S100A1 expression characterizes terminally differentiated superficial cells in the urothelium of the murine bladder and ureter.

The urothelium is a stratified epithelium that lines the inner surface of the components of the urinary drainage system. It is composed of a layer of basal cells, one or several layers of intermediate cells, and a layer of large luminal superficial or umbrella cells. In the mouse, only a small set of markers is available that allows easy molecular distinction of these urothelial cell types. Here, we analyzed expression of S100A1, a member of the S100 family of calcium-binding proteins, in the urothelium of the two major organs of the murine urinary tract, the ureter and the bladder. Using RNA in situ hybridization analysis, we found exclusive expression of S100a1 mRNA in luminal cells of the ureter from embryonic day (E)17.5 onwards and of the bladder from E15.5 to adulthood. Immunofluorescence analysis showed that expression of S100A1 protein is confined to terminally differentiated superficial cells of both the ureter and bladder where it localized to the nucleus and cytoplasm. We conclude that S100A1 is a suitable marker for mature superficial cells in the urothelial lining of the drainage system of the developing and mature mouse.