Although the mechanism of multinucleation in giant cell tumors of tendon sheath (GCTTS) remains unknown, two mechanisms have been proposed: one is cell fusion and the other amitotic division. The purpose of this study was to clarify the multinucleation process of cultured cells from GCTTS using an in vitro fluorescent cell membrane labeling technique. Cultured GCTTS cells obtained from a 7-year-old Japanese girl were divided into two groups, one for PKH-2 (green) staining and the other for PKH-26 (red) staining. After staining with the dyes, the cell populations were mixed and observed with a fluorescent microscope on the 4th and 14th days after mixing. On both the 4th and 14th days, the cultured GCTTS giant cells showed a mosaic of green and red colors, thus indicating cell membrane fusion. Images of double fluorescent labeled giant cells indicated cell fusion of mononucleated stromal cells that lead to multinucleated giant cells in these GCTTS cell cultures. These findings suggest that multinucleation in GCTTS results from the fusion of mononuclear stromal cells in vitro.
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