FIGURE 9

A model for the role of Pax1a in hyomandibular plate development. In wild types, Pax1a is necessary for the morphogenesis of the first pouch that forms at 18 hpf, with the loss of Pax1a leading to a distorted first pouch. After pouch formation, pax1a expression in the first pouch continues by 48 hpf, which is required to maintain efnb2a expression there. After the condensation of the skeletogenic mesenchyme in the hyoid arch at 36 hpf, Efnb2a in the first pouch would provide a survival signal to the barx1-positive osteochondral progenitors in the dorsal region of the hyoid arch that differentiate into chondrocytes contributing to the anterior plate of hyomandibular cartilage at 48 hpf. In pax1a mutants, reduced expression of efnb2a in the first pouch at 40 hpf would be insufficient to promote the survival of barx1-positive osteochondral progenitors in the dorsal region of the hyomandibular arch. Consequently, in pax1a mutants, osteochondral progenitors contributing to the anterior plate of hyomandibular cartilage die, and the anterior HM plate is defective in the larval head. p1, first pouch; MA, mandibular arch; HA, hyoid arch; HS, hyosymplectic cartilage; HM, hyomandibular cartilage.