Structure/Morphology
The mucosa of the anal canal forms the innermost layer of the wall of the anal canal.
The mucosa in the upper third of the anal canal is lined with simple columnar epithelial cells, like that in the rectum. It also contains secretory and absorptive cells and numerous glands (crypts). The glands secrete mucus onto the luminal surface of the anal canal. In the middle third of the anal canal, known as the anal transition zone, the epithelium transitions to a more stratified columnar epithelium and a stratified squamous epithelium. The inferior third of the anal canal, known as the squamous one, the epithelium is a nonkeratinized, stratified squamous epithelium. This becomes continuous with the perianal skin (Ross and Pawlina, 2006).
Related parts of the anatomy
Key Features/Anatomical Relations
The mucosal epithelium is in direct contact with the underlying lamina propria (loose connective tissue) and the muscularis mucosa, a thin layer of smooth muscle that marks the boundary with the submucosa.
In the middle part of the anal canal, there are 6–10 vertical mucosal folds called the anal columns. They frequently contain the terminal branches of the superior anorectal vessels. The distal ends of the anal columns form crescentic folds called the anal valves, above which are depressions called the anal sinuses. Together, the sinuses and valves form the pectinate (or dentate) line. When compressed by feces, the anal sinuses emit mucus that helps move feces from the anal canal.
References
Ross, M. H. and Pawlina, W. (2006) Histology: A text and atlas. Lippincott Williams & Wilkins.
