FGF4 is a limiting factor controlling the proportions of primitive endoderm and epiblast in the ICM of the mouse blastocyst.

The primitive endoderm (PE) and epiblast (EPI) are two lineages derived from the inner cell mass (ICM) of the E3.5 blastocyst. Although it has been shown that FGF signaling is necessary and sufficient for PE specification in the ICM, it is unknown what mechanisms control the PE/EPI proportion in the embryo. Because modulation of FGF signaling alone is sufficient to convert all ICM cells to either PE or EPI, a model has been proposed in which the amount of FGF in the embryo controls the PE/EPI proportion. To test this model, we reduced the amount of FGF4, the major FGF in the preimplantation embryo, using various genotypes of Fgf4 mutants. We observed a maternal contribution of Fgf4 in PE specification, but it was dispensable for development. In addition, upon treatment of Fgf4 mutant embryos with exogenous FGF4, we observed a progressive increase of PE proportions in an FGF4 dose dependent manner, regardless of embryo genotype. We conclude that the amount of FGF4 is limited and regulates PE/EPI proportions in the mouse embryo.