The developmental specification of the vertebrate skull

The initial form of the embryonic bony skull is determined in two ways; cranially, by the relative growth of the developing brain, and facially, by the chondrocranium. Both are essentially acting as structural templates around which the bony components of the skull are assembled. Assuming, therefore, that the specification of form and pattern in the facial skeleton occurs at the formation of the chondrocranium, this paper will focus on precisely how the chondrocranium forms. Any acceptable explanation of chondrocranial morphogenesis must satisfy at least two prerequisites. First, given the constancy of chondrocranial form in vertebrates, any model proposed should be equally applicable to all vertebrates. Second, it should enable us to answer questions of homology concerning the skull and, in particular, provide explanation for those instances where 'homologous' structures have a different (lineage) composition. From studies limited to a small number of amphibian, avian and mammalian species, it is apparent that chondrogenesis in the vertebrate skull is largely, if not entirely, elicited by epitheliomesenchymal tissue interactions. Analysis of such interactions (and of those promoting osteogenesis) reveals that these are matrix-mediated and, recently, the expression of certain 'relevant' matrix components has been shown to be developmentally regulated in a fashion that correlates with the location and timing of these interactions. From these, and related, observations a morphogenetic model, the so-called 'Flypaper Model', has been proposed to explain the specification of chondrocranial form. A number of predictions arising from that model are currently being tested experimentally and the current status of the model is reviewed. Finally, the ability of this model to satisfy the prerequisites defined above is assessed.