Figure 5.

Eomes and dose-dependent Nodal/Smad2/3 signalling levels control cardiac mesoderm and definitive endoderm specification during gastrulation.

(a) Smad4 and Foxh1 are critical Nodal pathway components for transducing high levels of signalling^{6, 12, 13}. Mesp1 is expressed normally in E6.5 and 7.5 Foxh1 null embryos and in embryos lacking Smad4 in the epiblast only (Smad4Δ). Mesp1 expression is also efficiently induced in Lhx1 mutant embryos, which display DE and midline mesoderm defects. However the failure of A-P axis rotation results in induction of Mesp1 throughout the proximal epiblast. (b) Eomes activity regulates formation of both cardiac mesoderm and DE progenitors during gastrulation. Eomes+ epiblast cells confined to the posterior side of the embryo prior to overt streak formation are exposed to low levels of Nodal signalling. Eomes-dependent activation of Mesp1/2 marks the earliest cardiac progenitors induced in the forming PS. Mesp1/2 expression leads to activation of the Nodal antagonist Lefty2²³ and direct repression of DE genes². As cells begin to migrate away from the PS Mesp1/2 expression is down-regulated via a negative feedback loop^{2, 23, 25}. In contrast the Eomes expression domain extends distally and overlaps with increased Nodal signalling levels as the PS elongates. Eomes, acting cooperatively with Nodal/Smad4/FoxH1 dependent signals in the APS, induces DE. (c) At later stages from E7.5 onwards Tbx6 expression in the pre-somitic mesoderm activates a second wave of Mesp1/2 expression in the pre-somitic mesoderm via occupancy of the conserved T-box regulatory elements.