The iPSCs were maintained as described previously . For inactive murine embryonic fibroblasts (MEFs) isolation, we used day 13.5 embryos. After the removal of the head, visceral tissues, and gonads, the remaining bodies were washed and dissociated with 0.25% trypsin-EDTA (Sigma-Aldrich, Saint Louis, MI, USA). Ten-million cells were plated on each gelatin-coated 100- mm dish and incubated at 37°C with 5% CO2. The next day, floating cells were removed by washing with PBS. In this study, MEFs were used within passage 4 to avoid replicative senescence. Normal iPSC line (HPS0063) was obtained from the RIKEN Bioresource Center . The harvested colonies were triturated to generate medium-sized small fragments, which were then seeded on new plates together with the mitomycin C-treated MEFs in complete ES medium composed of DMEM (Sigma-Aldrich) supplemented with 20% knockout serum replacement, 5 ng ml−1 recombinant human basic fibroblast growth factor (Peprotech), 20 mM HEPES buffer (pH 7.3), 0.1 mM 2-mercaptoethanol, 0.1 mM non-essential amino acids, 2 mM l-glutamine and 100 U ml−1 penicillin/streptomycin (all other materials were from Gibco). All images were prepared under the 100× objective of the phase contrast microscope in the BioStation CT system, using automatic Z-focus with a resolution of 1360 × 1024 pixels.