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Background  Mesendoderm induction during embryonic stem cell (ESC) differentiation  in vitro  is stimulated by the Transforming Growth Factor and Wingless (Wnt) families of growth factors.    Principal Findings  We identified the periods during which Bone Morphogenetic Protein (BMP) 4, Wnt3a or Activin A were able to induce expression of the mesendoderm marker,  Mixl1 , in differentiating mouse ESCs. BMP4 and Wnt3a were required between differentiation day (d) 1.5 and 3 to most effectively induce  Mixl1 , whilst Activin A induced  Mixl1  expression in ESC when added between d2 and d4, indicating a subtle difference in the requirement for Activin receptor signalling in this process. Stimulation of ESCs with these factors at earlier or later times resulted in little  Mixl1  induction, suggesting that the differentiating ESCs passed through ‘temporal windows’ in which they sequentially gained and lost competence to respond to each growth factor. Inhibition of either Activin or Wnt signalling blocked  Mixl1  induction by any of the three mesendoderm-inducing factors. Mixing experiments in which chimeric EBs were formed between growth factor-treated and untreated ESCs revealed that BMP, Activin and Wnt signalling all contributed to the propagation of paracrine mesendoderm inducing signals between adjacent cells. Finally, we demonstrated that the differentiating cells passed through ‘exit gates’ after which point they were no longer dependent on signalling from inducing molecules for  Mixl1  expression.    Conclusions  These studies suggest that differentiating ESCs are directed by an interconnected network of growth factors similar to those present in early embryos and that the timing of growth factor activity is critical for mesendoderm induction.

Differentiating Embryonic Stem Cells Pass through ‘Temporal Windows’ That Mark Responsiveness to Exogenous and Paracrine Mesendoderm Inducing Signals