Reinvestigation of the transitional epithelium (urothelium) of the human ureter.

The transitional epithelia (urothelia) of the ureters of 30 patients of different ages were studied by means of light and electron microscopical, histochemical and immunocytochemical methods. A great variability of the normal structural appearance of the urothelium was established. Structural features and the uptake of exogenous peroxidase by the surface epithelial cells provided high endocytotic activity. Urothelial cells take up many low and high molecular weight substances from the urine and further metabolize and transport these toward the subepithelial connective tissue. Lymphocytes, macrophages, monocytes, plasma cells and rarely polymorphonuclear leukocytes are distributed intra- and extraepithelially and are involved in the immunological response to agents which enter the intercellular spaces of the epithelium. These cells are also responsible for the elimination of aged and degenerating superficial squamous cells. The present investigation establishes the existence of immunological defence mechanisms in the adult human urothelium. The results obtained suggest that the human ureter contains three functional barriers directed against aggressive components of the urine: the first represented by structures of the superficial squamous cells, the second by the upper cells of the ureteric intermediate layer and the third comprising epithelial and immunological cells involved in immune defence mechanisms.