Stromal-epithelial Interaction in a Neoplasm (“Trichodiscoma”):
Trichodiscoma has been characterized as a mesenchymal tumor arising from the “hair disc.” The lesion basically is a small myxoma which, in its superficial location in the dermis, presses upon epithelial structures. Characteristically, it is in contact with one or more follicles - if for no other reason than its expanse, as related to the spacing of follicles. In the designation, trichodiscoma, there is the implication that the myxoma is derived from a specific follicular component, the “hair disk.” Trichodiscoma as it enlarges, becomes large enough to encompass regularly spaced, preexisting follicles of the skin; follicular relationships could be simply incidental. The putative origin from the hair disk is an attractive hypothesis, but the name, trichodiscoma, does not provide a prompt understanding of what is intended. How many pathologists have searched for, and found, the hair disk? The cells of “trichodiscoma”are immunoreactive for CD-34; this attribute is cited as evidence favoring an origin from perifollicular mesenchyme. In one approach, perifollicular mesenchyme is a component of the adventitial dermis.
The patterns in the myxoid component of a “trichodiscoma” are non-specific; the lesion is loosely cellular, and the matrix is fibromyxoid. Mucinous pools may be represented. In contact with a follicle, the lesion may induce proliferative changes in which rounded islands of loosely cellular myxoid matrix are outlined by interconnected cords of small squamous cells. The basic patterns - not the matrical features - might be compared to those of a fibro-folliculoma. In the expression of these patterns, the lesion qualifies as a variant of a fibrofollicular hamartoma; like fibro-folliculoma, this myxoid lesion is encountered in the setting of the Bert-Hogg-Dube syndrome, but also sporadically.
In addition to the fibrofollicular pattern, areas of basal cell hyperplasia may be encountered along the constant epithelial components of the involved follicles, and focally along the interface with the epidermis; Some of the latter areas are not clearly associated, or in continuity, with follicular components (at least in the available histologic sections). In a less speculative approach, the lesion qualifies as a myxofibrofolliculoma with basaloid germinative cell hyperplasia; the hyperplasia of germinative cells is interpreted as evidence of abortive follicular neogenesis.
The histologic features of a “trichodiscoma” are illustrated in chapters 14 & 15. The role of the “hair disk” (F5Ch11)as the site of origin for trichodiscomas is discussed.