The terms biliary sludge and cholesterol microlithiasis(hereafter referred to as microlithiasis)were originated from different diagnostic techniques and may represent different stages of cholesterol gall-stone disease.Although the pathogenesis of biliary sludge and microlithiasis may be similar,micro-lithiasis could be preceded by biliary sludge,followed by persistent precipitation and aggregation of solid cholesterol crystals,and eventually,gallstone formation.Many clinical conditions are clearly associated with the formation of biliary sludge and microlithiasis,including total parenteral nutrition,rapid weight loss,pregnancy,organ transplantation,administration of certain medications,and a variety of acute and chronic illnesses.Numerous studies have demonstrated complete resolution of biliary sludge in approximately 40％of patients,a cyclic pattern of disappearing and reappearing in about 40％,and progression to gallstones in nearly 20％.Although only a minority of patients with ultrasonographic demonstration of biliary sludge develop gallstones,it is still a matter of controversy whether micro-lithiasis could eventually evolve to cholesterol gallstones.Biliary sludge and microlithiasis are asymp-tomatic in the vast majority of patients;however,they can cause biliary colic,acute cholecystitis,and acute pancreatitis.Biliary sludge and microlithiasis are most often diagnosed ultrasonographically and bile microscopy is considered the gold standard for their diagnosis.Specific measures to prevent the development of biliary sludge are not practical or cost-effective in the general population.Laparoscopic cholecystectomy offers the most definitive therapy on biliary sludge.Endoscopic sphincterotomy or surgical intervention is effective for microlithiasis-induced pancreatitis.Ursodeoxycholic acid can effectively prevent the recurrence of solid cholesterol crystals and significantly reduce the risk of recurrent pancreatitis.

Cholesterol gallstone formation represents a failure of biliary cholesterol homeostasis in which the physical-chemical balance of cholesterol solubility in bile is disturbed.Lithogenic bile is mainly caused by persistent hepatic hypersecretion of biliary cholesterol and sustained cholesterol-supersaturated bile is an essential prerequisite for the precipitation of solid cholesterol monohydrate crystals and the forma-tion of cholesterol gallstones.The metabolic determinants of the supply of hepatic cholesterol molecules that are recruited for biliary secretion are dependent upon the input-output balance of cholesterol and its catabolism in the liver.The sources of cholesterol for hepatic secretion into bile have been extensively investigated;however,to what extent each cholesterol source contributes to hepatic secretion is still unclear both under normal physiological conditions and in the lithogenic state.Although it has been long known that biliary lithogenicity is initiated by hepatic cholesterol hypersecretion,the genetic mecha-nisms that cause supersaturated bile have not been defined yet.Identification of the Lith genes that determine hepatic cholesterol hypersecretion should provide novel insights into the primary genetic and pathophysiological defects for gallstone formation.In this review article,we focus mainly on the path-ogenesis of the formation of supersaturated bile and gallstones from the viewpoint of genetics and pathophysiology.A better understanding of the molecular genetics and pathophysiology of the formation of cholesterol-supersaturated bile will undoubtedly facilitate the development of novel,effective,and noninvasive therapies for patients with gallstones,which would reduce the morbidity,mortality,and costs of health care associated with gallstones,a very prevalent liver disease worldwide.