Intermediate zone

The intermediate zone of anal canal lies between the columnar zone and the cutaneous zone, acting as a transitional area.

Histological Characteristics

• Epithelium: The intermediate zone is lined by a non-keratinized stratified squamous epithelium, which is thicker than in the columnar zone but lacks the keratinization of the skin-like cutaneous zone.

• This epithelium represents a transitional epithelium, preparing for the external environment.

• It may contain basal cells and melanocytes, and show some variation in pigmentation, particularly in dogs and pigs.

Glandular Structures

• Unlike the columnar zone (which may have anal glands) and the cutaneous zone (which contains circumanal and sebaceous glands), the intermediate zone is typically poor in glands.

• Some species may have few small mucous glands, but this varies.

Species-Specific Notes

• Dogs: The intermediate zone is relatively narrow. It lies between the anorectal line and the anocutaneous line. It may show minor pigmentation. Dogs also have circumanal (hepatoid) glands, but these are more prominent in the cutaneous zone.

• Cats: Similar organization to dogs; however, the presence of glands and pigmentation varies slightly.

• Horses: The anal canal is relatively short, and the intermediate zone is well-defined with transitional epithelium.

• Ruminants: The zone may be less distinct but still shows transitional epithelium.

• Pigs: Often have a more prominent intermediate zone with pigmentation and a clear shift in epithelial type.

Functional Considerations

• This zone plays a protective role against the external environment while still allowing passage of feces.

• It provides structural integrity and flexibility, as it transitions between mucosal and cutaneous surfaces.

Clinical Relevance

• The intermediate zone is a common site for anal strictures, epithelial neoplasms, or trauma, especially due to its delicate epithelium.

• Surgical or pathological identification of this zone is important when considering perianal surgeries or biopsies.