Surgical Treatment and Pathology: Normal Histology

Abstract

There are a number of commonly used definitions of the extent of the anal canal. Perhaps the most widely accepted definition is that the anal canal starts at the pelvic floor and finishes at the anal opening. Surgeons usually define the upper limit of the anal canal at the level of the levator ani muscle (at the anorectal angle) and the lower limit at the anal orifice. The surgical anal canal varies from 3.0 to 5.3 cm in length and is on average slightly longer in the female. Pathologists typically define the anal canal as the area lying between the upper and lower borders of the internal anal sphincter whilst anatomists define the anal canal as lying between the dentate line and the anal verge. The anal canal can also be defined histologically based on the types of specialised mucosa present in the anal region (anal transitional zone and anal squamous zone). The surgical and pathological definitions of the anal canal extend more proximally than the anatomical/histological definitions. This results in the surgical and pathological anal canals including a ring of colorectal mucosa at their upper borders which is not present in the anatomical/histological anal canals. This variation in classification leads to some overlap and confusion in terminology particularly in respect to the nomenclature of pathology in the upper anus (e.g. rectal vs anal tonsil, lower rectum vs anal colorectal zone/anal cuff). For classification and staging of anal pathology, both the WHO and TNM systems use the surgical definition of the anal canal.