Embryonic Development of the First Branchial Arch

Development of the definitive jaws in vertebrates is preceded by establishment of restricted expression patterns of signalling molecules and homeobox genes within the first arch. It has been proposed that the nested pattern of Dlx expression identified in mouse first arch is conserved in gnathostomes as a common mechanism of patterning the arch. Similar to mouse nested expression of the Dlx genes was revealed in chick, however, differences in the extent of the mesenchymal expression, as well as marked differences in the epithelial expression patterns between these two species were observed. One possibility to explain these differences in expression may be attributed to differences in the Dlx cis-regulatory elements between chick and mouse. Another explanation is that differences in signalling between chick and mouse account for the differences observed in the Dlx expression patterns. Finally genes exhibiting disparate expression in these species like recently identified Lix1 may offer a further mechanism by which first arch patterning may be modulated to give species specific first arch characteristics.