Immunohistochemical study of proliferating mesenchymal spindle cells in heterogenous soft tissue lesions

Proliferation of mesenchymal spindle cells is a main event in a variety of lesions with similar morphological features but widely divergent biological behaviour. In order to identify criteria for precise histological diagnosis, 60 human soft tissue lesions, divided into 40 cases of fibroblastic cell proliferation, 10 smooth muscle cell tumours and 10 nerve sheath cell tumours, were examined for the immunohistochemical profile of the main lesional cell in addition to other histological features. The three groups could be differentiated by determining the lineage of the constituent spindle cell on the pattern of expression of vimentin, alpha-smooth muscle actin (ASMA) and macrophage antigen CD68 (MA-CD68). Smooth muscle cells expressed ASMA and vimentin but not MA-CD68, while nerve sheath cells were negative for ASMA but positive for vimentin and MA-CD68. The fibroblastic cell lesions as a group were easily differentiated on the basis of positive reactivity for all three markers but individual lesions could only be distinguished by additional assessment of histological features. Because of consistent co-expression of ASMA, vimentin and MA-CD68 in the spindle mesenchymal cell present in all varieties of lesions in this heterogeneous group, we suggest that this proliferating "fibroblastic" cell is phenotypically a fibromyohistiocyte.