Tumorigenesis in cells derived from induced pluripotent stem cells

Induced pluripotent stem (iPS) cells are an attractive source for potential cell-replacement therapy. However, transplantation of differentiated products harbors the risk of teratoma formation, presenting a serious health risk. Thus, we characterized Nanog-expressing (undifferentiated) cells remaining after induction of differentiation by cytological examination. To induce differentiation of iPS cells, we generated embryoid bodies (EBs) derived from iPS cells carrying a Nanog–green fluorescent protein (GFP) reporter and then injected GFP-positive and GFP-negative EBs into nude mice. GFP-positive EB transplantation resulted in the formation of immature teratoma grade 3, but no tumors were induced by GFP-negative EB. GFP-positive cells revealed significantly lower cytoplasmic area and higher nucleus/cytoplasm ratio than those of GFP-negative cells. Our results suggest that morphological analysis might be a useful method for distinguishing between tumorigenic and nontumorigenic iPS cells.