Ch23P15-L3 Tricholemmoma 
(blastematous component)

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F1Ch23 (P1112): The lesion has produced an elevation of the surface of the skin. Broad columns of squamous cells, predominantly of terminal differentiation (superficial unit-like) type, project from the epidermis into the dermis. To the right, a column has a central lumen; a column of keratinized debris extends from the lumen to protrude along the surface. On the right side of this follicular component, the epithelium shows a transition from squamous epithelium to basal cell epithelium of germinative type. There is an increased number of miniaturized follicles in the underlying dermis - most are cut in cross section. Centrally, a follicular shaped column of squamous cells is in continuity with a miniaturized follicle; a small component of basal cells is represented at the juncture.

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F2Ch23: The centrally located follicle of F1 is represented at higher magnification. Red arrows identify the germinative, basal cell epithelium in the pattern of a “bulge” in the transition zone. This bulge might be characterized as blastomatous. The small column of squamous cells, that extends from the transition zone to the lower margin of the field (see F1) can be identified as a component of the inconstant portion of this miniaturized follicle.

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F3Ch23 (for orientation see F1Ch23): This bulbous column clearly has follicular characteristics. Along a broad front (blue arrows), there is a row of palisaded basal (germinative) cells - a common feature of tricholemmomas. Along the interface identified by the four blue arrows to the right, the lesion is composed of cells with the cytologic features of “basal cells.” Red arrows identify whorls of tumor cells. The component among the whorls is composed of spindle cells. Near the bottom of the field, there are 3 closely spaced small follicles that have been cut in cross section; the follicles are increased in number in this localized area. The follicular component could be evidence that the lesion has had its origin in some type of follicular nevus; the possibility that the small follicles are part and parcel of the basic neoplasm cannot be ruled out.

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F4Ch23: There is a poorly stained hyaline membrane outlining the row of palisaded cells. The small, compact whorls and the background of spindle cells are well represented.

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F5Ch23: The column of keratotic debris shows parakeratosis. The adjacent epithelium at the base of the column shows evidence of dyskeratosis. Away from the column, the epithelium is cytologically monotonous; the cells have pale, or clear cytoplasm; each cell has a rounded nucleus with delicate chromatin patterns.

This pattern of apoptotic, terminal differentiation (individual cell necrosis) and parakeratosis is seen in limited anatomic sites in certain stages as normal follicles display their cyclic phenomena (see F4Ch10, F3Ch11, F3Ch12, & F5Ch13). These patterns are evidence of an altered type of terminal differentiation that, in normal follicles, is encountered at the junction of the inconstant follicular component with the isthmus of the constant component. In a normal follicle, the disruptive nature of the phenomena might be evidence of a process that eventuates in the desquamation of the keratinized debris contributed by a tripled layered column (i.e., the inner root sheath), and the hair. How remarkably like porokeratosis is this area.

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F5Ch23: Three minor columns project from the main portion of the lesion. The minor columns are composed of basal cells with a peripheral row of palisaded basal cells. These minor columns are germinative expressions, but are not sufficiently differentiated to characterize as to developmental potential.

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