Gallstones are classified as cholesterol stones, calcium bilirubinate stones (pigment stones), and calcium carbonate stones, which are extremely rare. Pure stones, however, are almost never found. All gallstones contain variable amounts of bile pigments, cholesterol, calcium carbonate, and apatite, and their core usually consists of bile pigments or mucoprotein, which is secreted by the inner lining of the gallbladder, also known as the epithelium.

Cholesterol stones

The basis of the common cholesterol stone appears to be a precipitation of cholesterol that cannot be absorbed into solution. Cholesterol, which is insoluble in water, is carried in solution in the bile. People with cholesterol stones sometimes secrete a bile that is saturated or supersaturated with cholesterol. The bile secreted in the liver is abnormal in patients with cholesterol stones. These patients have a reduced bile salt pool and it seems that the defect is a lack of bile salts rather than excessive cholesterol. Gallstones are also a common accompaniment of cirrhosis, as there is often a reduction in the bile salt pool and a decrease in the bile salt content of the bile.

The gallbladder really plays a secondary part in allowing the cholesterol crystals to form a stone. Stones can form in the bile ducts in people who have had their gallbladders removed (cholecystectomy), which proves that the gallbladder is not essential for stone production, however, this situation is really quite rare. There may also be dietary and genetic factors in cholesterol stone formation, as cholesterol stones are far more common in European and Western cultures, than in Oriental groups.

Pigment stones

Pigment stones are more common in African and Asian peoples than in Europeans. Further, they are a serious complication of some congenital hemolytic anemias but not of the acquired ones. Stones seem to beget stones. For instance, a gallbladder that has a stone can become obstructed and cause inflammation, which in turn causes more gallstones. Pigment stones consist of variable quantities of cholesterol, calcium bilirubinate, and calcium carbonate, and produce a predominantly brown colour. These mixed stones are typically found in multiple groups, and form around organisms and inflammatory debris.

Alexander Nagy, BSc, MD 
in association with the MediResource Clinical Team