The significance of double phenotypic patterns and markers in human sarcomas. A new model of mesenchymal differentiation.

Six soft-tissue sarcomas with two separate and juxtaposed histologic patterns were selected for immunohistochemical analysis. The first pattern was represented by five phenotypes (schwannian-skeletal muscle [Triton], cartilagenous, synovial, adipocytic, and smooth muscle). In each case the second histologic pattern resembled the fibrohistiocytic phenotype, ie, malignant fibrous histiocytoma (MFH). No other histologic patterns were identified. Appropriate cell markers were demonstrated in each of the first patterns; these were not detected in the second patterns. In contrast, the second pattern in all cases expressed alpha 1-antichymotrypsin, a marker commonly found in fibrohistiocytic lesions; this was not identified in any of the first patterns. This loss of one cell-specific marker and gain of another is termed the "antigenic shift" phenomenon and appeared to foretell the emergence of a true second phenotype (the same in each of these cases, which could be termed "dedifferentiated" sarcomas). Therefore, it is hypothesized that MFH is a final common pathway for some types of sarcomas and is the result of tumor progression or "dedifferentiation." The practical implications of this hypothesis concern the approach to sarcoma differential diagnosis and the meaning of an MFH pattern in both metastatic and primary sites. On a theoretic level, this hypothesis and the antigenic shift phenomenon force a reconsideration of the pathways of soft-tissue differentiation. A new model of mesenchymal differentiation incorporating these concepts is described and supported. It provides an explanation for a number of facts in soft-tissue pathology, and its predictions can be tested.