and circulating nutrients arrive through the portal vein or the hepatic artery and pass through the sinusoids and past the hepatocytes to the systemic circulation at the central vein. Biliary components are transported via the series of enlarging channels from the bile canaliculi through the bile ductule to the common bile duct.

Bile secretion is noted at the 12th week of human gestation. The major components of bile vary with stage of development. Near term, cholesterol and phospholipid content is relatively low. Low concentrations of bile acids, the absence of bacterially derived (secondary) bile acids, and the presence of unusual bile acids reflect low rates of bile flow and immature bile acid synthetic pathways.

The liver reaches a peak relative size of about 10% of the fetal weight at the 9th wk. Early in development, the liver is a primary site of hematopoiesis. In the 7th wk, hematopoietic cells outnumber functioning hepatocytes in the hepatic anlage. These early hepatocytes are smaller than at maturity (∼20 µm vs 30-35 µm) and contain less glycogen. Near term, the hepatocyte mass expands to dominate the organ, as cell size and glycogen content increase. Hematopoiesis is virtually absent by the 2nd postnatal month in full-term infants. As the density of hepatocytes increases with gestational age, the relative volume of the sinusoidal network decreases. The liver constitutes 5% of body weight at birth but only 2% in an adult.

Several metabolic processes are immature in a healthy newborn infant, owing in part to the fetal patterns of activity of various enzymatic processes. Many fetal hepatic functions are carried out by the maternal liver, which provides nutrients and serves as a route of elimination of metabolic end products and toxins. Fetal liver metabolism is devoted primarily to the production of proteins required for growth. Toward term, primary functions become production and storage of essential nutrients, excretion of bile, and establishment of processes of elimination. Extrauterine adaptation requires de novo enzyme synthesis. Modulation of these processes depends on substrate and hormonal input via the placenta and on dietary and hormonal input in the postnatal period.

Hepatic Ultrastructure

Hepatocytes exhibit various ultrastructural features that reflect their biologic functions (Fig. 346-3). Hepatocytes, like other epithelial cells, are polarized, meaning that their structure and function are directionally oriented. One result of this polarity is that various regions of the hepatocyte plasma membrane exhibit specialized functions. Bidirectional transport occurs at the sinusoidal surface, where materials reaching the liver via the portal system enter and compounds secreted by the liver leave the hepatocyte. Canalicular membranes of adjacent hepatocytes form bile canaliculi, which are bounded by tight junctions, preventing transfer of secreted compounds back into the sinusoid. Within hepatocytes, metabolic and synthetic activities are contained within a number of different cell organelles. The oxidation and metabolism of heterogeneous classes of substrates, fatty acid oxidation, key processes in gluconeogenesis, and the storage and release of energy occur in the abundant mitochondria.

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Figure 346-3 Schematic view of the ultrastructure and organelles of hepatocytes.

(Reprinted from Sherlock S: Hepatic cell structure. In Sherlock S, editor: Diseases of the liver and biliary system, ed 6, Oxford, 1981, Blackwell Scientific, p 10; with permission from Blackwell Scientific.)

The endoplasmic reticulum (ER), a continuous network of rough- and smooth-surfaced tubules and cisternae, is the site of various processes, including protein and triglyceride synthesis and drug metabolism. Low fetal activity of ER-bound enzymes accounts for a relative inefficiency of xenobiotic (drug) metabolism. The Golgi apparatus is active in protein packaging and possibly in bile secretion. Hepatocyte peroxisomes are single-membrane-limited cytoplasmic organelles that contain enzymes such as oxidases and catalase and those that have a role in lipid and bile acid metabolism. Lysosomes contain numerous hydrolases that have a role in intracellular digestion. The hepatocyte cytoskeleton, composed of actin and other filaments, is distributed throughout the cell and concentrated near the plasma membrane. Microfilaments and microtubules have a role in receptor-mediated endocytosis, in bile secretion, and in maintaining hepatocyte architecture and motility.

Metabolic Functions of the Liver

Protein Metabolism

During the rapid fetal growth phase, specific decarboxylases that are rate limiting in the biosynthesis of physiologically important polyamines have higher activities than in the mature liver. The rate of synthesis of albumin and secretory proteins in the developing liver parallels the quantitative changes in endoplasmic reticulum. Synthesis of albumin appears at approximately the 7th-8th wk in the human fetus and increases in inverse proportion to that of α-fetoprotein, which is the dominant fetal protein. By the 3rd-4th month of gestation, the fetal liver is able to produce fibrinogen, transferrin, and low-density lipoproteins. From this period on, fetal plasma contains each of the major protein classes at concentrations considerably below those achieved at maturity.

The postnatal patterns of protein synthesis vary with the class of protein. Lipoproteins of each class rise abruptly in the 1st wk after birth to reach levels that vary little until puberty. Albumin concentrations are low in a neonate (∼2.5 g/dL), reaching adult levels (∼3.5 g/dL) after several months. Levels of ceruloplasmin and complement factors increase slowly to adult values in the 1st yr. In contrast, transferrin levels at birth are similar to those of an adult, decline for 3-5 mo, and rise thereafter to achieve their final concentrations. Low levels of activity of specific proteins have implications for the nutrition of an infant. For example, a low level of cystathionine γ-lyase (cystathionase) activity impairs the trans-sulfuration pathway by which dietary methionine is converted to cysteine. Therefore, the latter must be supplied in the diet. Similar dietary requirements might exist for other sulfur-containing amino acids, such as taurine.

