Appraisal of differentiation markers in urothelial cells

The expression, subcellular localization, and supramolecular organization of structural and differentiation-related proteins were studied in differentiating superficial cells during regeneration of the rat urothelium. Cytokeratin 17 and epidermal growth factor receptor were expressed in undifferentiated cells, whereas in differentiated superficial cells, this expression ceased. beta-Actin was expressed in undifferentiated and in differentiated cells, but subcellular localization markedly changed by its withdrawal from apical region during differentiation. In differentiating cells, cytokeratin 20 was first observed as diffusely arranged spots distributed throughout the cytoplasm; later, it became organized as a network in the apical part of the cells. Urothelium specific proteins, uroplakins, were weakly expressed at early states of differentiation, but only in terminally differentiated cells did they organize into asymmetric unit membrane plaques. These findings show that in some cases, expression is a sufficient marker of cell differentiation. In other cases, however, the specific subcellular localization and supramolecular organization of proteins should be considered. It is evident that choosing the appropriate methods is crucial for precise evaluation of the urothelial differentiation states.