Intraepithelial asymmetric‐unit‐membrane plaques in mammalian urinary bladder

Asymmetric‐unit‐membrane (AUM) plaques are major components of the luminal surface of superficial cells in mammalian urinary bladder. We have now demonstrated ultrastructurally identical AUM plaques in intraepithelial locations, at the lateral and basal surfaces of superficial cells and at the apical surfaces of intermediate epithelial cells in the urinary bladder. Typically, luminal apposing AUM plaques are present in clusters. Similar plaque clusters are present on apposing surfaces of cell pairs within the epithelium. These intraepithelial plaque clusters are in register, although often separated by a wide interspace. In areas where they come into closer apposition, they form membrane complexes resembling but distinctly different from certain types of cell‐to‐cell junctions. Both luminal and intraepithelial AUM plaques serve as membrane attachment sites for cytoplasmic microfilaments. Their intraepithelial location and association with microfilaments support the hypothesis that AUM plaques serve a mechanical function and modulate cell surface area during the urinary bladder distention‐contraction cycle.