F1Ch14aaaP6aaa: It has been difficult to find an uncomplicated inverted follicular keratosis (catagen type tricholemmoma). By necessity, the search has been restricted to the material in teaching sets, and perhaps only complicated lesions have found their way into the sets. The configuration at the periphery of this example is fairly typical. The epidermis is hyperplastic and shows papillomatosis. Broad columns of squamous cells extend from the epidermis into the dermis. In the dermis, there is a tendency for the columns to become confluent. The lesion is composed mostly of cells of the so-called superficial unit-like type; they are plump and polygonal. Many of the cells have perinuclear vacuoles. In the deeper portion of the lesion, there are scattered whorls (squamous eddies). Cells of the basal unit type (germinative cells) form clusters, mostly in areas abutting upon the stroma. To the left of the red arrows, there are lytic clefts (markers for cytolysis). In areas, the defects have replaced much of the epithelium, sparing mostly the squamous eddies. The blue arrows identify a band of activated connective tissue. A cleft separates the connective tissue from aggregates of the acidophilic squamous eddies. The epithelium of basal unit-like type, including the basal layer, has undergone cytolysis. Cytolysis, that involves basal unit type epithelium, including a basal layer, results in defects that are likely to be inlaid with newly formed fibrous tissue. Islands of epithelium, particularly the whorls, that have resisted the cytolytic process are likely to become entrapped in these inlays.
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