The Molecular Determinants of Cranial Skeletal Development and Evolution

Abstract

The vertebrate head and face, one of the defining features of vertebrates, is an evolutionary novelty that enabled the major radiation of Vertebrata. The emergence of the multipotent neural crest cells, often mentioned as the “fourth germ layer”, which produces most of the bone, cartilage and dentine of the vertebrate head, was the key to this innovation. Unlike the appendicular and axial skeletons in the rest of the body, which have relatively uncomplicated anatomy and are generated from cells of mesodermal origin, most of the cranial bones have complex structures generated by a unique mechanism termed intramembranous ossification and originate from the cranial neural crest cells. The unique and innovative nature of the neural crest and cranial bone is also reflected in the morphogenic processes that create diversity in craniofacial shape throughout the course of vertebrate evolution, exemplified in the bird lineage. In this thesis, I seek to expand our knowledge of the development and evolution of craniofacial elements, by developing a 3-D platform to generate cranial bone in vitro and investigating the evolution of beak and tooth loss in birds. In the first chapter, I introduce the first system for generating intramembranous bone with mouse cranial neural crest cells in 3-D hydrogel culture. The flexibility of the platform will allow it to be utilized for generating 3-D cranial bones from neural crest cells using various sources, including birds and human patients with craniofacial disorders. In the second chapter, the process of beak evolution and the latent developmental potential to generate teeth in birds are analyzed using an intriguing chick mutant, Talpid2. The geometric morphometric analysis of, and the gene expression patterns observed in, the Talpid2 mutant reveal the atavistic nature of the mutant phenotypes, including ancestral beak shape and the possible resurrection of teeth development. Preliminary efforts to manipulate the gene responsible for the phenotypes are also addressed for future studies. Together, the results of these studies provide useful tools and knowledge to further advance our understanding in the craniofacial development in amniotes.