Ch14aaaP6aaa Inverted Follicular
 Keratosis

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H6951IFK010

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.

H6951IFK030

F2Ch14aaaP6aaa: In this example, the column of tumor cells extends, and attaches to, the epidermis. The whorls are more acidophilic; they have fewer vacuolated cells than the pale background epithelium. The acidophilic cells form a thin column that extends through the epidermis to the surface. At the interfaces between whorls and the background epithelium, there are small lytic defects.

H6951IFK040

F3Ch14aaaP6aaa: In areas in which the cytolytic process has destroyed the basal layer, a cleft has formed between the epithelium and the stroma. The epithelium to the right of the row of red arrows is activated. Centrally in the epithelial column, whorls are clustered. In this area, clefts have formed among the whorls; portions of the background epithelium have undergone lysis.

H6951IFK060

F4Ch14aaaP6aaa: Blue arrows identify the zone of activated connective tissue; red arrows identify the zones of cytolysis.

H6951IFK070

F5Ch14aaaP6aaa: Cytolysis mostly affects the cells of the background epithelium, among the whorls (cells of superficial unit-like type). Both the background epithelium of this field and the epithelium of the whorls are of superficial unit type, the cells of the background are pale.

H6951IFK080

F6Ch14aaaP6aaa: This type of activated connective tissue can follow defects into what was originally the epithelial domain; in the process, whorls and cords of acidophilic squamous cells can be entrapped to produce a pseudo-invasive pattern.

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