chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas GALLB LADDE R 645 inflamed gallbladders. Cells that appear identical to enteroendocrine cells of the intestine are also found in these glands.
The wall of the gallbladder lacks a muscularis mucosae and submucosa.
External to the lamina propria is a muscularis externa that has numerous collagen and elastic fibers among the bundles of smooth muscle cells. Despite its origin from a foregutderived tube, the gallbladder does not have a muscularis mucosae or submucosa. The smooth muscle bundles are somewhat randomly oriented, unlike the layered organization of the intestine. Contraction of the smooth muscle reduces the volume of the bladder, forcing its contents out through the cystic duct.
External to the muscularis externa is a thick layer of dense connective tissue (see Fig. 18.16). This layer contains large blood vessels, an extensive lymphatic network, and the autonomic nerves that innervate the muscularis externa and the blood vessels (cell bodies of parasympathetic neurons are found in the wall of the cystic duct). The connective tissue is also rich in elastic fibers and adipose tissue. The layer of tissue where the gallbladder attaches to the liver surface is referred to as the adventitia. The unattached surface is covered by a serosa or visceral peritoneum consisting of a layer of mesothelium and a thin layer of loose connective tissue.
In addition, deep diverticula of the mucosa, called Rokitansky-Aschoff sinuses, sometimes extend through the muscularis externa (Fig. 18.18 and Plate 67, page 660). They are thought to presage pathologic changes and develop as the result of hyperplasia (excessive growth of cells) and herniation of epithelial cells through the muscularis externa. Also, bacteria may accumulate in these sinuses, causing chronic inﬂammation that is a risk factor for the formation of gallstones.
Concentration of the bile requires the coupled transport of salt and water.
The epithelial cells of the gallbladder actively transport both Na, Cl and HCO3  from the cytoplasm into the intercellular compartment of the epithelium. ATPase is located in the lateral plasma membranes of the epithelial cells. This active transport mechanism is essentially identical to that described in Chapter 17 for the enterocytes of the small intestine and the absorptive cells of the colon. The epithelial cells of the gallbladder also express two types of aquaporins water channels (AQP1 and AQP8), integral membrane channel proteins that facilitate rapid passive movement of water (see Chapter 20, Folder 20.XX). The presence of water channels on apical and basolateral plasma membranes of the gallbladder epithelial cells suggests that they may be involved in both water absorption and secretion.