IMMUNOHISTOCHEMICAL STUDY OF PROLIFERATING MESENCHYMAL SPINDLE CELLS IN HETEROGENOUS SOFT-TISSUE LESIONS

Proliferation of mesenchymal spindle cells is a main event in a variet y of lesions with similar morphological features but widely divergent biological behaviour. In order to identify criteria for precise histol ogical 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 immunohistochemica l 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 o f vimentin, alpha-smooth muscle actin (ASMA) and macrophage antigen CD 68 (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 wer e easily differentiated on the basis of positive reactivity for all th ree markers but individual lesions could only be distinguished by addi tional assessment of histological features. Because of consistent co-e xpression of ASMA, vimentin and MA-CD68 in the spindle mesenchymal cel l present in all varieties of lesions in this heterogeneous group, we suggest that this proliferating ''fibroblastic'' cell is phenotypicall y a fibromyohistiocyte.