Atypical fibroxanthoma of the skin: an ultrastructural study of two cases.

The ultrastructure of two atypical" fibroxanthomas of the skin was studied. The first lesion was a spindle cell tumor on light microscopy, which electron microscopy revealed was composed of undifferentiated mesenchymal cells. The second lesion had a highly pheomorphic appearance on both light and electron microscopy. It was formed by atypical histiocytic cells and abundant "typical" Langerhans cells. The second lesion was considered to be a proliferation either of atypical Langerhans cells or of histiocytes related to Langerhans cells. Thus, atypical fibroxanthoma of skin may not be a homogeneous entity but could be a group of mesenchymal proliferative lesion in a similar clinical setting.