The anal transitional zone

The macroscopical appearance of the human anal canal was first described by Glisson (1597-1677) and Morgagni (1717), who mentioned the anal valves and anal columns, respectively. The first detailed light microscopic description of the three anal canal zones originates from Robin & Cadiat (1874). The present definition of the anal canal, extending from "the pelvic floor to the anal opening" was suggested by Symington (1888). There are no generally accepted names for the three epithelial zones of the anal canal. A review of the literature shows that no less than three different names have been used for the upper zone, 14 for the middle zone and 9 for the lower zone, as well as 10 names for the line comprising the anal valves and the base of the anal columns. In the present work the middle zone is termed the anal transitional zone (ATZ), and is defined as "The zone interposed between uninterrupted colo-rectal type mucosa above and uninterrupted squamous epithelium below, irrespective of the type of epithelium present in the zone itself". The line corresponding to the anal valves and anal sinuses is termed the dentate line (DL), as this name seems to be employed in more common textbooks. The location and extent of the ATZ has previously been measured by light microscopy on a small number of sections from a few anal canals. In the present work the extent of the zone has been elucidated from a large material, where macroscopic demonstration of the zone has been carried out by means of staining with Alcian dyes on fixed surgical specimens, by stereomicroscopy and finally by light microscopic control of both methods, achieved by systematic sectioning of the specimens. The results have shown that normally the ATZ reaches from the DL and almost 1 cm upward, but that it can be observed over a considerably larger area than previously reported, namely from 0.6 cm below to 2.0 cm above the DL, and eventually may be absent altogether. Further, it has been shown that the ATZ frequently has a map-like appearance. Light microscopically, the major part of the ATZ consists of a characteristic epithelium, which is provisionally termed the ATZ-epithelium. This appears to be composed of 5-9 cells layers. The surface cells can be columnar, cuboidal or somewhat more flattened. In the first case, signs are often seen of some mucin-production.(ABSTRACT TRUNCATED AT 400 WORDS)