SSX4 and synovial sarcoma: However, it is now believed that synovial sarcomas arise from pluripotent mesenchymal cells that are capable of differentiating into epithelial cells [1] and SS18-SSX1, SS18-SSX2, or SS18-SSX4 fusion oncoproteins [2]. The falciform ligament is a double layer of peritoneum that contains mesenchyme-derived loose connective tissue, small vessels, lymphatics, and variable amounts of fat [6].