Craniofacial development: discoveries made in the chicken embryo.

The aim of this review is to highlight some of the key contributions to our understanding of craniofacial research from work carried out with the chicken and other avian embryos. From the very first observations of neural crest cell migration to the fusion of the primary palate, the chicken has proven indispensable in facilitating craniofacial research. In this review we will look back to the premolecular studies where "cut and paste" grafting experiments mapped the fate of cranial neural crest cells, the role of different tissue layers in patterning the face, and more recently the contribution of neural crest cells to jaw size and identity. In the late 80's the focus shifted to the molecular underpinnings of facial development and, in addition to grafting experiments, various chemicals and growth factors were being applied to the face. The chicken is above all else an experimental model, inviting hands-on manipulations. We describe the elegant discoveries made by directly controlling signaling either in the brain, in the pharyngeal arches or in the face itself. We cover how sonic hedgehog (Shh) signals to the face and how various growth factors regulate facial prominence identity, growth and fusion. We also review abnormal craniofacial development and how several type of spontaneous chicken mutants shed new light on diseases affecting the primary cilium in humans. Finally, we bring out the very important role that the bird beak has played in understanding amniote evolution. The chicken, duck and quail have been and will continue to be used as experimental models to explore the evolution of jaw diversity and the morphological constraints of the vertebrate face.