Cirrhosis pathophysiology On the Web
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Cirrhosis occurs due to long term liver injury which causes an imbalance between matrix production and degradation. The pathological hallmark of cirrhosis is the development of scar tissue which leads to replacement of normal liver parenchyma, leading to blockade of portal blood flow and disturbance of normal liver function. When fibrosis of the liver reaches an advanced stage where distortion of the hepatic vasculature also occurs, it is termed as cirrhosis of the liver. The pathogenesis of cirrhosis involves inflammation, hepatic stellate cell activation, angiogenesis, and fibrogenesis. Kupffer cells are hepatic macrophages responsible for hepatic stellate cell activation during injury. Hepatic stellate cells (HSC) which are located in the subendothelial space of Disse, become activated in areas of liver injury and secrete transforming growth factor-beta 1 (TGF-β1), which leads to a fibrotic response and proliferation of connective tissue. Cirrhosis may also lead to hepatic microvascular changes including the formation of intra-hepatic shunts (due to angiogenesis and loss of parenchymal cells) and endothelial dysfunction. Fibrosis eventually leads to formation of septae that grossly distort the liver architecture which includes both the liver parenchyma and the vasculature, accompanied by regenerative nodule formation.
- When an injured tissue is replaced by a collagenous scar, it is termed as fibrosis. The development of fibrosis requires several months, or even years of ongoing injury.
- The pathological hallmark of cirrhosis is the development of scar tissue that leads to replacement of normal liver parenchyma, leading to blockade of portal blood flow and disturbance of normal liver function.
- When fibrosis of the liver reaches a point where distortion of the hepatic vasculature also occurs, it is termed as cirrhosis of the liver. If the damage progresses, panlobular cirrhosis may result.
- The cellular mechanisms responsible for cirrhosis are similar regardless of the type of initial insult and site of injury within the liver lobule.
- Viral hepatitis involves the periportal region, whereas involvement in alcoholic liver disease is largely pericentral.
- Cirrhosis involves the following steps:
Hepatic stellate cell activation
The role of hepatic stellate cells in the pathogenesis of cirrhosis is described below:
- Kupffer cells are hepatic macrophages responsible for hepatic stellate cell activation during injury.
- The stellate cell, (also known as the perisinusoidal cell or Ito cell) is a type of cell that normally stores vitamin A and plays a pivotal role in the development of cirrhosis.
- Hepatic stellate cells (HSC) are usually located in the subendothelial space of Disse and become activated to a myofibroblast-like cell in areas of liver injury. This contractile cell (known as a myofibroblast) obstructs blood flow in the circulation.
- The stellate cell secretes transforming growth factor-beta 1 (TGF-β1), which leads to a fibrotic response and proliferation of connective tissue.
- Connective tissue proliferation leads to the formation of extracellular matrix around hepatocytes that is composed of collagens (especially type I, III, IV), glycoprotein and proteoglycans.
- Collagen and non-collagenous matrix proteins responsible for fibrosis are produced by the activated hepatic stellate cells (HSC).
- Hepatocyte damage causes the release of lipid peroxidases from injured cell membranes leading to necrosis of parenchymal cells.
- Activated HSC induce the production of numerous cytokines and their receptors, such as platelet-derived growth factor (PDGF) and TGF-f31, which are responsible for fibrogenesis.
- The matrix formed due to HSC activation is deposited in the space of Disse and leads to loss of fenestrations of endothelial cells, through a process called capillarization.
- Stellate cell activation leads to disturbance of the balance between matrix metalloproteinases and the naturally occurring inhibitors (TIMP 1 and TIMP2). This is followed by matrix breakdown and replacement by connective tissue-secreted matrix.
- Matrix metalloproteinase (MMP) are calcium dependent enzymes that specifically degrade collagen and non collagenous substrate.
- MMP-2 and stromyelysin-1 are produced by stellate cells.
- MMP-2 degrades collagen and stromelysin-1 degrades proteoglycan and glycoprotein.
