Ch29aP21a Atypical Tricholemmoma
(Low-grade Basaloid Follicular CA)

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F1Ch29: At this magnification, the patterns have tricholemmoma-like qualities. Broad columns of keratinocytes extend from the epidermis, mostly in vertical orientation. At various levels the columns become confluent. The lesion is fairly symmetrical; it appears to sit upon the skin. There are moderate infiltrates of lymphoid cells. Near the right margin of the lesion, an angulated indentation along the surface contains compact keratinized debris.

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F2Ch29: There is a dual population of cells. The more superficial component is composed of acidophilic squamous cells. Tortuous columns of pale cells extend from the superficial component into the dermis. The supporting matrix is condensed fibrous tissue. Some of the fascicles of tumor cells are outlined by thick hyaline membranes. To the extreme right, there is an area of cytolysis involving the squamous cell component.

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F3Ch29: The columnar components, in vertical orientation, lend follicular qualities to the patterns. Blue arrows identify a hyaline membrane (glassy membrane) that outlines one of the follicular components. A prominent row of palisaded basal cells abut upon the membrane; the pattern of the palisade also lends a follicle-like quality. The cells of the column beyond the palisaded basal layer are polygonal and have clear cytoplasm - the cytologic features are outer root sheath-like. Basal cells form a bridge between this portion of the lesion and the neighboring, smaller   follicle-like component to the left. There are loose infiltrates of inflammatory cells. In the partially represented column to the right, the pale cells have migrated among the acidophilic cells to produce a “pagetoid” pattern. In addition, there is a rounded nest isolated in this column.

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F4Ch29: The interfaces between the population of acidophilic cells, and the component resembling the outer root sheath are well defined; they have the qualities of clonal interfaces; the acidophilic portion is superficial and the clear cell component is deep.

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F5Ch29: The cytologic features of the vacuolated cells of the column are remarkably outer root sheath-like.

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F6Ch29: Columns of squamoid cells are spaced in a fibrous matrix. The cells have pale cytoplasm. Hyaline membranes outline most of the nests of tumor cells. The lesion has sclerosing (desmoplastic) qualities, but this is not the pattern of so-called “desmoplastic” follicular hamartoma.

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F7Ch29: The lesion has basaloid qualities. In some of the nests, the cells have vacuolated cytoplasm.

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F8Ch29: Focally, the sclerosing patterns have complexly partitioned the fascicles. Short, thin cords and nests of cells are closely spaced; in some areas, only a hyaline membrane separates neighboring nests. The cells have round nuclei, diffuse, delicate chromatin patterns, and variable nucleoli; some of the nuclei have prominent nucleoli.

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