ACTIN AND CYTOKERATIN IN SUPERFICIAL TRANSITIONAL EPITHELIAL-CELLS OFTHE RABBIT URINARY-BLADDER - A CONFOCAL AND ELECTRON-MICROSCOPIC STUDY

Transitional epithelial cells undergo dynamic morphological alteration s during the contraction-expansion cycle in the mammalian urinary blad der. In this study, the localization of two major cytoskeletal element s, actin and cytokeratin, was examined by fluorescence and electron mi croscopic methods to elucidate the relationship of these components in the transitional epithelial cells of non-distended bladders. Using ph alloidin-FITC and confocal laser scanning microscopy (CLSM), a strong staining for filamentous actin was observed at subplasmalemmal regions of the superficial cells. Cytoplasmic regions of these cells were cha racterized by punctate regions of staining that increased at sites of cell-cell contact. Ultrastructural observations using immunogold and h eavy meromyosin (HMM) labelling techniques for detecting actin filamen ts revealed a rich actin filament network in superficial cells most no tably in the subplasmalemmal regions. These filaments occurred in orie ntations that were both parallel and perpendicular relative to the sur face membrane. Intermediate and basal cells contained a less dense net work of filamentous actin in comparison to superficial cells. Immunola belling for cytokeratin 18 revealed abundant thread-like filaments cou rsing throughout the cytoplasm and making contact with the plasmalemma in superficial cells. By electron microscopy, the cytokeratin filamen ts formed a dense interlacing network arranged into bundles that inter connected with membrane-bound structures. These observations indicate that actin and cytokeratin are major cytoskeletal elements of transiti onal epithelial cells and that they are related to the configurational alternations of transitional epithelial cell borders and their conten ts. It is postulated proteins stabilize and support membrane of superf icial cells from hydrostatic pressure within the bladder.