Plexiform fibrohistiocytic tumor

Two groups of cells, one rounded or oval and the other spindle-shaped, were found in a case of plexiform fibrohistiocytic tumor. Immunohistochemistry was strongly positive for alpha smooth muscle actin in most cells of both types but stronger in the spindle-shaped cells, suggesting a myofibroblastic origin. This was in correlation with electron microscopic findings which revealed fibroblasts, myofibroblasts and some undifferentiated mesenchymal cells. However, factor XIIIa was negative suggesting that this tumor does not originate from dermal dendrocytes.