HALL, B. K.; Dalhousie University, Halifax: The Neural Crest: New Germ Layer, Skeletons, and Developmental Processes as Innovations

The neural crest (NC) arises embryonically at the boundary between neural and epidermal ectoderm. As a craniate (hagfishes and vertebrates) synapomorphy, the neural crest qualifies as an innovation; in this instance, regarding a new feature as an innovation. A rationale for the NC as a fourth germ layer will be presented, NC, like mesoderm, being a secondary germ layer, ectoderm and endoderm being primary germ layers � a new germ layer as an innovation. Transformation of epithelial ‘neural’ ectoderm to mesenchymal NC cells and embryo-wide migration of those cells illustrates developmental process as innovation. Cells and tissues can also be innovations; many of the cells and tissues derived from the NC are either not seen in other metazoans � osteoblasts and bone; dentine and teeth � or represent a more highly derived and diversified versions of cells and tissues found elsewhere (chondroblasts and cartilage). Endo- and exoskeletons within the same organism and derived from ‘similar’ tissues is only found in conjunction with NC cells � organ systems as innovation. Developing organ systems in vertebrates are more closely linked developmentally (epigenetically) than is true for other animals. Explanations reside in: (i) the origin and diversification of mesenchyme; (ii) the modular organization of mesenchymal cell populations as condensations; (iii) epithelial-mesenchymal interactions to activate specific cell populations, and; iv) subsequent hierarchical control of differentiation and morphogenesis. These four aspects of developmental organization will be explored using endo- and exoskeletal systems derived from the NC. I will place some emphasis on populations of NC cells as developmental and evolutionary modules and comment on the sense in which modules are innovations. Supported by NSERC (Canada), the Canada Council, and Dalhousie university