S3C12P1-1a (lichenoid keratosis): The pattern at the dermal-epidermal interface is a cell-rich lichenoid reaction, but small, lytic defects containing lymphocytes, histiocytes, and necrotic keratinocytes are present on the epidermal side of a mostly intact basement membrane. There are small, focal breaks in the basement membrane and, in some of the defects, lymphocytes and histiocytes have been caught in transit from the dermal domain into the epidermis.

S3C12P1-2 (left) (lichen planus): The lytic complex of the established lichenoid reaction is illustrated. The entire thickness of the surviving epidermis has features of a component committed to terminal differentiation. Some of the epidermal interstitial avenues are open; some of the open ones contain a few lymphocytes, and migratory histiocytes. Green arrows identify the superficial margin of the epidermal defect. Red arrows identify colloid bodies. Some of the colloid material appears to be intracytoplasmic in a histiocyte. In the repair of this defect, fibrous tissue will be inlaid. In the inlay, necrotic cells will be incorporated in the fibrous tissue (accretive fibrosis).

S3C12P1-3 (right) (lichen planus): Reduplicated basement membrane material in seen near the top of the field on the right. Fibrocytes have extended into the defect and fibrous tissue, loosely and focally, has been inlaid in the defect. Disruption of the basement membrane may allow fibrocytes to invade what was formerly epidermal domain.

S3C12P1-4 (left) (lichen planus): Here, the inlay of fibrous tissue is rather complete. It is unlikely that the prominent clusters of colloid bodies have been delivered to the dermis in the cytoplasm of histiocytes. Colloid bodies are markers for what once was epidermal domain. In this field, they have been entrapped during the process of a fibrous inlay of a defect in the epidermal domain. A thin, lytic cleft remains at the new dermal-epidermal interface. There is no morphologic, functional basal unit as such in this field. The cells of the epidermis have all acquired the characteristics of a hyperplastic, and hypertrophic superficial unit (with cytologic features of cells that are committed to terminal differentiation).

S3C12P1-5 (to the right) (lichen planus): Immuno-fluorescence shows prominent clusters of rounded bodies at the dermal-epidermal interface. This is the characteristic lichen planus-like pattern. The rounded bodies correspond to the colloid bodies of an H & E stained section.

S3C12P1-6 (left) (lichenoid keratosis): The epidermis shows compact hyperkeratosis, and a thin superficial unit. The basal unit is partially preserved and, focally, even hyperplastic. Keratinocytes of the basal unit are outlined in green. The green material outlining keratinocytes of the hyperplastic basal unit is representative of the mucinous, epidermal interstitium of the functional basal unit of the epidermis. The red arrows outline a lytic defect in the basal unit of the epidermis. The defect appears clear (presumably the defect contained a watery matrix). The upper portion of the dermis shows mucinous changes; normally, the papillary dermis is mucinous. This is a selected field in a lichenoid keratosis.                                         

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