Origins and patterning of craniofacial mesenchymal tissues.

Three mesenchymal tissues participate in the formation of orofacial tissues; these are the neural crest, paraxial mesoderm, and lateral mesoderm. Interactions both among these populations and between them and surrounding epithelial tissues are an essential feature of facial development. Perturbation of these interactions may result in craniofacial malformations and dysmorphologies. This review outlines the origins and early morphogenetic movements of each of the three mesenchymal populations, then describes experiments which reveal some of the interactions that control their development. Spatial organization within cephalic mesenchyme is manifest initially in connective tissue precursors. In the facial region these are derived from the neural crest; in contrast, much of the neurocranium is derived from paraxial mesoderm. Most crest populations become spatially programmed prior to their emergence from the neuroepithelium, presumably during the primary induction of the neural plate. As they migrate to form the branchial arches, the crest populations bring spatial information to these peripheral regions. Connective tissue-forming populations within cephalic paraxial mesoderm display a similar inherent spatial programming, but it is not known when or how they acquire this information.