Distinct proteins of the hepatocyte sinusoidal and canalicular domains. (A) Left: Electron micrograph reveals the ultrastructure of the sinusoidal plasma membrane with numerous microvilli protruding into the perisinusoidal space (marked “Sin,” arrow) adjacent to sinusoidal endothelial cells (marked “E”). The sinusoidal domain contains abundant coated endocytic pits marked by arrowheads, highlighting the striking endocytic capacity of the hepatocyte. The corresponding cartoon (right) illustrates some of the prominent sinusoidal resident proteins that include a wide variety of receptors, signal transduction proteins, and transporters. (B) Left: The canalicular domain also contains microvilli that protrude into the bile canaliculus (marked “BC,” arrow), where bile transport and lysosomal secretion occurs (arrows denote electron-dense lysosomes). As shown in the cartoon (right), the canalicular domain is dominated by ectoenzymes and numerous transporters that shuttle water, bile acids, lipids, and other organic and inorganic molecules across the membrane and into the bile canaliculus. Electron micrographs (magnification of 9,100) are reprinted with permission from Schroeder and McNiven (2009). AQP, aquaporin; MATE-1, multidrug and extrusion protein 1; AP-N, alanine aminopeptidase.