TRICHOEPITHELIOMA
This is the tumor that is traditionally designated as trichoepithelioma. It is composed of epithelial subunits, and each epithelial subunit is surrounded by moderately cellular fibroblastic stroma. Immature differentiation of several components of the follicle is present, and these components are arranged as in a foreshortened follicular structure. Two and sometimes three follicular components are represented, though not necessarily in the same epithelial subunit. These include: One or more of the subunits may be connected to the epidermis. The subunits with their mesenchymal investments are often in juxtaposition, particularly in the superficial part of the tumor. They may appear to be separated by reticular dermis collagen bundles, particularly in the deep aspect of the tumor. The architectural variant, trichoblastic fibroma, is a sharply circumscribed, single stromal nodule that contains trichoepithelioma-like or trichoblastoma-like subunits.

Tumors that have benign profiles but which are lacking one or more of these subunits (particularly the microcysts) are sometimes designated as trichoblastomas. This is at variance with Headington's definition of trichoblastoma (no special stroma should be present).

It should be noted that Ackerman, et alref 1 use trichoblastoma as a global term to include the traditional trichoepithelioma as well as the traditional trichoblastoma, desmoplastic trichoepithelioma, and trichoblastic fibroma. 


From the face of a 22 year old male: Three scanning power views of the same tumor plus higher power views are presented.
 
Scan  #1 showing an epidermal connection (EC) by the tumor. Microcysts are present and one of these has an angulated lumen (AL). Multiple epithelial subunits are intimately associated with fibroblastic stroma. These epithelial-mesenchymal structures are in apposition in some foci and separated by reticular dermis collagen in others. There are holes in the dermis that are artifactual tears in the tissue. 
A composite low power view of Scan #1 (above).  Subunit #1 (SU1) connects the epidermis to a microcyst. The epithelial-mesenchymal relationship is nicely illustrated by subunit #2 (SU2). SU2 has interconnecting strands of basaloid epithelium radiating from the lumen in a cribriform pattern. Other patterns of proliferation are also seen in this picture. 
Scan  #2. Some of the epithelial subunits and associated stroma, particularly those in the deep dermis, are separated by normal reticular dermis.
Medium power view from the black rectangle in Scan #2 (above). Specialized connective tissue forms a dermal hair papilla (HP). A few apoptotic cells (AP) of the type found in telogen phase follicles are barely visible. Greatly foreshortened follicular differentiation is seen. Note the investment of moderately cellular fibroblastic tissue intimately associated with the epithelial structure. Note also the normal reticular dermis in the lower left side of the picture.
Scan  #3. Foreshortened follicular structures are seen in the upper part of the picture. Clefts are sometimes formed between the stromal components of the subunits. Note the separation of the epithelial-mesenchymal structures  by reticular dermis collagen in the deep part of the lesion.
Medium power view of the area in the black rectangle in Scan #3 (above).

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