Developmental mechanisms and evolution of vertebrate masticatory systems

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Combining the experimental embryology, comparative embryology, and molecular biology, developmental mechanisms and evolution of the vertebrate jaw and tongue structures were investigated. By observing normal developmental patterns of sturgeons, shark, and the lanmprey, it was shown that basic morphological architecture was conserved in early embryonic patterns between gnathostome and agnathan species. Especially, mesodermal configuration has shown that agnathans were at low level of developmental programing of the hypobranchial system that leads to the tongue formation. In the topographical relation between specific parts of the ectoderm and ectomesenchyme, however, a small differerence was detected with regard to the positions of nasal and hypophysial placodes. This difference involved the positions of initial Fgf8 gene expression domain. The lattter gene expression is responsible for the patterning of mandibular arch ectomesenchyme. By local application of growth factor-soaked beads, we could show that in a stage-, and developmental context-dependent manner, the craniofacial ectomesenchyme can be repattened. Thus, epithelial-mesenchymal interactions play fundamental roles in initial shaping of the craniofacial ectomesenchyme. Expanding this concept, we applied the same technique to the lamprey embryo and shoed that the gnathostome jaw was obtained through repatterning of tissue interaction