DEVELOPMENTAL PATTERNING AND EVOLUTION OF THE MAMMALIAN VISCEROCRANIUM - GENETIC INSIGHTS INTO COMPARATIVE MORPHOLOGY

The vertebrate cranium is generally classified into the neurocranium a nd the viscerocranium, The latter is derived from the neural crest and so is the prechordal portion of the neurocranium. A view we favor con siders the prechordal neurocranium as the premandibular component of t he viscerocranium, and the vertebrate skull to consist of the neural c rest-derived viscerocranium and the mesodermal neurocranium, Of these developmental units, only the viscerocranium appears to have completel y segmented metamerical organization, The Hox code which is known to f unction in specification of the viscerocranium does not extend rostral ly into the mandibular and premandibular segments, By genetic manipula tion of rostrally expressed non-Hox homeobox genes, the patterning mec hanism of the head is now demonstrated to be snore complicated than is omorphic registration of tile Hox code to pharyngeal arches, The pheno type by haplo-insufficiency of Otx2 gene, in particular, implies the p remandibular cranium shares a common specification mechanism with the mandibular arch, Our interpretation of the metamerical plan of the vis cerocranium offers a new scheme of molecular codes associated with the vertebrate head evolution. (C) 1997 Wiley-Liss, Inc.