Ch39aP31a Tricholemmoma 
(Lymphoepithelial Change)

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F1Ch39a (K1493): The common features of follicular hamartomas of the butterfly area of the face are represented; they include some degree of papillomatosis, focal parakeratosis, and a dual population of cells (i.e., representation of a superficial unit-like component and some pattern of a basal unit-like component). This lesion is distinguished by a prominent component of vacuolated basal cells; this feature would lead the pathologist to a diagnosis of ‘tricholemmoma.” Immature (miniaturized) follicles are present in the underlying dermis.

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F2Ch39a: Two distinctive populations of cells are represented. To the left and above, the epithelium is of the superficial unit type (a population committed to terminal differentiation). There is one whorl of terminally differentiating cells with a small central zone of keratinization - this pattern can be taken as an abortive expression of an attempt to form a lumen. Below the population of terminally differentiating cells, the cells have features of “basal cells.” They have scanty cytoplasm and small round nuclei. There is a prominent row of palisaded basal cells defining an interface with stroma. Many of the small cells above the basal layer have clear cytoplasm; these cytologic features constitute the basic requirement for a diagnosis of tricholemmoma. There is moderate cytologic atypia.

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F3Ch39a: With rare exceptions, sclerosing entrapment is a process affecting components with the cytologic features of a superficial unit component. In this field, sclerosing entrapment is involving the population of cells of basal unit type. A portion of the basal unit component projects into the fibrotic stroma; it is outlined by laminated, brightly acidophilic fibrous tissue. Perhaps, this pattern merely is the expression of cytopathic changes in a superficial unit component in which the cytolysis and dyskeratosis has extended to the interface with a basal unit component.

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F4Ch39a; This is a neighboring field. The patterns are compatible with a process of sclerosing entrapment involving a basal unit component

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F5Ch39a: An additional uncommon feature of this lesion is an area in which lymphocytes and histiocytes form a rich infiltrate among keratinocytes of basal unit type; some of the inflammatory cells have collected in lytic defects; there are scattered dyskeratotic cells. To the left, in the stroma adjacent to the epithelial component, there is a small cluster of loosely arranged histiocytes. The pattern is of lymphoepithelial type.

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F6Ch39a: The interface between the lymphoepithelial pattern, and less involved epithelium of outer root sheath-like type is represented. The basement membrane along the interface of the clear cell component is hyalinized; it focally is laminated.

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F7Ch39a: The combination of features qualify as a lymphoepithelial pattern. The plump, pale cells ar altered squamous cells; in areas, poorly preserved intercellular bridges are represented. Lymphocytes and histiocytes loosely infiltrate the altered epithelium. Just below the center of the field, plump, pale keratinocytes are loosely spaced; a few intercellular bridges are intact; the cytologic features of these cells are characteristic of changes seen in lymphoepitheliomatous lesions. It is of interest that similar changes affect melanocytes in halo nevi. Perhaps some “lymphoepitheliomas” of the skin are examples of this distinctive process involving a follicular hamartoma.

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