Figure 3.

Fgf–Erk1/2 signaling regulates the composition of the inner cell mass. The inner cell mass of the blastocyst is composed of epiblast (green) and primitive endoderm (red) cells. Decreasing Fgf or Erk1/2 signaling through pharmacological inhibition or genetically disrupting components of the pathway produces blastocysts with fewer primitive endoderm cells. Conversely, the composition of the inner cell mass can be shifted toward the primitive endoderm cell fate by culturing embryos in an excess of exogenous Fgf ligands.