Why mice have muzzles and birds have beaks

Although the faces of vertebrate embryos look remarkably similar at early stages of development, they rapidly adopt species-specific characteristics. But how is regional specification of the face achieved? The answer, suggest Brugmann and colleagues on p. 3283, is Wnt signalling. Using transgenic Wnt reporter embryos, the researchers show that domains of Wnt-responsive cells in the developing mouse face correspond to the facial prominences (for example, the characteristic muzzle) that develop later. These domains of Wnt responsiveness, the researchers note, generally coincide with areas where there is marked cell proliferation. Furthermore, they report, genetic or biochemical disruption of Wnt signalling in mouse embryos produces animals with unusually wide-set eyes and flattened midfaces. Similar investigations in chick embryos reveal that Wnt signalling is an evolutionarily conserved mechanism that determines facial features by regulating differential craniofacial growth. In other words, the radically different facial features of vertebrates might all be explained by species-specific, regional variations in Wnt signalling during craniofacial development.