The anal transitional zone. A method for macroscopic demonstration.

The anal canal extends from the upper to the lower border of the internal sphincter. The canal can histologically be divided into three zones according to the epithelial lining, being of colo-rectal type in the upper part and squamous in the lower part, while the middle part, where the epithelium varies, is called the anal transitional zone (ATZ). This zone can be demonstrated macroscopically using whole mount staining with Alcian dyes, which reveal it as light green or blue, contrasting to the dark stained rectal type mucosa above and the unstained squamous epithelium below. Comparison with histological section shows that this zone corresponds with reasonable accuracy to the anal transitional zone.