From Dog

Tricholemmoma (Trichilemmoma) is a benign neoplasm of canine keratinocytes that involve the outer root sheath of hair follicles[1].

Tricholemmomas are neoplasms of older dogs (5 - 13 years) with characteristic upregulation of CD34 expression with some functionally similar conditions to the terminal hair follicles in the anagen phase[2]. The cause of this neoplastic transformation is unknown, although in humans has an hereditary origin.

Clear cell squamous cell carcinomas histopathologically mimic or are difficult to distinguish from tricholemmal carcinoma[3].

The Afghan Hound breed appears predisposed and these tumors appear commonly on the head and neck as firm, ovoid and lobulated masses without involvement of deeper subcutaneous tissue.

Diagnosis is based on histological examination of biopsied material, with histological features involving positive PAS staining of keratinocytes due to their glycogen content. These tumors are often surrounded by thickened basement membranes[4].

Treatment is usually curative with wide-margin excision.


  1. Abramo F et al (1999) Survey of canine and feline follicular tumours and tumour-like lesions in central Italy. J Small Anim Pract 40(10):479-481
  2. Misago N et al (2011) CD34 expression in human hair follicles and tricholemmoma: a comprehensive study. J Cutan Pathol 38(8):609-615
  3. Misago N et al (2012) Tricholemmoma and clear cell squamous cell carcinoma (associated with Bowen's disease): immunohistochemical profile in comparison to normal hair follicles. Am J Dermatopathol 34(4):394-399
  4. Scott, DW et al (2001) Muller & Kirk's small animal dermatology. 6th edn. Saunders. San Diego, CA