Histologic Findings
Tumor of the follicular infundibulum is a histologic diagnosis. Most cases typically resemble the original description by Mehregan and Butler. [2] A platelike fenestrated subepidermal tumor extends horizontally under the epidermis with multiple cordlike connections to the overlying epidermis. Upon serial sectioning, connections between the pale-staining, glycogen-containing, tumoral keratinocytes and the external root sheath of adjacent hair follicles usually can be found. Peripheral palisading of the basal cells is present.
Peripheral palisading of basaloid cells frequently is observed. Pale staining results from the presence of glycogen analogous to the external root sheath and is confirmed by periodic acid-Schiff (PAS) stain with diastase digestion. A dense band or brushlike network of elastic fibers frequently is demonstrated at the border of the tumor, again analogous to the normal hair follicle. This can be observed readily using Verhoeff, van Gieson, or orcein stains.
A case report from 2001 noted foci of sebaceous differentiation within the fenestrated epithelium. [12]
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Tumor of the follicular infundibulum shows a platelike dermal tumor with anastomosing islands and cords with connections to the overlying epidermis and horn cysts (hematoxylin and eosin stain, 40X magnification).
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Tumor of the follicular infundibulum shows epidermal connections, horn cysts, and anastomosing islands (hematoxylin and eosin stain, 100X magnification).
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Tumor of the follicular infundibulum shows epidermal connections, peripheral palisading, and horn cyst (hematoxylin and eosin stain, 400X magnification).
