Abnormalities in the external ear, which is composed of the auricle and the external auditory canal (EAC), are frequent in newborns. However, little is known about the molecular mechanisms that govern external ear morphogenesis. Here, Filippo Rijli and colleagues report that Hoxa2 is a key transcriptional regulator that controls auricle morphogenesis in mice (). The researchers show that the mouse auricle derives from Hoxa2-expressing neural crest mesenchyme of the second pharyngeal arch, and not from the first and second arches as previously proposed. Furthermore, they report, the lining of the EAC derives from Hoxa2-negative first arch mesenchyme. Their analysis of gene expression patterns in wild-type and Hoxa2 mutant mice suggests that Hoxa2 organises patterns of cell proliferation during ear morphogenesis, acting in part through BMP signalling. Finally, they find that ectopic expression of Hoxa2 in the neural crest of the first arch results in auricle duplication, suggesting that Hoxa2 is able to induce and maintain the molecular programme that underlies auricle formation.
