Actin filaments during terminal differentiation of urothelial cells in therat urinary bladder

The localisation of actin filaments was studied in rat urothelial cells dur ing differentiation which accompanied regeneration after cell damage induce d by cyclophosphamide (CP). By immunofluorescence it was established that a ctin filaments equally stained along the cell circumference in basal and in termediate cells, while basolateral cell membrane expression was found in t erminally differentiated superficial cells. During regeneration, after CP t reatment, simple urothelial hyperplasia developed with smaller cuboidal sup erficial cells, in which actin filaments were equally distributed under the apical and basolateral plasma membranes. As demonstrated by immunoelectron microscopy, the apical surface of these superficial cells was covered with microvilii containing bundles of actin filaments. Within 1 week, the uroth elium reverted to its normal three-layer thickness. Superficial cells becam e larger and flattened and the unthickened apical plasma membrane matured i nto a thick asymmetric unit membrane. Concomitantly actin filaments disappe ared from apical areas of superficial cells while remaining abundant at bas olateral areas. Our results indicate that in the urothelium subcellular dis tribution of actin filaments can be considered as a marker of cell differen tiation.