UC Berkeley

The neural crest (NC) is a vertebrate-specific population of multipotent cells, often referred to as the fourth germ layer because of its unique ability to migrate to all parts of the body and contribute to a variety of tissues. Positional information allows neural crest cells to determine their location relative to each other and in complex 3D environments, and to respond accordingly. A central question in developmental biology is how these positional cues are imparted to cells, and how the cells interpret them for the appropriate response. The work in this thesis sets out to determine 1) how Wnt and BMP gradients define cells as neural crest, 2) how the position along the AP axis determines neural crest fate and 3) how abolishing one positional cue results in a craniofacial phenotype.

Precise control of BMP and Wnt signaling in both space and time is necessary for proper neural crest induction. While the Wnt signaling pathway has been extensively studied, it is not clear how the cells interpret and respond to the Wnt signals. Knowing the importance of these pathways in setting up the embryo axes, I wanted to further our understanding of the precise way these pathways initiate the neural crest program. An important first step toward determining how the intersection of the Wnt and BMP signaling pathways set up the neural crest cell program in space and time was to develop a method to rapidly and uniformly manipulate the signaling pathways in Xenopu laevis embryos. I was able to validate that small molecules can replace the use of mRNA injections in order to manipulate signaling pathways.

The next chapter of my thesis determines how neural crest cells are specified at different axial levels. The derivatives and migratory paths of neural crest cells in the cranial and non-cranial regions are very distinct, so the gene regulatory networks are likely to differ as well. Previous work has focused on the cranial neural crest cells, and therefore, regulation of the non-cranial neural crest program remains poorly understood. By taking advantage of transgenic zebrafish lines, I show that a neural crest cell’s position along the AP axis during early development determines its fate.

In the final chapter of this thesis, I examine how abolishing one cue (Wnt signaling) affects patterning of neural crest-derived cells without affecting earlier aspects of neural crest formation, such as proliferation and differentiation. Analysis of the craniofacial structures in mutants for multiple components of non-canonical Wnt signaling provides evidence that each component is required for proper formation in different axes.