Genetic and epigenetic control of connective tissues in the craniofacial structures.

The connective tissues of the craniofacial region can neither develop nor be maintained independently of their local environment. I have stressed such epigenetic control in the present brief review of the origin, differentiation, morphogenesis, and growth of selected craniofacial tissues. I began by discussing the origin of these tissues in the neural crest. The extensive migration of neural crest cells to form the craniofacial region of the embryo is epigenetically regulated by extracellular matrix products such as fibronectin, and by basal laminae of the epithelia along which some cells migrate. The trigger which initiates differentiation of these mesenchymal cells is provided by epithelia with which they come into contact either before, during, or after migration, as illustrated by the differentiation of both Meckel cartilage and mandibular dermal bones. Subsequent differentiation of secondary cartilage is also shown to be epigenetically initiated, in this case by the mechanical environment created by muscle action. Both secondary cartilages on avian dermal bones and the condylar cartilage of the mammalian dentary are discussed. The interplay between genetic and epigenetic factors in morphogenesis and growth of craniofacial connective tissues is then very briefly discussed.