Staining patterns of keratins in the human urinary tract.

The keratins, members of the intermediate filament family, are characteristically expressed in epithelial cells. In the various types of epithelia, the keratin expression pattern is characterized by cell-type specific combinations of the keratin isotypes with a plain pattern in monolayered (simple) epithelia and more complex patterns in stratified and pseudostratified epithelia. Here we demonstrate that the transitional epithelium of the human urinary tract holds an exceptional position between the pseudostratified and stratified epithelia. We show that the simple epithelia keratins 7, 8, 18 and 19 are expressed throughout the whole epithelium as known from pseudostratified epithelia. In addition, we demonstrate expression of keratins 5, 14 and 17, otherwise present in basal cells of multilayered epithelia, and keratins 4 and 13, present in suprabasal areas of non cornified multilayered epithelia. Moreover, we report differences in expression in the various morphological parts of the urinary tract which might be related to their specific functions. Keratin 20, a typical component of the simple epithelia of the digestive tract, is present in bladder and ureter but not in the renal pelvis. Keratin 6, typical for stratified epithelia, is found only in parts of the renal pelvis. We further show that changes in keratin pattern occur during the development from embryonic to adult bladder urothelium. In contrast to adult tissue, the simple type keratins 7, 8 and 18 are not synthesized in basal embryonic cells. Further, keratin 20, present in cells facing the bladder lumen in adult urothelium, is expressed in all but the basal cells in embryonic bladder.