Evolutionary changes in brain organisation may partly explain why different species have different faces, suggest Diane Hu and Ralph Marcucio, who have been studying upper jaw development (see ). The growth of this part of the facial skeleton, which is formed from neural crest-derived cells, is controlled by Sonic hedgehog (SHH) signals from the frontonasal ectodermal zone (FEZ). Variations in the FEZ (the establishment of which requires Shh expression in the developing forebrain) underlie the divergent facial morphogenesis of birds and mammals. The researchers now report that activation of the SHH pathway in chick brains superimposes a mammalian-like morphology onto the chick's upper jaw and splits its single FEZ into right and left domains that resemble those of mice. The expression of several other signals (including Fgf8) in the brain and in the nasal pit is also altered in the SHH-treated chicks, note Hu and Marcucio. Thus,they suggest, the brain establishes multiple signalling centres within the developing upper jaw that regulate craniofacial morphogenesis.
