Ch35P27-L4 Pilar Cyst

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F1Ch35: The lesion is a keratin-filled cyst. The keratinized debris is compacted and uniformly acidophilic. Centrally, areas of calcification are fragmented; a delicate fibrous matrix surrounds the zones of calcification. A few smaller zones of calcification are present in the keratinized debris. The epithelial lining is pale; it varies in contour at the interface with the keratinized debris.


F2Ch35: The lining is stratified squamous epithelium. The basal layer is a well-defined row of cuboidal cells. There is a thin, but well-defined, “glassy” membrane. The number of cell layers above the basal layer is variable. The variability is contributed by variations in the number of cell layers in the basal unit (as opposed to what appears to be more uniformity in the number of cell layers in the superfcial unit). The cells of the superficial unit are plump, pale, and polygonal. Nuclear characteristics in this area are uniform at all levels; chromatin is delicate. At the terminal level, the plump cells bulge and interdigitate with more intensely stained (more definitively keratinized) cellular debris. Some of the bulging pale forms are devoid of a nucleus. Some of the intensely keratinized debris near the viable zone contain pyknotic nuclei. This type of keratinization is reminiscent of the changes seen in a catagen follicle, particularly in the region of the deep extremity of a clubbed hair (F3Ch12 & F2Ch13a).


F3Ch35: The distinctive form of keratinization, as seen in the epithelial lining of this lesion recapitulates the patterns seen in a regressing catagen follicle, about the end of the clubbed hair - in what remains of the inconstant portion of the follicle. In this approach, a “pilar” cyst is a keratinizing cyst in which the process is a distorted counterpart of the process which facilitates the loss of a hair in a catagen follicle. It is not altogether correct to characterize the collection of changes as a process affecting the outer root sheath. For a regressing follicle, changes of this type would be found in the bulge that forms about the club of a clubbed hair; it would appear to be a special adaptation of what was once the bulb of an anagen follicle, but phenotypes that characterize an anagen follicle mostly are lost in the development of catagen patterns; the patterns are altered to a degree that comparisons with the normal patterns of an anagen follicle are inappropriate. In the field above, there is a thin “glassy membrane.”


F4Ch35: As additional evidence of the phenotypic diversity of follicular epithelium, this field demonstrates the induction of an abortive hair “bulb” in the lining of this “pilar” cyst. The row of basal cells is accentuated; A cellular, primitive papilla distorts the contour of the basal row of germinative cells.


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