Understanding of the fundamental mechanisms that govern unintentional differentiation of human induced pluripotent stem cells (hiPSCs) provides key strategies to maintain their undifferentiated state during cell expansion. This study focused on deviation from the undifferentiated state in hiPSC colonies during culture with feeder cells. Deviated cells from the undifferentiated state of hiPSCs in cultures with SNL and MEF feeder cells were observed at the center and periphery of the colonies, respectively, accompanied by dramatic changes in the cell morphology from small to large flattened shapes. It was found that the deviation of undifferentiated hiPSCs in culture with SNL feeder cells caused deviated cells in the center of the colony through spontaneous occurrence in a colony size-dependent manner, whereas the deviation of undifferentiated hiPSCs in culture with MEF feeder cells caused deviated cells in the periphery of the colonies through accidental events during migration in a colony size-independent manner. Based on a kinetic analysis of time-lapse images of single hiPSC colonies, the specific growth rate for replication of deviated cells from the undifferentiated state in culture with SNL feeder cells was 1.83 and 3.57 times higher than those of undifferentiated cells and transformation, respectively, meaning that the deviation of undifferentiated hiPSCs dramatically expanded through replication of deviated cells from the undifferentiated state and transformation once deviation from the undifferentiated state had occurred. In the case of MEF feeder cells, the specific growth rates for replication of deviated cells from the undifferentiated state was 3.12 times higher than that of undifferentiated cells, whereas the rate by transformation exhibited a negligible level compared with the rates of replication for undifferentiated cells and deviated cells from undifferentiated state, meaning that deviation of undifferentiated hiPSCs dramatically expanded only through replication of deviated cells from the undifferentiated state. These results suggest that once deviation has occurred in a colony, the deviated cells from undifferentiated state undertake dramatic invasion to occupy the colony. Maintenance of the undifferentiated state in subcultures inevitably requires vigilant care to remove any colonies that include deviated cells from the undifferentiated state. Biotechnol. Bioeng. 2014;111: 1128–1138. © 2014 Wiley Periodicals, Inc.
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