Cellular mechanisms of neural fold formation and morphogenesis in the chick embryo

The mechanisms underlying neural fold formation and morphogenesis are complex, and how these processes occur is not well understood. Although both intrinsic forces (i.e., generated by the neuroepithelium) and extrinsic forces (i.e., generated by non-neuroepithelial tissues) are known to be important in these processes, the series of events that occur at the neural ectoderm-epidermal ectoderm (NE-EE) transition zone, resulting in the formation of two epithelial layers from one, have not been fully elucidated. Moreover, the region-specific differences that exist in neural fold formation and morphogenesis along the rostrocaudal extent of the neuraxis have not been systematically characterized. In this study, we map the rostrocaudal movements of cells that contribute to the neural folds at three distinct brain and spinal cord levels by following groups of dye-labeled cells over time. In addition, we examine the morphology of the neural folds at the NE-EE transition zone at closely-spaced temporal intervals for comparable populations of neural-fold cells at each of the three levels. Finally, we track the lateral-to-medial displacements that occur in the epidermal ectoderm during neural groove closure. The results demonstrate that neural fold formation and morphogenesis consist of a series of processes comprising convergent-extension movements, as well as epithelial ridging, kinking, delamination, and apposition at the NE-EE transition zone. Regional differences along the length of the neuraxis in the respective roles of these processes are described.