The neural crest: what can it tell us about cell migration and determination?

Of all vertebrate structures, the nervous system has perhaps the greatest cellular diversification and complexity of organization. The study of developmental neurobiology, therefore, sharply probes the central issue of embryogenesis; i.e., how does a complex system arise form a single cell? During development, ectodermal tissue becomes determined along neuronal lines via contact or "primary induction" from the roof of the archenteron (Mangold, 1933; Spemann, 1938). A major problem is how the morphologically indistinguishable stem cells of the neuroectoderm become progressively determined along divergent developmental lines such that they give rise to a myriad of cell types in the mature nervous system. Another question posed by this system is how the cells migrate to their final locations and become organized in precise spatial arrays. Once the pattern is composed, what mechanisms govern the establishment of the proper connections between neurons and other neurons, as well as between neurons and their target organs?