FOLLICULAR LESION HAVING COMBINED FEATURES OF PILAR SHEATH ACANTHOMA AND TRICHOFOLLICULOMA (TRICHOGENIC PILAR SHEATH ACANTHOMA?)
The distinction between follicular lesions or tumors composed of outer root sheath type epithelium depends upon the architecture of the lesion. Such lesions or tumors include trichilemmoma, pilar sheath acanthoma (small trichogenic focus in this particular one), and the tumor of follicular infundibulum. Other follicular epithelial and/or specialized stromal components are seldom demonstrated.

Trichofolliculomas, on the other hand, have one distinctive type of architecture characterized by a central lumen lined by squamous (infundibular?) epithelium from which radiate fairly well-formed proximal (trichogenic) follicular subunits. The trichogenic subunits are prototypically associated with moderately cellular fibroblastic stroma similar to that seen in traditional trichoepitheliomas.

The case below has architecture of the type found in pilar sheath acanthomas as well as in trichofolliculomas. Both elements are represented. If one wished to add another 'entitity' to the list of  benign follicular lesions, one could propose trichogenic pilar sheath acanthoma as the term.


 
Scan power view showing follicular structures radiating into (or out of) a central lumen that is lined by squamous epithelium. A trichogenic area is in the black rectangle. Buds of hyperplastic outer root sheath type epithelium also radiate from the wall of the lumen.
Low power view of the area in the black rectangle shown above. An imperfect hair shaft (Trich) is present as well as a properly formed hair shaft (HS). There is an area of hyperplasia of outer root sheath type  epithelium (ORS) which distinguishes this from a normal follicle that happened to be included in the lesion..
A high power view of the hair shaft shown above.
A low power view of hyperplastic outer root sheath epithelium that radiates from the lumen. A squamous eddy is present, and there is hypergranulosis at the luminal junction. Such hypergranulosis is occasionally seen in infundibular epithelium and is not presumptive evidence of papilloma virus infection.  This picture could be interpreted as showing the junction of hyperplastic outer root sheath epithelium with infundibular epithelium.
A medium power view showing peripheral palisading, a slightly thickened basement membrane, minimal clear cell differentiation centrally, and a few necrotic or apoptotic keratinocytes. This combination is typical of tumors having outer root sheath differentiation.

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