Of 210 dermatofibromas, 54 (25.7%) were covered by a flattened epidermis akin to that seen above scars. The epidermis was acanthotic in the manner of lichen simplex chronicus in 24 dermatofibromas (11%). The epidermis was acanthotic, simulating features of seborrheic keratosis in 132 dermatofibromas (62.9%) and in all those cases basal epidermal hyperpigmentation was encountered, too. Interestingly, the epidermis showed areas of clear and pale cells similar to those seen in clear cell/pale cell acanthoma in 40 cases (18.6%).

Induction of adnexal differentiation either in the form of follicular or in the form of sebaceous differentiation was encountered in a total of 89 cases (41%).

Follicular differentiation was seen in 23 (10.9%) dermatofibromas, all of which had at least foci of follicular germinative cells. In 20 cases (9.5%), follicular germinative cells were associated with mesenchymal aggregations in the form of well-circumscribed papillae. In 9 cases (4%), formation of a complete hair follicle with a bulb of matrical cells contiguous with a follicular papilla and with outer and inner sheath differentiation were encountered. In one dermatofibroma, extensive follicular structures simulating a trichoblastoma were seen within the fibrohistiocytic proliferation as well as in the overlying epidermis.

Sebaceous differentiation in the form of mantle induction was identified in 54 (25.7%) dermatofibromas. In 35 (16.7%) dermatofibromas, sebaceous lobules were seen connected to the overlying epidermis. Not uncommonly, those sebaceous structures were arranged in a reticulate pattern similar to that seen in reticulated acanthoma with sebaceous differentiation (26 cases = 12%).

Illustrations of histopathologic findings are given in Figures 1–16.