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In this paper we performed morphological studies of Bednar tumor, which is a rare pigmented variant of dermatofibrosarcoma protuberans (DFSP) for evidence of histogenetic origin. Presence of melanosome-containing cells is the only histologic criterion of Bednar tumor, which is different from the ordinary DFSP. Immunohistochemical and ultrastructural examinations revealed that the melanosome-containing cells gave positive reactivity to antibodies to vimentin and S-100 protein, but negative to HMB-45, CD38, Factor VIII, NSE, and KP-1. The melanosomes are in Stage II, III, and IV, suggesting that the pigment was produced within the cell rather than phagocytosed from other cells. The rest of the tumor had similar pattern of immunoreactivity to DFSP such as positivity for CD34 and vimentin, indicating that Bednar tumor and DFSP originated from the same cell line. The ultrastructural feature of non-pigmented cells was similar to perineural cells in the first group and similar to fibroblasts in the second group. DNA flow cytometry in our cases revealed diploid cells with low to intermediate S-phase fraction and no aneuploid cells which were reported in a previous study.
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