Morphologically and chemically, gallstones are of two main types, cholesterol and pigment stones. Cholesterol gallstones are more common (90% of all gallstones) and contain more than 50% cholesterol by weight.[9] Mechanisms of cholesterol lithogenesis include biliary cholesterol hyper secretion, super saturation of bile with cholesterol, nucleation of crystals and formation of gallstones.[10] All these processes are under genetic control and/ or influenced through intermediate pathogenic steps linked to a variety of environmental factors. Hyper- secretion of cholesterol in the bile is considered to be an important prerequisite for gallstone formation.[11], [12]
First step in gallstone formation is initiation of cholesterol monohydrate crystals, which grow in size and agglomerate to form macroscopic stones in the gall bladder. Bile cholesterol is mostly derived from preformed cholesterol circulating in the plasma and bound to different apo-lipoproteins. Cholesterol transporting molecules like apo E and apo B control the availability of cholesterol for bile secretion. Dietary cholesterol is absorbed in the intestine and transported in chylomicron remnant as part of low and high-density lipoproteins. After hepatic uptake, chylomicron remnant cholesterol becomes a substrate for bile acid synthesis and is secreted into bile both as bile salt and un-esterified cholesterol.
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