**3.1 Cholesterol gallstones**

Various defects occur simultaneously for the nucleation and crystallization of cholesterol monohydrate viz unphysiological supersaturation with cholesterol, accelerated nucleation and gallbladder hypomotility [16]. The mucin glycoprotein hypersecretion follows and lead to the stone formation [84].

Excessive secretion of cholesterol into bile leads to cholesterol supersaturation [85] owing to multiple biochemical defects either from increased input (de novo synthesis, lipoprotein uptake) or decreased disposition (de novo bile salt and cholesteryl ester synthesis) [84]. However single defects in hepatocellular processing of cholesterol are known viz


These changes resulting in the increased levels of cholesterol in microsomes [87] and cytosol [88] of hepatocytes in patients with gallstones. Sterol carrier protein 2 concentration also increases simultaneously [89].

At puberty hypersecretion of cholesterol into bile begins [90]. In liver excess cholesterol is delivered to bile by a relay station [91]. Postprandially the cholesterol is completely micellized by bile salts and lecithin during healthy condition [92].

Cholesterol molecules are absorbed by gallbladder mucosa efficiently from micelles of supersaturated bile [93] where the activity of acyl coenzyme A: cholesterol acyl transferase esterifies the sterol [87]. But the molecules that remain free are dissolved in plasma and intercellular membranes where they become intercalated with and stiffen the phospholipid molecules [86]. Reverse diffusion when gallbladder bile become unsaturated or esterification is the only means of escape of membrane cholesterol as gallbladder mucosa does not produce cholesterol [92].

The abnormity in diurnal variation of gallbladder bile due to hypersecretion of cholesterol lead to the further trapping of excess free cholesterol molecules into gallbladder mucosa and muscle membranes [94]. There occurs the divergence of hepatic bile into gastrointestinal tract due to impaired motor and mucosal functions of gallbladder [95]. This in turn lead to increased bacterial catabolism and turnover of primary bile salts [92], which ultimately increase the production of secondary bile salts (hydrophobic bile salts) and suppress de novo bile salt synthesis [96].

Rapid recirculation and hydrophobicity are harmful for the stability of bile as these lead to the augmentation of secretion of cholesterol and hydrophobic lecithin molecules, which in turn shorten the time of nucleation of supersaturated bile [91]. The hydrophobicity of bile salt pool also increases secretion of total proteins into bile [97] and also altering the ratio of pro and anti-nucleating activities for cholesterol crystallization [97]. Multiple stimuli trigger mucin glycoprotein hypersecretion like cholesterol in itself [87], 'cytotoxic' filamentous cholesterol crystals [98], prostanoids [99], and hydrophobic bile salts e. g Deoxycholate and lithocholate conjugates [100]. This lead to the development of mucin gel on gallbladder wall and then as a crescent in gallbladder lumen, setting stage for accelerated nucleation. This together with hypomotility lead to its accumulation as biliary sludge, failure of complete evacuation of which lead to gallstone formation [84]. However, the end result may become inevitable by an environmental perturbation that inauspiciously tips the delicate pathophysiological and physical-chemical balance of supersaturated bile towards nucleation and stone growth [86].

#### **3.2 Pigment gallstones**

Black stones occur in sterile environment [101] with increased frequency in patients of chronic haemolysis. A shift in ratio of bilirubin diconjugates to the favour of monoconjugates especially monoglucuronides occur due to hypersecretion of bilirubin conjugates in bile [102]. Bile pigment output increases by 10-fold with haemolysis [102] and predominantly becomes monoconjugates that are more hydrolysed by endogenous β glucuronidase [101]. This gives rise to very high levels of unconjugated pigment exceeding biliary solubility [101]. The insoluble acid calcium salt [Ca (HUCB)2] forms at pH values typical of gallbladder bile [103]. Unphysiological supersaturation occurs with an elevation in the ion product of calcium and monoacid species of unconjugated bilirubin [101] facilitating factors like in cholesterol stones are important for stone formation. The prevalence of pigment stones increases with age and is gender dependent; and all haemolytic patients does not develop pigment stones [101]. In experimental studies it was shown that there is no gallbladder motility defect in black pigment stone formation [104]. However, mucin hypersecretion occurs [105] in response to the high levels of unconjugated bilirubin [106] or mucosal cell cytotoxicity of earliest precipitates [107]. Nucleation is initiated at glandular crypts of gallbladder mucosa where mucin gel accumulates first [107]. Biliary supersaturation (owing to defective acidification of hepatic bile [101] with organic salts occur which is indicated by presence of crystalline calcium carbonate and phosphates in black pigment stones [101]. 3–20% of black pigment stones are composed of mucin glycoprotein [108]. Various factors came into play

#### *Pathophysiology of Gallstones DOI: http://dx.doi.org/10.5772/intechopen.100553*

and through different mechanisms give rise to pigment stone formation in alcoholic cirrhosis, ileal dysfunction and in aging Viz bile salt hyposecretion, defective solubilization of unconjugated bilirubin, impaired calcium ion binding, haemolysis, defective bilirubin conjugation [101], enteric hyperbilirubinobilia [86].

Chronic anaerobic infection with functional stasis of bile ducts is necessary for brown pigment stones. Stasis in bile duct occur due to secondary stones from gallbladder, or due to sphincter of Oddi dysfunction or parasitic infections (mostly underdeveloped countries) [84]. Commencing with stasis of bile and then followed by anaerobic bacterial infection lead to accumulation of both mucin gel and bacterial cytoskeleton in bile ducts. Bacterial enzymes (phospholipase A, β glucuronidase) hydrolyse the biliary lipids that contain amide or ester linkages which results in the nucleation of cholesterol monohydrate crystals A trap is laid down in the form of culture medium for anaerobic bacteria by mucin gel and solid components making their exit difficult. This is a vicious cycle formed between growing stone, stasis of bile and bacterial infection [84, 109].
