Figure 3.

Cartoon illustrating the interacting roles of FGF2, ACTIVIN A, and BMP4 in lineage specification, especially in directing TB differentiation from epiblast type ESC and iPSC. The upper part of the figure shows how inhibitors can be used to block FGF2 signaling (PD173074, SU5402) and ACTIVIN A/TGFB/NODAL signaling (A83–01, SB431542) in conjunction with BMP4-directed differentiation to TB. Blocking signaling via both the SMAD2/3 and MEK1/2 pathways in the presence of BMP4 appears to be optimal for unidirectional differentiation toward TB. The lower part of the cartoon illustrates the various combinations of reagents, including medium conditioned by mouse embryonic fibroblasts (CM) that have been employed to drive hESC to TB, mesoendoderm, and mixtures of TB and mesoendoderm. Names (adjacent to a drawing of an implanting day 9–10 human conceptus) are relevant references with citation numbers listed in Supplementary Table S1, except Vallier et al. [63] and James et al. [65] that are only cited in the main text. In the conceptus, note the pluripotent, bilaminar epiblast (blue and mauve), extra-embryonic mesoderm (green), progenitor cytoTB (maroon), and the advancing mass of STB (pink) enclosing lacunae filled with maternally derived fluid acting as a nutrient support external to the embryo proper. At this stage of pregnancy, the conceptus has mainly passed through the uterine epithelium (yellow) and lodged in the uterine wall.