F1Ch16 (R5256): The epidermis shows mild, compact hyperkeratosis; rete patterns are partially preserved. The rete ridges are composed of small keratinocytes of the basal unit type. The epidermis, away from the rete ridges shows an inconspicuous basal unit; mostly, only a basal layer is represented; the epidermis, away from the rete ridges, consists of an hyperplastic superficial unit. There is mild melanocytic hyperplasia, as evidenced by an increased number of “clear cells” in the basal layer. The upper portion of the dermis shows solar elastosis; in general, solar elastosis is a marker for the upper portion of the reticular dermis. A normal papillary dermis is not represented; the domain of the papillary dermis consists of a fibrous mat of coarse collagen bundles. The mat is hypocellular, but there is an increased number of small cells, some of which are dendritic in outline even at this magnification, irregularly spaced at a short distance from the basal layer of the epidermis. The coarse fibrous mat continues to the left along the infundibulum of a follicle. It extends to form the adventitia of dilated vessel, near the right-hand margin of the field. There are spotty, mild perifollicular infiltrates of lymphoid cells.
The lesion is a thin fibrous papule. The affected anatomic unit is papillary dermis; the normal fibrous stroma of the papillary dermis is not represented. The abnormal fibrous component also affects the perifollicular connective tissue sheath, and the adventitia of a vessel; on the basis of the extent of involvement, it is reasonable to characterize a fibrous papule as a fibrous dysplasia of the adventitial dermis. In normal skin, the fibrocytes of the adventitial dermis would appear to have special qualities; the special attributes combine to qualify the components of the adventitial dermis as stroma. In this approach, stroma is ideally suited to offer sustentacular functions for related epithelium. An alteration in the character of the papillary dermis may find expression in the character of the overlying epidermis. The epidermal changes over a fibrous papule are variable from mild to marked. In the more extreme expressions, the changes have a lichenoid quality but are distinguished by their cell-poor character (with the lymphocyte as the deficient component). For some examples, lymphocytic infiltrates are a prominent component of a dermal, rather than an cellular interface reaction.