Molecular and embryological mechanisms of neural crest induction: The role of BMP signaling and underlying mesoderm in Danio rerio

Signals from the non-neural ectoderm, the neural ectoderm and the underlying mesoderm have all been implicated in the induction of neural crest. Bone Morphogenetic Protein (BMP) signaling in particular has an important role in this process; however, it is unclear whether this activity of BMP is due to its effects on patterning the underlying mesoderm, to its ability to establish a competent neural plate boundary zone, or to the direct specification of neural crest at intermediate levels of activity within a BMP gradient. The "vertical signals" model proposes that BMP signaling is important for patterning the mesoderm, which then directs neural crest development in the overlying ectoderm. The "two-step" model predicts that BMP signaling establishes a boundary zone between the neural and the non-neural ectoderm that is then competent to respond to secondary neural crest inducing signals. The "BMP gradient" model argues that ectodermal cells encountering intermediate levels of BMP signaling within a BMP activity gradient are directly specified to become neural crest. In Chapter 2, I show that neural crest induction proceeds after genetically preventing mesoderm involution during gastrulation in zebrafish embryos, indicating that underlying mesoderm and the vertical signals derived from it are dispensable for this process. Neural crest induction proceeds in the absence of the secreted BMP antagonists known to be expressed during gastrulation, eliminating one potential mechanism for establishing a BMP signaling gradient. In Chapter 3, using a transgenic zebrafish line expressing an inducible cell-autonomous inhibitor of BMP signaling, I determine when BMP signaling is required for neural crest induction and whether neural crest precursor cells require BMP signaling directly. Inactivating BMP signaling during early gastrulation results in the failure of neural crest induction, but this requirement is lost by mid-gastrulation stages. Embryonic mosaics in which transgenic cells are introduced into the neural crest domain of wild-type embryos indicate that neural crest precursor cells may not require BMP signaling directly. These results support the "two-step" model and are less consistent with the BMP gradient model for neural crest induction.