Urothelial cell differentiation was studied on cell culture, in animals, and human urinary bladder of patients with selected diseases. The main research results are following: The expression, subcellular localisation, and supramolecular organisation of structure and differentiation - related proteins (especially uroplakins) were studied in differentiated superficial cells during growth and regeneration of urothelium. The finding shows that in some cases, expressing is sufficient marker of cell differentiation. In other cases, however, the specific subcellular localisation and supramolecular organisation of proteins should be considered. Subapical cytokeratins network elucidates the until now undecided supramolecular organization of cytokeratins in the apical region of surface urothelial cells. We established in vitro culture model that closely resembles whole mouse urothelial tissue - the primary explants culture. It was shown that non-differentiated mouse urothelial cells growing on a porous membrane show a high level of de novo differentiation. The recovery of a normal three-layered urothelium from hyperplastic urothelium induced by cyclophosphamide treatment has been investigated. The urothelium responds to the damage by intense proliferation of undifferentiated cells followed by differentiation, necrosis and apoptosis. Succession events in desquamation of superficial cells as a response to damage were evaluated. Dysfunction of tight junction and after that adherens junction was confirmed. It was established that ischemia of urinary bladder primary induce breakdown of the blood-urine permeability barrier. It causes interruption of the contact between the cells, which is followed by detachment and desquamation of viable urothelial cells. The damage occurs as a funnel-shaped wounds extending into the lamina propria. The expressing and distribution of uroplakins, and inducible nitric-oxide synthase (iNOS) in the urinary bladder of patients with bladder outlet obstruction (BOO) caused by benign prostatic hyperplasia (BHP) was evaluated. It was confirmed that in patients with BOO associated with BHP some superficial cells lack asymmetric membranes, suggesting focal compromise of the blood-urine barrier. In such undifferentiated zones there was an increase of the expression of iNOS. The study of localisation of tissue - type plasminogene activator (tPA) was evaluated in human histologically normal bladder urothelium and non-invasive tumours of the urinary bladder. The results suggest a positive correlation between tPA and cell differentiation.