Ch14bP6bTricholemmoma

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F1Ch14bP6b: The lesion has a distinct vertical orientation; it is polypoid. The well-defined columnar patterns, and the cytoplasmic pallor of a preponderance of the cells set the lesion apart from seborrheic keratosis. The lesion is supported by an expanded papillary dermis (stroma). For this example, dilated vessels are a prominent feature of the stroma. Most of the columns show central cavities that contain necrotic cellular debris. About the cavities, keratinocytes show dyskeratosis.

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F2Ch14bP6b: Characteristically, the columns of a tricholemmoma where they join the epidermis are composed of common squamous cells; these short segments of ordinary squamous cells have a central canal containing keratinized debris; they have the features of the infundibular portion of a normal follicle. The deeper portion of this column is somewhat bulbous; it is composed of small cells that have pale or clear cytoplasm; these distinctive cells can be compared to the cells of the outer root sheath of an anagen follicle. There is one small whorl of squamous cells.

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F3Ch14bP6b: The junction between the infundibular component, and the tricholemmal component is represented. Many of the cells of the tricholemmal component have vacuolated or clear cytoplasm. There is some variation in nuclear size, staining, and orientation; there is mild atypia. Focally, basal cells form prominent palisades. The stroma is a loose, delicate fibrous matrix. The basement membrane is coarse and hyalinized.

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F4Ch14bP6b: Loose infiltrates of lymphoid cells hug the epithelial component. Focally, the basement membrane is prominent. Blue arrows identify an area of fibrosis in the stroma. Small aggregates of tumor cells are entrapped in the fibrous tissue. In this area, an area of stromal invasion, showing regressive changes, cannot be ruled out: this portion of the lesion has lympho-epithelial qualities.

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F5Ch14bP6b: Many of the cells have clear cytoplasm. Cell membranes are sharply defined. The cells, but for the cytoplasmic pallor or clearness, have features of the basal unit of the epidermis. The cytoplasmic features are of the type seen in cells of the outer root sheath of an anagen follicle. There is mild nuclear atypia. At the bottom of the field on the left, basal cells form prominent palisades. The basement membrane is coarsened and hyalinized.

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F6Ch14bP6b: The clear cell (outer root sheath-like) qualities are uniform. There is moderate nuclear atypism with some degree of pleomorphism. Some degree of nuclear atypia is common in the lesions of this category. Cell membranes are sharply defined.

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F7Ch14bP6b (H9505): In this small, “young,” lesion, the basic features of a tricholemmoma are represented. Columns of squamous cells extend from the epidermis into a tumor stroma. The lesion is confined to the tumor stroma (a widened, richly vascularized papillary dermis). At a certain depth, the epithelial component becomes confluent. There are irregular clusters of vacuolated cells in the confluent portion. In areas, where the vacuolated cells extend to the margin of the epithelial component, the basal layer shows prominent palisading of nuclei. Two small follicles (blue arrows) have been distorted by the expansile tumor. As is often the case, small follicles appear to increased in number, at the margins, or beneath, a tricholemmoma.

 

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