Histogenesis of cloacogenic carcinoma. Fine structure of anal transitional epithelium and cloacogenic carcinoma.

The fine structural morphology of the anal transitional zone is described and is compared to the ultrastructure of urothelium and anal squamous epithelium. Electron microscopic observations in eight cases of cloacogenic carcinoma are also presented. Anal transitional epithelium is not highly specialized and incorporates features of both urothelium and squamous epithelium; slight urothelial differentiation is considered vestigial. Cloacogenic carcinoma is a specific morphologic entity, which can be distinguished from urothelial and squamous carcinoma. The "transitional" variant is composed of cells similar to the intermediate zone cells of anal transitional epithelium, an observation confirming the concept that the transitional form of cloacogenic carcinoma is derived from the anal transitional zone. The "basaloid" and pleomorphic variants appear to represent less differentiated forms of cloacogenic carcinoma.