Histology and histogenesis of two different types of inverted urothelial papillomas.

Inverted papillomas are rare tumors of the lower urinary tract. Among 1829 reclassified tumors of the urinary bladder, renal pelves, ureters, and urethra, there were 40 (2.2%) inverted papillomas. The great majority of these were localized in the bladder. Because of distinctive histologic features and patterns of growth it is possible to differentiate between two basic types of inverted papillomas which were termed "trabecular" and "glandular." The trabecular type consists of widely branched, anastomosing cords of urothelial cells originating directly from the overlying transitional epithelium. The trabeculae are arranged horizontally or perpendicularly to the surface epithelium and occasionally exhibit peripheral palisading of the cells. The glandular type is characterized by multiple round to oval islands of proliferated urothelial cells together with pseudoglandular and true glandular structures which are often still connected with the surface urothelium. The gland-like structures are lined by stratified urothelium, the true glands by mucus secreting columnar epithelium. Sometimes glandular metaplasia of an intestinal type with goblet cell formation could be observed. Inverted papillomas of the trabecular type arise histogenetically from a proliferation of the basal cells of the urothelium. The glandular type develops apparently from a proliferative cystitis cystica and glandularis which, therefore, should be considered a potentially preneoplastic lesion. The predominant view of the biological behavior of inverted papillomas is that of a benign neoplastic lesion. Morphologic findings supplied some arguments in favor of a low grade malignant potential of these tumors. However, their malignant transformation seemed to be much lower than that of exophytic papillomas.