Columnar zone

The columnar zone is the uppermost part of the anal canal, situated immediately below the anorectal junction. It is named for its lining by simple columnar epithelium and the presence of mucosal folds called anal columns. These folds are arranged longitudinally and contain a vascular core. This zone extends from the termination of the rectum to a landmark called the anocutaneous junction ( anocutaneous line; pectinate line), where the epithelium transitions to a stratified squamous type.

Histological characteristics

• Epithelium: The columnar zone is lined by simple columnar epithelium, similar to the rectum. Goblet cells are also present, contributing mucus for lubrication. As the anal canal progresses distally, the epithelium changes to non-keratinized stratified squamous epithelium in the intermediate zone, and then to keratinized stratified squamous epithelium at the perianal skin.

• Anal columns and anal sinuses: The mucosa forms vertical ridges called anal columns. Between these columns are small depressions known as anal sinuses. In some species, ducts of anal glands open into these sinuses.

• Glandular structures: Mucous glands are found in this region and assist in lubrication. In species like the dog, anal glands may open into the anal sinuses at this level.

Species-specific features

Dog and cat The columnar zone is well-developed. Anal columns and sinuses are prominent. Anal sac ducts often open at the level of the anal sinuses. This area is clinically important due to conditions like anal sacculitis, perianal fistulae, and glandular tumors.

Horse The columnar zone is present but less prominent than in dogs. The mucosa is relatively smooth with minimal glandular development.

Cattle, sheep, and goats The columnar zone is short. The transition to squamous epithelium is gradual, and the anal canal is less distinctly divided into zones. Anal glands are generally poorly developed.

Pig The columnar zone is clearly distinguishable. The anal canal in pigs is long and has distinct epithelial and glandular zones. Mucous glands are more abundant.

Functional significance

• Acts as a transitional region from the rectal mucosa to the skin-like epithelium of the anal canal

• Provides mucus for lubrication during defecation

• Contains vascular and glandular elements that are prone to inflammation, impaction, or neoplastic changes in certain species

Clinical importance

In small animals, especially dogs, this zone is the site of entry for anal sac ducts. Diseases of the anal sacs (such as impaction or infection) often involve this region. Surgical procedures around the anal canal must consider the position of the columnar zone to avoid damaging important glandular or vascular structures.