Current concepts of cholesterol transport and crystal formation in human bile. The presence of vesicles in human bile probably accounts almost entirely for the frequently observed, but hitherto unexplained, phenomenon of metastable cholesterol supersaturation. This, in turn, largely explains the prolonged stability of cholesterol solubilized in supersaturated human bile. Under certain overall compositional conditions for a supersaturated native bile, the vesicular phase in its contribution to total cholesterol transport also becomes supersaturated in cholesterol. Because of this, the vesicles also become unstable, leading to formation of cholesterol crystals. A simple but common example of one factor affecting composition in this way is concentration of total solutes, especially the biliary lipids. Conversely, dilution of bile (e.g., hepatic bile) markedly reduces the cholesterol saturation level in biliary vesicles. The result is that such vesicles become much more stable. Under these conditions, cholesterol crystal formation becomes unlikely and rarely, if ever, occurs. 