Anocutaneous junction

The anocutaneous junction is the boundary between the intermediate zone (non-keratinized stratified squamous epithelium) and the cutaneous zone (keratinized skin).

It corresponds to the distalmost limit of the anal canal proper, where the gastrointestinal tract ends and external perineal skin begins.

It is located just inside the anal opening, and is visible externally in some species as the anal verge.

Glandular Structures Around the Junction

• Circumanal glands (also called hepatoid glands): Found in the cutaneous zone, close to the anocutaneous junction, especially in dogs.

• These glands are modified sebaceous glands, and they are androgen-sensitive, making them clinically relevant in perianal tumors (particularly in intact males).

• The anal sacs (paranal sinuses) open into the anal canal near this region but are anatomically distinct, being located laterally or ventrolaterally in the cutaneous zone.

Species-Specific Notes

• Dog:

  • The junction is clearly demarcated.

  • The surrounding skin contains circumanal glands, which are prominent.

  • The anal sacs open just caudal to this junction.

• Cat:

  • Less prominent circumanal glands, but the junction is still distinguishable.

  • Anal sacs are present and open near this region.

• Horse, Ruminants, Pig:

  • The anocutaneous junction may not be as clearly defined externally, but histologically the transition is present.

  • Fewer glandular structures than in dogs.

Clinical Relevance

• The anocutaneous junction is a critical surgical landmark during:

  • Perianal surgery (e.g., tumor excision, fistula repair).

  • Anal sacculectomy (surgical removal of anal sacs).

• Perianal tumors (especially hepatoid tumors) often arise just adjacent to this junction.

• Anal furunculosis, common in German Shepherds, often affects this zone.