- The anus forms the last few centimeters of the GI Tract and represents a histological transition zone between the architecture of the GI Tract and that of normal skin.
- Epithelial Transition
- The simple columnar epithelium of the rectum abruptly transitions to a stratified squamous epithelium. This abrupt histological transition is very similar to that occurring in the cervix and indeed these two areas are prone to the same infectious diseases, especially with Human Papilloma Virus.
- Muscular Transition
- The inner anus possesses a thick ring of smooth muscle cells continuous with the rectal muscularis propria, forming the involuntary internal anal sphincter. This layer of muscle transitions to that of skeletal muscle cells which forms the voluntary external anal sphincter.