Fig. 9. Model of the regulation of differentiation of ESCs into ExE cells by transcription factors. The ExE differentiation process is divided into at least two stages. In the first stage, ESCs differentiate into COUP-TF1-positive PrE cells, and in the second stage, they differentiate further into XEN, VE and PE cells. We have demonstrated that Sox17 plays a substantial role in the late-stage differentiation.