bile canaliculus
Summary
A bile canaliculus is a tiny channel formed by the apical (canalicular) membranes of adjacent hepatocytes into which bile is secreted. Canaliculi drain toward the portal triad into the canals of Hering and then bile ductules, and bile flows opposite to blood flow (periphery of the lobule toward the portal tract, from central vein region outward).
Detail
STRUCTURE: Bile canaliculi are 1-2 µm channels with no wall of their own. They are formed by grooves in the apical (canalicular) surface of adjacent hepatocytes, sealed by tight junctions (zonula occludens) that separate bile from the sinusoidal blood and perisinusoidal space of Disse. The canalicular membrane has microvilli and is surrounded by a pericanalicular actin network that allows contractions and propels bile.
BILE FLOW: Hepatocyte canaliculus → canal of Hering (lined partly by hepatocytes and cholangiocytes; a stem/progenitor cell niche) → bile ductule → interlobular bile duct (portal triad) → left/right hepatic ducts → common hepatic duct → (via cystic duct to gallbladder) → common bile duct → ampulla of Vater → duodenum. Bile flows in the opposite direction to blood flow (blood flows from the portal triad to the central vein; bile flows from the central vein region toward the portal triad).
PHYSIOLOGY: The canalicular membrane is the site of the rate-limiting step of bile formation. Key transporters (ATP-binding cassette): - BSEP (ABCB11): bile salt export pump; main driver of bile acid-dependent bile flow. - MDR3 (ABCB4): phosphatidylcholine (phospholipid) flippase. - MRP2 (ABCC2): conjugated bilirubin (and glutathione, drug conjugates) export. - ABCG5/ABCG8: cholesterol export. - MDR1 (ABCB1): organic cations/drugs. Bile contains bile acids, phospholipids, cholesterol, conjugated bilirubin, electrolytes and water. Cholangiocytes later modify the bile (secretin-stimulated bicarbonate and water secretion via CFTR and Cl-/HCO3- exchange).
CLINICAL CORRELATES (high-yield): - Dubin-Johnson syndrome: AR, MRP2 (ABCC2) defect → impaired excretion of conjugated bilirubin into canaliculi → conjugated hyperbilirubinemia; grossly black liver due to pigment accumulation; benign; normal liver enzymes. Rotor syndrome is similar but with a nonpigmented liver (OATP1B1/1B3 defect, impaired hepatic reuptake/storage). - Progressive familial intrahepatic cholestasis (PFIC): type 2 = BSEP defect (low GGT, cholestasis in infancy); type 3 = MDR3 defect (high GGT, ↓ phospholipid → bile duct injury, cholesterol gallstones). - Benign recurrent intrahepatic cholestasis: milder ATP8B1/BSEP variants. Intrahepatic cholestasis of pregnancy: ABCB4/ABCB11 variants + estrogen effects; pruritus, ↑ bile acids. - Drug-induced cholestasis: BSEP inhibition (e.g., bosentan, cyclosporine, rifampin, glyburide, some OCPs/estrogens, anabolic steroids, chlorpromazine); canalicular cholestasis often with ↑ ALP/GGT and conjugated bilirubin. - Cholestasis histology: bile plugs in dilated canaliculi, feathery degeneration of hepatocytes. - Alagille syndrome (JAG1) involves paucity of interlobular bile ducts (not canaliculi); primary biliary cholangitis targets small intrabile ducts (anti-mitochondrial antibodies), primary sclerosing cholangitis involves medium/large ducts (p-ANCA, beading). - Cholestasis lab pattern: ↑ conjugated (direct) bilirubin, ↑ ALP, ↑ GGT, ↑ cholesterol, pruritus, pale stools, dark urine (bilirubinuria). - Mnemonic: BSEP = bile salts; MDR3 = phospholipids; MRP2 = bilirubin.
Sources
- First Aid for the USMLE Step 1 (Gastrointestinal: Liver tissue architecture, Hyperbilirubinemia)
- Robbins & Cotran Pathologic Basis of Disease (Liver and Biliary Tract chapter)
- Junqueira's Basic Histology (Liver chapter)
- Guyton and Hall Textbook of Medical Physiology (Liver secretion of bile)
- Costanzo Physiology (GI physiology: Bile formation)
- Pathoma (Liver pathology)
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