More than 90% of bladder carcinomas are transitional cell carcinomas derived from the uroepithelium. About 6% to 8% are squamous cell carcinomas, and 2% are adenocarcinomas.[1] Adenocarcinomas may be either of urachal origin or of nonurachal origin; the latter type is generally thought to arise from metaplasia of chronically irritated transitional epithelium.[2] Pathologic grade, which is based on cellular atypia, nuclear abnormalities, and the number of mitotic figures is of great prognostic importance.

References

1. Mostofi FK, Davis CJ, Sesterhenn IA: Pathology of tumors of the urinary tract. In: Skinner DG, Lieskovsky G, eds.: Diagnosis and Management of Genitourinary Cancer. Philadelphia, Pa: WB Saunders, 1988, pp 83-117. 
   
   
2. Wilson TG, Pritchett TR, Lieskovsky G, et al.: Primary adenocarcinoma of bladder. Urology 38 (3): 223-6, 1991.  [PUBMED Abstract]