The gallbladder is a blind pouch that leads, via a neck, to the cystic duct. Through this duct it receives dilute bile from the chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas GALLB LADDE R 643 hepatic duct. The gallbladder can store and remove about  Localized concentrations of mitochondria in the apical 90% of the water from the incoming bile, which results in an and basal cytoplasm increase of bile salts, cholesterol, and bilirubin concentrations  Complex lateral plications up to 10-fold. Hormones secreted by the enteroendocrine cells of the small intestine, in response to the presence of fat in the proximal duodenum, stimulate contractions of the smooth muscle of the gallbladder. Because of these contractions, concentrated bile is discharged into the common bile duct, which carries it to the duodenum.
Mucosa of the gallbladder has several characteristic features.
The empty or partially filled gallbladder has numerous deep mucosal folds (Fig. 18.16). The mucosal surface consists of simple columnar epithelium (Fig. 18.17). The tall epithelial cells exhibit the following features:  Numerous, but short and not well-developed apical microvilli  Apical junctional complexes that join adjacent cells and form a barrier between the lumen and the intercellular compartment
These cells closely resemble the absorptive cells of the intestine.
Both cells share the above characteristics, as well as localization of Na/K–activated ATPase on their lateral plasma membranes and secretory vesicles filled with glycoproteins in their apical cytoplasm.
The lamina propria of the mucosa is particularly rich in fenestrated capillaries and small venules, but there are no lymphatic vessels in this layer. The lamina propria is also very cellular, containing large numbers of lymphocytes and plasma cells. The characteristics of the lamina propria resemble those of the colon, another organ specialized for the absorption of electrolytes and water.
Mucin-secreting glands are sometimes present in the lamina propria in the normal human gallbladder, especially near the neck of the organ, but they are more commonly found in
FIGURE 18.16 • Photomicrograph of the wall of the gallbladder. The mucosa of the gallbladder consists of a lining of simple columnar epithelial cells and a lamina propria of loose connective tissue, which typically exhibits numerous deep folds in the mucosa. Beneath this layer is a relatively thick layer, the muscularis externa. There is no muscularis mucosae or submucosa. The smooth muscle bundles of the muscularis externa are randomly oriented. External to the muscle is an adventitia containing adipose tissue and blood vessels. The portion of the gallbladder not attached to the liver displays a typical serosa instead of an adventitia. 175.
FIGURE 18.17 • Electron micrographs of gallbladder epithelium. a. The tall columnar cells display features typical of absorptive cells, with microvilli on their apical surface, an apical junctional complex separating the lumen of the gallbladder from the lateral intercellular space, and numerous mitochondria in the apical portion of the cell. 3,000. b. During active fluid transport, salt is pumped from the cytoplasm into the intercellular space, and water follows the salt. Both salt and water then diffuse into the cell from the lumen. As this process continues, the intercellular space becomes greatly distended (arrows). Fluid moves from the engorged intercellular space (arrows) across the basal lamina into the underlying connective tissue (CT) and then into blood vessels. The increase in size of the lateral intercellular space during active fluid transport is evident with the light microscope. 3,000.