Model of pharyngeal cartilage development in the presence (A) and absence (B) of vgll2a. Expression of vgll2a within the pharyngeal epithelia is regulated positively by FGFs and negatively by RA (A). Within the endoderm (orange cells) and perhaps the ectoderm (blue cells), vgll2a regulates expression of a survival signal secreted by these epithelia and required for survival of neural crest cells (green cells) around 24 hpf. In the presence of vgll2a expression, these neural crest cells go on to form the pharyngeal cartilages by 5dpf. In the absence of vgll2a expression (B), the pharyngeal epithelia fails to express the vgll2a-regulated survival signal and a subset of neural crest cells undergo caspase-mediated cell death around 24 hpf. By 5 dpf, only neural crest cells in close enough association with one-another meet the critical mass required for chondrogenesis, resulting in cartilage elements that are hypoplastic or absent.