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.