FGF Signaling In The Blastocyst Seems To Function As A Positional Cue For Sorting Primitive Endoderm Cells At The Blastocoel Cavity

Understanding what molecular cues cause cells to specialize is important for developing methods to direct pluripotent cells toward certain cell fates for both experimental and therapeutic endeavors. The blastocyst stage of mouse embryo development serves as an ideal system to analyze cell fate specification. Within the blastocyst, the Inner Cell Mass (ICM) gives rise to the primitive endoderm (PE) and to the pluripotent epiblast (EPI). Evidence shows that Fibroblast Growth Factor (FGF) signaling plays a role in directing cells toward forming the PE by increasing expression of PE-specific transcription factor, Gata6, and suppressing an EPI-specific transcription factor, Nanog. To test the extent to which PE formation is induced by this signaling pathway, we examined expression of other PE-specific genes following FGF4 treatment. The PE-specific transcription factors Sox17 and Sox7 maintain PE cell fate.  Sox17 is expressed in all PE cells, while Sox7 is only detected in PE cells adjacent to the blastocoel cavity. We examined FGF4-treated and untreated embryos by immunofluorescence and confocal microscopy and determined relative numbers of cells expressing Sox proteins or Nanog. We found that FGF signaling was sufficient to induce expression of both Sox proteins in all cells of the ICM. This result supports the literature on FGF-dependent segregation of EPI and PE, but also points to a more specific role for FGF signaling. Our results suggest that FGF signaling in the blastocyst may function as a positional cue that induces Sox7 once PE cells come to occupy their final position adjacent to the blastocoel cavity.