Schematic depicting the major signaling pathways and some of the important genes involved in regionalization and specification of various cranial placodes from the pre-placodal region. Activation (green) and repression (magenta) of signaling pathways are color coded and the arrows indicate whether a signaling pathway is activated (↑) or inhibited (↓). BMP, Bone Morphogenetic Protein; FGF, Fibroblast Growth Factor; PDGF, Platelet Derived Growth Factor; RA, Retinoic Acid; SHH, Sonic hedgehog.