Anal columns

Anal columns are longitudinal ridges of mucosa found in the upper portion of the anal canal, specifically within the columnar zone, just distal to the anorectal junction. Each column is formed by a core of connective tissue and smooth muscle, and sometimes contains a small blood vessel. These columns typically terminate at the level of the anocutaneous or pectinate line.

Between adjacent columns are small grooves or recesses known as anal sinuses, which may receive the openings of anal gland ducts in some species.

Structure and histology

• Epithelium The surface of the anal columns is lined by simple columnar epithelium, often with goblet cells that secrete mucus.

• Core contents Each column has a core made of loose connective tissue. Small blood vessels, lymphatics, and nerve endings may be present within the core. In some species, these blood vessels can form small venous plexuses.

• Associated structures At the base of the anal columns, the anal sinuses are found. These are shallow depressions or pockets that may trap mucus or debris and can be the site of inflammation or infection. In dogs and some other species, anal gland ducts open into these sinuses, contributing to lubrication or scent marking.

Species-specific considerations

Dog and cat Anal columns are well developed and clearly visible in these species. The anal sinuses between them receive the ducts of anal sacs (also called paranal sinuses), which are common sites for clinical problems such as impaction or infection. This region is significant in surgical procedures and in the diagnosis of perianal diseases.

Horse The anal columns are not well defined. The transition from rectum to anal canal is smoother, and distinct mucosal folds are generally absent or minimal. Anal sinuses and glands are also poorly developed.

Ruminants (cattle, sheep, goats) Anal columns are either absent or very poorly defined. The transition from columnar to squamous epithelium occurs over a short distance without distinct folds or sinuses.

Pig Anal columns are present but not as prominent as in dogs and cats. The mucosa may show some degree of longitudinal folding, and glandular structures can be observed near this region.

Functional significance

• The anal columns allow for some expansion of the anal canal during the passage of feces.

• Goblet cells in the columnar epithelium contribute mucus, which helps lubricate the canal.

• In species where anal glands open into the anal sinuses, the columns and sinuses play a role in secretion and scent marking.

• The vascular structures within the columns may assist in maintaining closure of the canal through a cushion effect.

Clinical relevance

In carnivores, the anal columns and their associated sinuses are prone to several disorders:

• Anal sac impaction: Blockage of the ducts that open into the sinuses.

• Infection or abscessation: Occurs when trapped material becomes secondarily infected.

• Perianal fistulae: May involve the sinuses and require careful surgical intervention.

• Neoplasia: Perianal gland tumors can occur near the openings of the sinuses.