Ch39cP31c Tricholemmoma (Sclerosing  
Entrapment & Lymphoepithelial Change)

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K4455trichlem190

F1Ch39c (K4455): In the central area of fibrosis, the epithelium along the periphery is of basal unit type. To the left, portions of two whorls are represented. There is mild cytologic atypia. At the left, at the bottom of the field, there are several dyskeratotic cells. To the right, a bulbous nest of squamous epithelium of superficial unit-like type protrudes from the epithelial component into the fibrous stroma. Just to the right and above, there is smaller nest that is isolated in the stroma. These two nests show moderate atypia. Neither nests shows evidence of a preserved basal layer. The neck of the larger nest, at the point of attachment, shows lysis and dyskeratosis of cells, as if there is an impending separation. A even smaller nest near the bottom of the field is composed of cells showing intense cytoplasmic acidophilia. In the fibrous stroma to the left, among ectatic vessels, there are clusters of loosely aggregated “colloid bodies” (necrotic, dyskeratotic keratinocytes). Once entrapped, the keratinocytes of the nests undergo progressive lysis and dyskeratosis.

K4455trichlem201

F2Ch39c: The sheet of squamous cells to the right is composed mostly of cells of the superficial unit type. Near the bottom of the field, at the interface with the stroma, there is an inner portion, consisting of 3-5 cell layers in which the cells have the features of cells of basal unit-like type. Some of the cells of superficial unit-like type show cytolytic and dyskeratotic features. At about the middle of the field, the basal unit plays out; above this level, there is no evidence of a basal layer. Keratinizing cells abut upon the fibrous stroma. Along this entire interface, activated connective tissue cells are loosely spaced in a clear matrix (connective tissue activation). Cytologic atypia is moderate. Near the bottom of the field, in the sheet of cells to the right, one enlarged cell contains colloid bodies in the cytoplasm. Some observers might interpret these bodies as viral inclusions. A bulbous nest projects from the sheet of cells into the stroma to the left; it has a narrow stalk. There is moderate atypia in this nest ; much of the nest is without a basal layer. Atypia is moderate to moderately severe.

K4455trichlem220

F3Ch39c: Centrally, there is a confluence of cords of atypical squamous cells; the degree of atypia is moderate to moderately severe; a blue arrow points to an atypical mitotic figure. To the right of the blue arrow, colloid bodies, representative of necrosis of dyskeratotic cells, are loosely clustered. The stroma is fibrotic.

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F4Ch39c: The separate lesion as seen in F1Ch39b is represented at a higher magnification. The character of the nests and cords of cells, the prominent basal layer at the periphery of the nests and cords, and the central areas of calcification all combine to provide patterns of follicular differentiation. The prominent basal layer might be cited as evidence of differentiation along the lines of the inconstant portion of a catagen follicle. The lesion is interpretated as an early catagen-liketricholemmoma; there is mild atypia. Some of the keratotic debris is calcified.

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