As originally described by Headington these are very rare tumours that tend to be larger and more deeply located than trichoepithelioma. They are formed of varying combinations of basaloid germ cells and follicular mesenchyme with varying degrees of primitive hair bulb formation (stromal induction).

Trichoblastomas are not aggressive unless they contain an element of basal cell carcinoma.

Clinical presentation: Presents as a well circumscribed, solitary lesion (more than 1cm in diameter).

Site:  Head, particularly the scalp is a common site.

Microscopic features:

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Large circumscribed basaloid tumour ; No epidermal connection ; Located in mid & lower dermis ;  May extend to the subcutis ; Nests of basaloid cells ; Variable stromal condensation & pilar differentiation ; Primitive hair bulbs (stromal induction) ; Apocrine & sebaceous differentiation ; Stromal amyloid & merkel cells.

Variants:

Clear cell ;  Pigmented ; Adamantinoid ; Rippled-pattern. Rippled-pattern trichoblastoma is characterized by peculiar arrangement of the basaloid cells in linear rows parallel to one another similar to the palisading of nuclei of Verocay bodies seen in schwannomas. 

Cutaneous lymadenoma is an adamantinoid variant of trichoblastoma.

Dependent on the degree of differentiation toward other follicular components and the relative prominence of stromal and epithelial elements this group was classically subdivided into trichoblastoma nos, trichoblastic fibroma  and trichogenic trichoblastoma. 

Trichoblastic fibroma include both germ and fibrous stroma.
Trichogenic trichoblastoma most mature form and include complete hair follicle.

Differential diagnosis: 

Basal cell carcinoma;Classic trichoepithelioma (more keratocysts are present; distinction is very difficult)             

High-grade trichoblastic carcinoma arising in trichoblastoma : a rare adnexal neoplasm often showing metastatic spread. Am J Dermatopathol. 2005; 27(1):9-16.

Fibroepithelioma of pinkus is a fenestrated trichoblastoma.Am J Dermatopathol. 2005 Apr;27(2):149-54.

Trichoblastoma with apocrine and sebaceous differentiation.Am J Dermatopathol. 2005;27(1):6-8.

Androgen receptor expression helps to differentiate basal cell carcinoma from benign trichoblastic tumors.Am J Dermatopathol. 2005;27(2):91-5

Vulvar Trichogenic Tumors: A Comparative Study With Vulvar Basal Cell Carcinoma. Am J Surg Pathol. 2005 ;29 (4):479-484.

Low-grade trichoblastic carcinosarcoma of the skin. Am J Dermatopathol. 2004;26(4):304-9

Trichoblastic neoplasm with apocrine differentiation.Am J Dermatopathol 2002;24(4):358-60

Melanotrichoblastoma: immunohistochemical study of a variant of pigmented trichoblastoma. Am J Dermatopathol 2002 Dec;24(6):498-501

Trichoblastoma arising within an apocrine poroma.Am J Dermatopathol 2002 Feb;24(1):59-62

Clear Cell Trichoblastoma in Association With A Nevus Sebaceus . Am J Dermatopathol 2001;23:143-145

Sebaceous carcinoma, trichoblastoma, and sebaceoma with features of trichoblastoma in nevus sebaceus.Am J Dermatopathol. 2001 Oct;23(5):456-62.

Cytokeratins as Markers of Follicular Differentiation. An Immunohistochemical Study of Trichoblastoma and Basal Cell Carcinoma . Am J Dermatopathol 2001;23:501-509

Trichoblastoma of the skin occurring in the breast. A case report.Acta Cytol. 2001 May-Jun;45(3):435-40.

Rippled-pattern sebaceous trichoblastoma. J Cutan Pathol 2000;27(9):455-9

A rippled-pattern trichoblastoma: an immunohistochemical study. J Cutan Pathol 2000;27(9):460-5

Trichoblastic carcinoma ("malignant trichoblastoma") with lymphatic and hematogenous metastases. Mod Pathol 2000 ;13(6):673-8

Trichoblastoma is the most common neoplasm developed in nevus sebaceus of Jadassohn:a clinicopathologic study of a series of 155 cases. Am J Dermatopathol 2000;22(2) :108-18

Cytokeratin expression in trichoblastic fibroma (small nodular type trichoblastoma), trichoepithelioma and basal cell carcinoma. Br J Dermatol 1999;140(1):8-16

Immunohistochemical comparison of cutaneous lymphadenoma, trichoblastoma, and basal cell carcinoma: support for classification of lymphadenoma as a variant of trichoblastoma. J Cutan Pathol 1999 Mar;26(3):119-24.

Nodular desmoplastic variant of trichoblastoma. Am J Surg Pathol 1994;18(5):495-500

Giant trichoblastoma on the scalp. Am J Dermatopathol 1993;15(5):497-502

Pigmented trichoblastoma. Am J Dermatopathol 1992;14(4) : 345-9

Hair Germ Differentiation:

Trichoepithelioma 

Desmoplastic Trichoepithelioma

Trichofolliculoma

Trichoblastoma

Cutaneous lymphadenoma

                               
Infundibular differentiation:

Trichoadenoma

Dilated Pore of Winer

Pilar Sheath Acanthoma

Tumour of Follicular Infundibulum

Outer root sheath differentiation:
                    
Trichilemmoma

Trichilemmal Carcinoma

Proliferating Trichilemmal Cyst (Pilar Tumour)

Matrical differentiation:
                  
Pilomatrixoma and Pilomatrix Carcinoma