Corpus: Anal canal

The anal canal, about 3 to 4 cm long, is the lumen of the anus lined with mucous membrane. It extends from the upper edge of the sphincter ani internus muscle to the linea anocutanea. When unfilled, the anal canal has a slit-shaped cross-section, the longitudinal axis of which runs from ventral to dorsal. The ventral and dorsal lines of the anal canal are known as the anterior and posterior commissure respectively. The outer boundary of the anal canal is the anocutaneous line.

Embryologically, the anal canal is of ectodermal origin and develops from the anus.

The anal canal connects the rectum with the outside world and begins at the linea anorectalis, which represents the boundary between the colonic mucosa of the rectum and the irregular mucosa of the anal canal.

Histologically, the anal canal presents a very varied picture and can be divided into several sections, although the classification in the anatomical literature is inconsistent. This is aggravated by the fact that clinically different names and orientation markers are sometimes used than anatomically.

Zona columnaris[Bearbeiten]

Synonyms: transitional zone, transitional zone, zona transitionalis

The zona columnaris characterises the upper 2/3 of the anal canal. It extends from the linea anorectalis (junctio anorectalis) to the linea dentata, which runs in serpentine lines. This section contains the anal columnae running in 6 to 10 longitudinal folds, which are raised by the corpus cavernosum recti (haemorrhoidal plexus) located in the submucosa.

The zona columnaris, also known as the transitional zone, has a transitional epithelium that lies between the endodermal mucosa of the rectum and the anoderm. The anal papillae and anal crypts are located in the transition zone. The latter contain the orifices of the anal glands, which are lined with transitional or cylindrical epithelium.

Zona alba[Bearbeiten]

Synonyms: Zone squamosa, Pecten analis, anoderm, Hilton zone

The zona alba of the anal canal runs from the linea dentata to the linea anocutanea (Hilton line). It is lined by several layers of uncornified squamous epithelium, the whitish colour of which gives this section its name. In the lowest area, the tissue merges into keratinised squamous epithelium without a clear boundary. The zona alba is also known as the anoderm or pecten analis and is of ectodermal origin. However, it has no skin appendages, in particular no hair. It is supplied with sensation by the inferior rectal nerves.

Zona cutanea[Bearbeiten]

The zona cutanea or perianal skin begins at the linea anocutanea and forms the connection to the skin of the perianal region. It is highly conspicuous and pigmented. In its marginal area, it has more hair as well as sweat and sebaceous glands.

Other zones[Bearbeiten]

In addition to the 3 zones mentioned above, a zona colorectalis is also histologically defined in the oral section of the anal canal, which lies between the anorectal junction and a supratransitional line at the oral end of the zona columnaris. Its histology corresponds to that of the rectal mucosa with a single-layered, highly prismatic epithelium.

• Image source for podcast: © Jacob Rice / Unsplash