Figure 1: In vivo and in vitro cardiogenesis. (A) Schematic illustration of a transverse section of early stage mammalian embryo showing counteracting signals which shape the cardiac precursor zone in vivo. Cardiogenic secreted factors for cardiac mesoderm formation are emitted by the underlying anterior endoderm and by the anterior ectoderm including bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs) sonic hedgehog (Shh), non-canonical Wnts and canonical Wnt inhibitors. These signals counteract with cardiac inhibitors like canonical Wnts and BMP inhibitors released by the neural plate. The cardiac mesoderm is defined by a region with high BMP and low Wnt activity. (B) Sequential steps in cardiac differentiation in vitro from pluripotent stem cells to functional cardiomyocytes. Positive (blue) and negative (red) acting signals influence cardiomyogenesis in vitro and display parallels to embryonic development. Early mesodermal cells are induced by canonical Wnt, activin, BMP and FGF signaling and inhibited by insulin. Diversification to cardiac mesodermal cells can be inhibited by canonical Wnts, notch and insulin signaling. Cardiac progenitor cell specification is promoted by non-canonical Wnts and negatively regulated by canonical Wnt, activin and BMP signaling. Notch signaling promotes cardiac progenitor cell differentiation and inhibits cardiomyocyte differentiation. Neurogenin 1 (Ngn1), retinoic acid (RA) and specific microRNAs (miRNAs) direct cardiomyocyte subtype-specific differentiation. Structural and functional maturation is not well understood. Maturation might be provoked by hormones, electrical and mechanical stimulation and organization in 3D engineered heart tissues (EHTs). Typical markers and characteristics for the different cell types are indicated.