In vivo tumorigenesis. (A) Human adipose tissue-derived stem cells (hASCs) cultivated in basal medium supplemented with allogeneic human serum (aHS) were injected into the dorso-lateral subcutaneous region of nude mice and did not form tumors. (B) Histology of the regions of cell transplantation showed normal skin layers (scale bar 150 μm). (C) Histology of tumor formed by transplantation of MDA-MB-231 revealed pleomorphism, mitotic activity and well-delimited tumor (scale bar 30 μm). (D) The same numbers of FN052 iPS cells formed teratoma after 12 weeks of transplantation. These tumors involved (E) transitional epithelium (endoderm-like tissue; scale bar 150 μm), (F) neural ectoderm (ectoderm-like tissue; scale bar 50 μm), and (G) skeletal muscle (mesoderm-like tissue; scale bar 150 μm), as shown by histological staining with hematoxylin and eosin. Asterisks indicate the dorso-lateral subcutaneous region of cell transplantation.