Urinary Bladder and Urothelium

Urinary Bladder and Urothelium Joseph Alroy, D.V.M. The urinary bladder is an organ that has to change its luminal volume. This is achieved by the structures of both the smooth muscle, i.e. detrusor, and the urothelium. Unlike the gastrointestinal tract in which the smooth muscle cells are arranged in circular and longitudinally to enable the peristaltic movement of the digestive system, in the bladder the muscle bundles are interlaced to enable the dilation and contraction of the organ. Likewise, the urothelium is also able to contract and distend. The superficial and the intermediate cells contain plaques of asymmetric unit membrane, in which the outer leaflet is thicker than the inner leaflet. The plaques are joined by “hinges” of interplaque plasma membrane. These plaques are synthesized in the Golgi apparatus. The luminal membrane folds to form fusiform vesicles when the organ is contracted. The following 4 illustrations are taken from “Intraepithelial asymmetric-unit-membrane plaques in mammalian urinary bladder”, by Joseph Alroy and Ronald Weinstein. The Anatomical Record 197:75-83 (1980).