MECHANISTIC INSIGHTS INTO SECONDARY NEURULATION

The central nervous system is formed by a series of movements referred to as neurulation. There are two modes of neurulation, known as primary and secondary neurulation. Primary neurulation occurs in anterior regions and involves an epithelial folding process. During secondary neurulation, which takes place in posterior regions, mesenchymal cells coalesce into a rod-like structure and subsequently undergo a mesenchymal-to-epithelial transition (MET) to create a lumen and form the neural tube. Although extensive studies have furthered our understanding of primary neurulation, mechanisms underlying secondary neurulation are not clearly understood. Several pieces of evidence from this research dissertation and published by other laboratories suggest that the zebrafish embryo undergoes a mode of secondary neurulation. Moreover, the optical properties of zebrafish embryos have enabled a detailed and real-time analysis of the dynamic cell behaviors that drive neurulation. These observations establish the zebrafish as a prime model system to study the molecular underpinnings of secondary neurulation. Studies outlined in this dissertation have revealed several molecules implicated in different stages of neurulation. During neural convergence, cells migrate in a polarized manner towards the midline. Generation of cell traction and polarization are essential during migration. Evidence suggests that the cell adhesion molecule N-cadherin promotes cell movement by stabilizing cell-cell contacts, thereby enabling cell traction. Polarization of cell movement appears to be mediated by the microtubule network, as treatment with the microtubule-destabilizing drug nocodazole randomizes the orientation of membrane protrusions. A central role for microtubules is further supported by the analysis of linguini mutants in which microtubules appear unstable and cells behave in a similar manner as in nocodazole-treated embryos. During MET, the centrosome migrates from a central position close to the nucleus to the apical cortex, where it docks and becomes a nucleating center for cilia. In par3-deficient cells, the centrosome fails to migrate to the apical surface and as a consequence cilia form intracellularly. This study identifies a previously undescribed role for Par3 in centrosome migration. Overall, these studies establish the zebrafish as a model system to study secondary neurulation and have identified key players that are required for different phases of secondary neurulation.Additional files available: 7 .avi files, stored on CONTENTdm server with ETD .pdf