Midfacial retrusion and loss of facial appendages caused by mutation of Pax9 in zebrafish

Loss of dentition has occurred repeatedly throughout vertebrate evolution. Cyprinid fish, including zebrafish, form teeth only deep within the pharynx, not on their oral jaws. However, zebrafish still robustly express transcription factors associated with mammalian tooth development in the neural crest-derived mesenchyme surrounding the mouth. We investigated whether this expression is vestigial or whether these factors contribute to the formation of non-tooth mesenchymal structures in the oral region, using Pax9 as a test case. Zebrafish homozygous for two different pax9 mutant alleles develop the normal complement of pharyngeal teeth but fail to form the premaxilla bone, most of the maxilla, and nasal and maxillary barbels. Lack of most of the upper jaw complex does not preclude effective feeding in the laboratory environment. We observed a significant deficit of sp7:EGFP+ osteoblasts and adjacent alx4a:DsRed+ condensing mesenchyme where the maxilla forms, and no accumulation of either in the premaxillary domain. These phenotypes are not presaged by major disruptions in early facial patterning, Wnt signaling, proliferation, or cell death; however, loss of a small population of Wnt-responsive cells around the maxilla correlates with its stalled growth in mutants. We conclude that Pax9 is not unequivocally required for all vertebrate tooth development, but instead may be broadly involved in the development of a variety of organs forming through mesenchymal condensation around the mouth.