The anal transitional zone: a scanning and transmission electron microscopic investigation of the surface epithelium.

The ultrastructure of the anal transitional zone (ATZ) is described by means of scanning and transmission electron microscopy. The technique used permits histologic reexamination of the scanning electron microscopic biopsies. The surface of the ATZ shows, in addition to areas of normal colorectal mucosa and squamous epithelium, a characteristic picture: there are cells of varying size, arranged in a cobblestone pattern, and a surface covered with short microvilli that tend to form rows, indicating beginning microridge formation. This corresponds histologically to the so-called ATZ epithelium, and it is suggested that this might be metaplastic squamous epithelium rather than urothelium. The deepest part of the epithelium contains endocrinelike cells with granules of at least two types.