ABSTRACT
The lower urinary tract of experimental animals has been actively studied since the search began for etiological factors in bladder carcinogenesis following the report in 1895 by Rehn that workers in the German aniline dye industry developed bladder cancer. The mucosa of the lower urinary tract has been referred to as a transitional cell epithelium or more recently as an urothelium. In the urinary bladder, it consists of three cell layers, although frequently in light microscopic sections the epithelium appears to be only one or two cells thick. However, in the trigone region there is occasionally four to five cell layers due to an increase in the number of intermediate cells. This distends the bladder mucosa, eliminating the artifactual folds of the normal mucosa which can be difficult to distinguish from early papillary lesions. The bladder mucosa may be extensively involved with these lesions and is then referred to as papillomatosis. Generally, papillomas are large enough to be seen macroscopically.
