Adnexal tumors with hair follicle differentiationD23.L/C44.L

Heterogeneous group of tumours that originate from the hair follicle epithelium and mimic the cells of a developing embryonic hair germ. With a rather monomorphic and uncharacteristic clinical picture they show an extraordinarily large histological variability, which often makes their exact classification difficult. In addition, there are difficulties due to historically determined, different nomenclatures.

The classification presented here, which has been varied and extended according to Kazakov, is essentially based on the classification principle presented by Ackerman and colleagues.

• Tumours with predominantly follicular germ cell differentiation
  • Basal cell carcinoma
• Biphasic epithelial-mesenchymal tumors with differentiation towards follicular germ cells and the specific follicular stromas
  • Fibroepithelioma premalignes (Pinkus tumor)
  • Trichoblastom (with different variants)
  • Trichoblastic "carcinosarcoma
• Tumours with predominant matrix differentiation
  • Pilomatrixoma (previously: Epithelioma calcificans malherb)
  • Melanocytic matrixoma
  • Pilomatrix carcinoma
• Tumours with predominant differentiation towards the outer hair root sheath
  • Tricholemmoma
  • Tricholemmal carcinoma
  • Trichilemmal Keratosis
  • Tumor of the follicular infundibulum
  • Vaginal acanthoma
  • Proliferating trichilemmal tumor
• Tumours with predominantly infundibular differentiation
  • Trichoadenoma
  • Panfolliculoma
  • Dilated pore
  • comedonic nevus
• Tumours with predominantly panfollicular differentiation
  • Trichofolliculoma
    • sebaceous trichofolliculoma
    • Folliculosebaceous cystic hamartoma
  • Panfolliculoma
  • Fibrous papule (fibrous nasal papule)
• Follicular cysts
  • Infundibular cyst
  • Trichilemmal cyst
  • Hybrid cyst
  • Verrucous cyst
  • Vellus hair cysts
  • Milia
• Non-neoplastic or hamartomatous changes
  • Basaloid follicular hyperplasia (follicular tumorlet)
  • Hair follicle nevus
  • Ectopic hair follicles
  • Trichoepithelioma (previously: Epithelioma adenoides cysticum)
  • Tumor of the follicular infundibulum (infundibuloma).

Biphasic, epithelial-mesenchymal neoplasias comprise a group of follicular tumors that develop a double differentiation towards the follicular germinal epithelium and the specific follicular mesenchyma. The prototype of such a biphasic hair follicle tumor is the trichoblastoma. The premalignant fibroepithelioma (Pinkus tumor) also belongs to the group of biphasic epithelial mesenchymal neoplasias, although this tumor is usually listed among the variants of basal cell carcinoma.

Tumours with predominantly matrix differentiation mimic the structures of the normal follicular bulb. The prototype is the pilomatrixoma (previously: Epithelioma calcificans Malherbe) and a related melanocytic variant, the melanocytic matrixoma.

Cystic structures are often expressed during infundibular differentiation. In epithelial differentiation a stratum granulosum is expressed. A typical representative is the trichoadenoma.

Tumours with panfollicular differentiation express all parts of the hair follicle. An example of this is the trichofolliculoma.

In addition to the true follicular neoplasias, there is a heterogeneous group of hamartomas as well as cystic and non-cystic follicular hyperplasias.

1. Ackerman B et al (2001) Neoplasms with follicular differentiation. Ardor Scribendi, New York
2. Headington JT (1976) Tumors of the hair follicle. A review. On J Pathol 85: 481-503
3. Kaddu S et al (2003) Adnexal tumors with hair follicle differentiation. In: Kerl H et al (ed.) Histopathology of the skin. Springer, Berlin Heidelberg New York, S. 651-669
4. Kazakov d (2016) Adnexal tumors with sebaceous gland and hair follicle differentiation. In: Cerroni L et al (eds.) Histopathology of the skin. Springer, Berlin Heidelberg New York, S 670-671