- Formation of intra hepatic shunts (due to angiogenesis and loss of parenchymal cells)
- Hepatic endothelial dysfunction
- Sinusoidal endothelial cells are also important contributors of early fibrosis. Endothelial cells from a normal liver produces collagen, laminin and fibronectin.
- The endothelial dysfunction is characterised by:
- The liver responds to injury with new blood vessel formation. Mediators involved in angiogenesis include:
- Angiogenesis in cirrhosis results in the production of immature and permeable vascular endothelial growth factor (VEGF) induced neo-vessels that further exacerbate liver injury.
The role of fibrosis in the pathogenesis of cirrhosis is described below:
- Fibrosis eventually leads to formation of septae that grossly distort the liver architecture which includes both the liver parenchyma and the vasculature.
- A cirrhotic liver compromises hepatic sinusoidal exchange by shunting arterial and portal blood directly into the central veins (hepatic outflow).
- Vascularized fibrous septa connect central veins with portal tracts leading to islands of hepatocytes surrounded by fibrous bands without central veins.
- These mechanisms simultaneously occurring in the liver lead to fibrous tissue band (septa) and regenerative hepatocyte nodule formation, which eventually replace the entire liver architecture, leading to decreased blood flow throughout.
- The formation of fibrotic bands is accompanied by regenerative nodule formation in the hepatic parenchyma.
- Advancement of cirrhosis may lead to parenchymal dysfunction and development of portal hypertension.
- The pathological hallmark of cirrhosis is the development of scar tissue that replaces normal parenchyma, leading to blockade of portal blood flow and disturbance of normal liver function.
- Due to portal hypertension, the spleen becomes congested, which leads to hypersplenism and increased platelet sequestration.
Pathogenesis of cirrhosis according to cause
Pathogenesis of cirrhosis based upon the underlying cause is as follows:
- Alcoholic liver disease: Alcohol seems to injure the liver by blocking the normal metabolism of protein, fats, and carbohydrates. Patients may also have concurrent alcoholic hepatitis with fever, hepatomegaly, jaundice, and anorexia. Liver damage due to alcoholic hepatitis may progress to cirrhosis.
- Chronic hepatitis C: Infection with the hepatitis C virus causes inflammation and low grade damage to the liver that may eventually lead to cirrhosis after decades.
- Non-alcoholic steatohepatitis (NASH): In NASH, fat builds up in the liver and eventually causes scar tissue. This type of hepatitis appears to be associated with diabetes, protein malnutrition, obesity, coronary artery disease, and treatment with corticosteroid medications.
- Primary sclerosing cholangitis (PSC): PSC is a progressive cholestatic disorder presenting with pruritus, steatorrhea, fat soluble vitamin deficiencies, and metabolic bone disease.
- Autoimmune hepatitis: Immunologic damage to the liver leads to inflammation, scarring and cirrhosis.
- Portal hypertension may result from a combination of the following:
Pathophysiology of Cirrhosis due to Alcohol
- Ethanol intake leads to elevated accumulation of intracellular triglycerides by:
- Alcohol is converted by alcohol dehydrogenase to acetaldehyde.
- Due to the high reactivity of acetaldehyde, it forms acetaldehyde-protein adducts which cause damage to cells by:
- Reactive oxygen species begin to form as a result of hepatocyte damage that activate Kupffer cells.
- Kupffer cell activation leads to the production of profibrogenic cytokines which in turn, stimulates stellate cells.
- Stellate cell activation leads to connective tissue formation due to deposition extracellular matrix and collagen.
- Portal triads develop connections with central veins due to connective tissue formation in pericentral and periportal zones, leading to the formation of regenerative nodules.
- Shrinkage of the liver occurs over years due to repeated insults that lead to:
Pathophysiology of Portal Hypertension due to Cirrhosis
- Portal hypertension is related to elevation of resistance in the portal vasculature.
- Increased resistance in portal system may be due to both intra-hepatic and also portosystemic collateral resistance.
- Intra-hepatic resistance
- The main factor responsible for intra-hepatic resistance is hepatic vascular compliance, which is greatly decreased in liver fibrosis or cirrhosis.