Biotransformation

Newborn infants have a decreased capacity to metabolize and detoxify certain drugs, owing to underdevelopment of the hepatic microsomal component that is the site of the specific oxidative, reductive, hydrolytic, and conjugation reactions required for these biotransformations. The major components of the mono-oxygenase system, such as cytochrome P450 (CYP), cytochrome-c reductase, and the reduced form of nicotinamide-adenine dinucleotide phosphate (NADPH), are present in low concentrations in fetal microsomal preparations. In full-term infants, hepatic uridine diphosphate (UDP) glucuronosyltransferase and enzymes involved in the oxidation of polycyclic aromatic hydrocarbons are expressed at very low levels.

Age-related differences in pharmacokinetics vary from compound to compound. The half-life of acetaminophen in a newborn is similar to that of an adult, whereas theophylline has a half-life of ∼100 hr in a premature infant, as compared to 5-6 hr in an adult. These differences in metabolism, as well as factors such as binding to plasma proteins and renal clearance, determine appropriate drug dosage to maximize effectiveness and to avoid toxicity. Dramatic examples of the susceptibility of newborn infants to drug toxicity are the responses to chloramphenicol (the “gray baby” syndrome) or to benzoyl alcohol and its metabolic products, which involve ineffective glucuronide and glycine conjugation, respectively. The low concentrations of antioxidants (vitamin E, superoxide dismutase, glutathione peroxidase) in the fetal and early newborn liver lead to increased susceptibility to deleterious effects of oxygen toxicity and oxidant injury through lipid peroxidation.

Conjugation reactions, which convert drugs or metabolites into water-soluble forms that can be eliminated in bile, are also catalyzed by hepatic microsomal enzymes. Newborn infants have decreased activity of hepatic UDP glucuronosyltransferase, which converts unconjugated bilirubin to the readily excreted glucuronide conjugate and is the rate-limiting enzyme in the excretion of bilirubin. There is rapid postnatal development of transferase activity irrespective of gestational age, which suggests that birth-related, rather than age-related, factors are of primary importance in the postnatal development of activity of this enzyme. Microsomal activity can be stimulated by administration of phenobarbital, rifampin, or other inducers of cytochrome P450. Alternatively, drugs such as cimetidine can inhibit microsomal P450 activity.

Hepatic Excretory Function

Hepatic excretory function and bile flow are closely related to hepatic bile acid excretion and enterohepatic recirculation. Bile acids, the major products of cholesterol degradation, are incorporated into mixed micelles with cholesterol and phospholipid. These micelles act as an efficient vehicle for solubilization and intestinal absorption of lipophilic compounds, such as dietary fats and fat-soluble vitamins. Secretion of bile acids by the liver cells is the major determinant of bile flow in the mature animal. Accordingly, maturity of bile acid metabolic processes affects overall hepatic excretory function, including biliary excretion of endogenous and exogenous compounds.

In humans, the 2 primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized in the liver. Before excretion, they are conjugated with glycine and taurine. In response to a meal, contraction of the gallbladder delivers bile acids to the intestine to assist in fat digestion and absorption. After mediating fat digestion, the bile acids themselves are reabsorbed from the terminal ileum through specific active transport processes. They return to the liver via portal blood, are taken up by liver cells, and are re-excreted in bile. In an adult, this enterohepatic circulation involves 90-95% of the circulating bile acid pool. Bile acids that escape ileal reabsorption reach the colon, where the bacterial flora, through dehydroxylation and deconjugation, produce the secondary bile acids, deoxycholate and lithocholate. In an adult, the composition of bile reflects the excretion of the primary and also the secondary bile acids, which are reabsorbed from the distal intestinal tract.

Intraluminal concentrations of bile acids are low in newborn infants and increase rapidly after birth. The expansion of the bile acid pool is critical because bile acids are required to stimulate bile flow and absorb lipids, a major component of newborns’ diets. Nuclear receptors, such as farnesoid X receptor (FXR), control intrahepatic bile acid homeostasis through several mechanisms including regulation of expression of the genes encoding 2 key proteins, cholesterol 7α-hydroxylase (CYP7A1) and bile salt export pump (BSEP). These proteins are critical for bile acid synthesis and canalicular secretion, respectively. Neonatal expression of these nuclear receptors varies depending on the studied animal model and is largely unknown for humans.

Because of inefficient ileal reabsorption of bile acids and the low rate of hepatic clearance of bile acids from portal blood, serum concentrations of bile acids are commonly elevated in healthy newborns, often to levels that would suggest liver disease in older persons. Transient phases of “physiologic cholestasis” and “physiologic steatorrhea” can often be observed in low birthweight infants and in full-term infants following perinatal stress, such as hypoxia or infection, but are otherwise uncommon in healthy full-term newborns.

Many of the processes related to immaturity of the newborn in liver morphogenesis and function as discussed earlier are implied in the increased susceptibility of infants to liver disease associated with parenteral nutrition. The reduced bile salt pool, hepatic glutathione depletion, and deficient sulfation contribute to production of toxic lithocholic bile acids and cholestasis, whereas deficiencies of essential amino acids including taurine and cysteine can cause hepatic steatosis in these infants. Beyond the neonatal period, disturbances in bile acid metabolism may be responsible for diverse effects on hepatobiliary and intestinal function (Table 346-2).