Gallstones can be due to precipitation of cholesterol. Under normal conditions, bile can dissolve all the cholesterol excreted by the liver. If the liver produces more cholesterol than bile can dissolve, the excess cholesterol may precipitate as crystals which are then trapped in gallbladder mucus, producing sludge. This is known as cholesterol supersaturation. These crystals may grow and form stones. Cholesterol stones are the most common type (80-90%), especially in Western countries.
Cholesterol stones are usually radiolucent meaning they are not readily visible on X-ray imaging (10% are radiopaque due to associated calcium).
Black pigment stones result from hemolysis and consist primarily of calcium bilirubinate. This can occur in hemolytic disorders like sickle cell anemia, for example.
Extravascular hemolysis such as that occurring in sickle cell anemia increases bilirubin in the bile, increasing the risk of pigmented (black) gallstones formation.
Another type of pigmented stones are brown stones. These stones are associated with a bacterial or parasitic infection of the biliary system.
Brown stones are radiolucent and are associated with infections. Brown pigment stones are associated with bacterial infections and parasites (e.g. Clonorchis sinensis) affecting the biliary tract. Infection causes the release of beta-glucuronidase (a lysosomal enzyme that deconjugates direct bilirubin) by injured hepatocytes and bacteria, which hydrolyzes conjugated bilirubin and lecithin in the bile increasing unconjugated bilirubin and fatty acids. This leads to precipitation of calcium carbonate, cholesterol and calcium bilirubinate (dark color) in bile.
Pigmented gallstones are usually radiopaque in contrast to cholesterol stones which are usually radiolucent.
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