Characterizing the role of FGF signaling in the mouse blastocyst

Understanding the molecular cues that direct cells to specialize is important for developing methods to induce pluripotent cells towards specific cell fates. The mouse embryo is an ideal system to analyze embryonic stem cell fate specification. For example, at the late blastocyst stage of development cells of the Inner Cell Mass (ICM) specialize into epiblast (EPI) and primitive endoderm (PE). Evidence shows that the Fibroblast Growth Factor (FGF)/MAPK signaling pathway is important for directing cells towards a PE fate. Stimulation of FGF signaling increases expression of the PE-specific transcription factor Gata6, and suppresses the EPI-specific transcription factor, Nanog; the reverse is seen when the pathway is inhibited. It is unclear whether FGF signaling is important for the initial differentiation or for the maintenance of mature PE. The maturation of the PE is marked by the sequential expression of the PE-specific transcription factorsGata6, Sox17, and Sox7. We investigated the requirement of FGF signaling for PE maintenance by analyzing the expressions of Sox17 and Sox7 in response to inhibition or stimulation of FGF signaling in vitro. We examined the embryos by immunofluorescence and confocal microscopy and used a cell count system to measure protein expression. We found that FGF signaling is sufficient to induce the expression of both Sox proteins, but required only for Sox7 expression- a marker of mature PE cells.  These results suggest that the role of FGF signaling in the specification of PE and EPI is to help maintain mature PE after they have specialized.