Ch38P30Proliferating PilarTumor

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F1Ch38 (G7504): To begin, the reader should refer to the photographs of catagen follicles, and then concentrate on the character of the epithelium and the keratinized debris about the clubbed hair of a catagen follicle (F3Ch12, F2Ch13a); the patterns of the keratinized debris in the region of a clubbed hair are similar to the character of the keratinized debris, and of the neighboring epithelium at the interface that is identified by green arrows. The lesion that is represented in this field is a proliferating pilar (tricholemmal) cyst.

This lesion often is both solid and cystic. This field is from a solid area. Pegs of stroma project into the plane of the section; the epithelial-stromal interface in these areas is identified by the prominent rows of basal cells with their palisades of nuclei. Two components of squamous cells are represented. Bridges of epithelium, composed of plump, pale squamous cells, extend among collections of small squamous cells. The population of plump, pale acidophilic cells is representative of the superficial unit of squamous epithelium - a population of cells committed to terminal differentiation. This characterization is accurate for a wide range of patterns as manifested in squamous epithelium of different anatomic sites. For example, the patterns of keratinization along a follicle, and during the cycling of follicles, is divergent, but a superficial unit always contributes the keratinized debris. Even the character of the debris may differ in keeping with variations in the patterns in the superficial unit. The population of small squamous cells are of a type seen in the basal unit of squamous epithelium. The basal unit is a germinative unit; the basal layer of the basal unit has an important role in maintainance of a basement membrane. The basal unit has an important role in the metabolic functions of squamous epithelium. Generally, it provides a residence for Langerhans cells and, as such, provides a domain for immune reactions. Near the bottom of the field on the right, there are areas of cytolysis and dyskeratosis in the superficial unit component. The patterns in the basal unit component are in keeping with follicular epithelium of inconstant type, and with catagen-like alterations.

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F2Ch38: A portion of the field seen in F1 is seen at higher ermagnification. The distinctive pattern of keratinization is characterized by cobbling of plump, pale cells at the interface between viable cells and keratinized cells. The luminal row of cells are polygonal, or oblong; individual, dyskeratotic cells are identifiable in the densely packed, brightly acidophilic keratinized debris. A microvesicle is present below the area of keratinization. To the left, a coarse, irregular hyaline membrane defines a boundary between an island of stroma, and the row of prominent basal cells.

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F3Ch38: Centrally, a zone of keratinization is irregular in outline. To the far right, a population of small keratinocytes of basal unit type extends to the fibrotic stroma. A blue arrow marks a boundary; to the right of the arrow the interface with stroma is protected by a row of basal cells; to the left, all the way to the opposite margin, epithelium of the superficial unit type (cells committed to terminal differentiation) impinge upon the stroma without a basal layer as intermediary. Cells of this superficial unit, that press upon the fibrotic stroma, show degenerative changes; nuclei are pyknotic, and cytoplasms are condensed and brightly acidophilic. Cells of the superficial unit are not adapted to survive in contact with stroma.

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F4Ch38: Cytolysis has resulted in the formation of microcysts. There are scattered dyskeratotic cells in the defects. The pink material is proteinaceous fluid. Note that the cysts are within the domain of the superficial unit, or at the interface between the superficial unit and the prominent basal unit. There is a mild degree of nuclear atypia in the basal unit. The degree of atypia in a lesion of this type can be widely variable.

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F5Ch38: Cytolysis and individual cell dyskeratosis seem to have a role in the formation of the irregular defect in the superficial unit. Some of the degenerating cells have cytoplasmic vacuoles. At the interface between the superficial and basal units, blue arrows identify individual cells showing dyskeratosis.

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F6Ch38: Individual cell dyskeratosis is a prominent feature of this portion of a superficial unit. To the right, one nucleus contains an acidophilic inclusion; this probably represents an intranuclear inclusion of cytoplasm. Some of the keratinized material has a lamellar quality.

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F7Ch38: A microcystic component is represented to the left, near the top of the field. Near the bottom of the field, dyskeratotic changes are becoming confluent; cells keratinize without the formation of keratohyaline, or trichohyaline, granules.

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