To the Editor:
Follicular hybrid cyst (HC) is a rare cutaneous cyst that comprises 2 or more epithelial components, arising from separate regions of the pilosebaceous unit.1,2 By definition, the wall of such cyst is composed of 2 or more epithelial zones, set side by side but showing distinct types of follicular differentiation.1,2 Theoretically, any combination of hair follicle components may be seen in a follicular HC.1,2 In practice, however, the association of infundibular and trichilemmal epithelium is the pattern observed most frequently.1,2 More rarely, hybrid follicular cysts containing either infundibular and pilomatrical or isthmic-catagen and pilomatrical components also occur.1–3 Herein, we describe a follicular HC intimately associated with a syringocystadenoma papilliferum (SCAP), a benign adnexal tumor of probable apocrine origin.4 The epithelial lining of this HC exhibited trichilemmal keratinization and “shadow” or “ghost” cells (GC), indicating a combination of isthmic-catagen and pilomatrical differentiations.1,2
The patient, a 47-year-old man of Italian extraction, progressively developed a cystic lesion in the neck area that clinically resembled infundibular cyst. He mentioned episodes of inflammation and some mechanical attempts to drain the content of the cyst. The lesion was surgically excised for diagnostic and therapeutic purposes. The cyst measured 0.7 cm width. It was located within the dermis and did not demonstrate any connection to the overlying epidermis. This unicystic lesion comprised an otherwise typical SCAP and a verrucous structure protruding into the lumen of the cyst, like a cutaneous horn (Fig. 1A). Both verrucous structure and SCAP were merged (Fig. 1A). The lumen of the cyst contained cellular debris and neutrophils but no lamellar keratin. The verrucous outgrowth was composed of a squamous epithelium that showed acanthosis and papillomatosis. The basal cell layer exhibited some palisade arrangement (Fig. 1B). The squamous cell layer was thick and contained keratinocytes with eosinophilic cytoplasm (Fig. 1B). No distinct granular cell layer could be demonstrated (Fig. 1B). Strikingly, the most superficial portion of this verrucous structure was composed of a column of pale and eosinophilic cells resembling GC of pilomatricoma (Fig. 1B). No mitotic activity, cell atypia, koilocytosis, dyskeratosis, or parakeratosis could be noted (Fig. 1B). Cells of this verrucous outgrowth strongly expressed keratins 5/6, 14, and 17 (Fig. 1C). By contrast, immunohistochemical studies using antibodies directed against keratin 7 were negative. Beta catenin was expressed in the cell membranes of keratinocytes within the squamous layer, but no nuclear expression could be documented. Notably, GC did not express this protein. Morphologically, SCAP was composed of true papillae filled with plasma cells (Figs. 1A, D). Cells of SCAP exhibited apocrine-like features (Fig. 1D). They were positive for keratin 7 and, to a lesser extent, for keratin 17 (Fig. 1C). Based on these findings, a diagnosis of follicular HC with isthmic-catagen, pilomatrical, and SCAP components was rendered.
In our opinion, the pathological findings in this case indicate distinct but contiguous lesions originating from the same hair follicle, in a zone extending from the upper portion of the isthmus to the lowest part of the infundibulum (infrainfundibulum). Indeed, keratin 17 is normally expressed in the outer root sheath of hair follicle and the infrainfundibulum.5,6 In pilomatricoma, its expression is restricted to areas with infundibular-type or trichilemmal keratinization-type epithelium.6 By contrast, this keratin is not expressed in the infundibulum.6 Therefore, the strong expression of keratin 17 by the verrucous outgrowth in this case may be a clue to its origin from the outer root sheath or the infrainfundibulum. Interestingly, this immunohistochemical finding is also consistent with the fact that squamous epithelium associated with SCAP normally express keratin 17, a characteristic suggesting that SCAP may differentiate, at least partially, toward the infrainfundibulum.5,6
The presence of GC is not restricted to pilomatricoma.7 Indeed, GC may be noted in various conditions such as alopecia areata, mixed tumors, adnexal tumors with follicular differentiation, squamous cell carcinoma, or proliferating trichilemmal tumors.7,8 Moreover, these peculiar cells have also been demonstrated in follicular HC with infundibular, isthmic-catagen, and pilomatrical differentiation.2 According to Jacobson and Ackerman,7 the presence of such cells within a cutaneous lesion should be considered as a clue to follicular differentiation.
The origin of SCAP still remains a matter of debate.4 The frequent connection to the infundibulum of hair follicle supports an apocrine origin.4 In addition, the occasional presence of features suggestive of human papillomavirus (HPV) infection in tubular adenoma,4 a tumor-like condition closely related to SCAP, and the sporadic association of SCAP and HPV-induced verrucous lesions, including verrucous cysts,9 have led some authors to consider tubular adenoma and SCAP as HPV-related disorders or alternatively, as reactive and nonneoplastic processes that develop in response to HPV.4,9 Interestingly, a diagnosis of verrucous cyst associated with SCAP was initially considered in our patient because of the verrucous appearance of the epithelial lining. However, the absence of other characteristics indicative of HPV infection such as hypergranulosis with large and irregular granules, koilocytic changes, raised mitotic activity, cell atypia, dyskeratosis or parakeratosis speaks against such diagnosis.
In summary, our findings illustrate further the spectrum of epithelial combinations that can be seen in follicular HC. They also underscore the relationship between SCAP and the infrainfundibulum, as previously suggested by Kurokawa et al.5
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