- Portal hypertension occurs when compliance is decreased and blood flow is increased in liver.
- Pre-hepatic and post-hepatic portal hypertension arise due to some secondary obstruction before or after liver vasculature, respectively.
- Alcoholic hepatitis causes both sinusoidal and post-sinusoidal pathologies.
- Hepatic vascular endothelium synthesizes and secretes both vasodilators (e.g., nitric oxide, prostacyclins) and vasoconstrictors (e.g., endothelin and prostanoids).
- Increased resistance due to the elevation of vascular tone may be caused by excess of vasoconstrictors or lack of vasodilators.
- It is postulated that in cirrhotic liver the nitric oxide level is lower and the response to endothelin in myofibrils is stronger than in normal liver.
- Portosystemic collateral resistance
- Collateral blood circulation develops as a consequence of portal hypertension which is the main contributor to esophageal and gastric varices
- The main purpose of the collaterals is to decompress and bypass portal blood flow.
- However, portosystemic collaterals may not lead to a complete decompression.
- Portosystemic circulation occurs between the short gastric, left gastric vein, and the esophageal, azygos and the intercostal veins; the superior, the middle, and the inferior hemorrhoidal veins; the paraumbilical venous plexus, the venous system of abdominal organs juxtaposed with the retroperitoneum and abdominal wall; the left renal vein, the splanchnic, the adrenal, and the spermatic veins.
- Intra-hepatic resistance
Hyperdynamic circulation in portal hypertension
- Peripheral vasodilatation is the basis for decreased systemic vascular resistance and mean arterial pressure, plasma volume expansion, elevated splanchnic blood flow, and elevated cardiac index.
- Systemic vasodilation
- Plasma volume
- Certain TERT (Telomerase reverese transcriptase) gene variants resulting in reduced telomerase activity have been found to be a risk factor for sporadic cirrhosis
- An uncharacterized nucleolar protein, NOL11, has a role in the pathogenesis of North American Indian childhood cirrhosis
- Loss of interaction between the C-terminus of a protein called Utp4/cirhin and other SSU processome proteins may cause cirrhosis in children
On gross examination, the liver may initially be enlarged, but with progression of the disease, it becomes smaller. Its surface is irregular, the consistency is firm, and the color is often yellow (if associates steatosis). Depending on the size of the nodules there are three macroscopic types: micronodular, macronodular and mixed cirrhosis.
- In the micronodular form (Laennec's cirrhosis or portal cirrhosis) regenerating nodules are under 3 mm.
- In macronodular cirrhosis (post-necrotic cirrhosis), the nodules are larger than 3 mm.
- The mixed cirrhosis consists of a variety of nodules with different sizes.
Images of gross pathology of cirrhosis
- Microscopic pathology reveals the four stages of cirrhosis as it progresses:
- Chronic nonsuppurative destructive cholangitis: inflammation and necrosis of portal tracts with lymphocyte infiltration leads to the destruction of the bile ducts
- Development of biliary stasis and fibrosis
- Periportal fibrosis progresses to bridging fibrosis
- Increased proliferation of smaller bile ductules leads to regenerative nodule formation
- Microscopically, cirrhosis is characterized by regeneration nodules surrounded by fibrous septa.
- In these nodules, regenerating hepatocytes are present.
- Portal tracts, central veins and the radial pattern of hepatocytes are absent.
- Fibrous septa are present and inflammatory infiltrate composed of lymphocytes and macrophages) are also visible.
- If the underlying cause is secondary biliary cirrhosis, biliary ducts are damaged, proliferated or distended leading to bile stasis.
- Dilated ducts contain inspissated bile which appears as bile casts or bile thrombi (brown-green, amorphous).
- Bile retention may be found also in the parenchyma and are referred to as "bile lakes".
The main microscopic histopathological findings in portal hypertension are related to cirrhosis, esophageal varices, hepatic amyloidosis, and congestive hepatopathy due to heart failure or Budd-Chiari syndrome.
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