Smooth-muscle proliferation in dermatofibromas.

Although a variety of epithelial changes have been associated with dermatofibromas, mesenchymal proliferations induced by dermatofibromas appear to be rare. We report three dermatofibromas that were associated with proliferation of smooth muscle within the adjacent dermis. The lesions ranged from a large, densely cellular dermatofibroma with "monster cells" to a sparsely cellular, fibrous lesion. In all three cases, the smooth muscle could be discerned in hematoxylin- and -eosin-stained sections, because its presence caused an interruption in the pattern of reticular dermal collagen bundles with blunt-ended vesicular nuclei. Well-formed fascicles were apparent in immunoperoxidase-stained sections using antisera to muscle-specific actin (HHF-35) and desmin. The smooth muscle failed to stain with either antisera to S-100 protein or Leu-7, precluding determination of whether it immunophenotypically resembled vascular or pilar smooth muscle. We believe that the smooth-muscle proliferation in our cases was induced by the spindled cells of dermatofibroma, just as epithelial changes in association with dermatofibroma are. We further believe that it is most likely of vascular derivation, based on a previous report of angioleiomyoma arising in a dermatofibroma as well as the continuity of fascicles of smooth muscle with a thick-walled vessel in one of our cases. Less likely explanations include collisions between dermatofibromas and leiomyomas or divergent differentiation in a proliferation arising from primitive mesenchymal cells.