Gastrointestinal stromal tumors and KIT-positive mesenchymal cells in the omentum

Gastrointestinal stromal tumor (GIST) is currently considered to be derived from the interstitial cells of Cajal (ICC). To test the hypothesis that omental mesenchymal tumor is also a type of GIST, we evaluated the expression of specific molecules in GIST, and c‐kit gene mutation in omental mesenchymal tumors, and we identified a possible counterpart of ICC in the omentum. Immunohistochemically, all of the omental mesenchymal tumors (n = 5) were positive for both KIT and CD34, and three of the five tumors were also positive for an embryonic form of smooth‐muscle myosin heavy chain (SMemb). Polymerase chain reaction–single‐strand conformational polymorphism analysis (PCR–SSCP) and direct sequencing revealed mutations in c‐kit gene exon 11 in all five tumors. As for the ICC counterparts in the omentum, there were some KIT‐positive mesenchymal cells resembling ICC at the surface of the omentum. Double fluorescence immunostaining, using anti‐KIT polyclonal antibodies and monoclonal antibodies against other molecules, demonstrated that KIT‐, CD34‐ and SMemb‐positive cells were present just beneath the mesothelial cells of the omentum. These results show that omental mesenchymal tumor corresponds to GIST of the omentum, and that KIT‐positive bipolar mesenchymal cells may be a counterpart of ICC in the gastrointestinal tract. Identification of a new type of KIT‐positive mesenchymal cell in the omentum may lead to the discovery of a new physiological role for this organ.