The anal transitional zone. Location and extent.

The location and extent of the anal transitional zone (ATZ) were investigated in the age group typical for anal canal carcinomas. The methods used were macroscopic determination after whole-mount staining with Alcian-dyes as well as conventional histological technique. The results show that the epithelial variants may be found over a larger area than previously reported, namely from 6 mm below to 20 mm above the dentate line. Variations in location and extent of the ATZ are described, as well as the frequent finding of mature squamous epithelium high in the anal canal. The significance of the findings in relation to the special types of anal canal carcinomas is discussed, and on the basis of the macroscopic definition of the canal, as well as the histological observations in this study, it is proposed that anal canal carcinomas should be defined as tumours partly or totally located within a distance of 2 cm above the dentate line.