Rhombomeric origin and rostrocaudal reassortment of neural crest cells revealed by intravital microscopy.

Neural crest cell migration in the hindbrain is segmental, with prominent streams of migrating cells adjacent to rhombomeres (r) r2, r4 and r6, but not r3 or r5. This migratory pattern cannot be explained by the failure of r3 and r5 to produce neural crest, since focal injections of the lipophilic dye, DiI, into the neural folds clearly demonstrate that all rhombomeres produce neural crest cells. Here, we examine the dynamics of hindbrain neural crest cell emigration and movement by iontophoretically injecting DiI into small numbers of cells. The intensely labeled cells and their progeny were repeatedly imaged using low-light-level epifluorescence microscopy, permitting their movement to be followed in living embryos over time. These intravital images definitively show that neural crest cells move both rostrally and caudally from r3 and r5 to emerge as a part of the streams adjacent to r2, r4, and/or r6. Within the first few hours, cells labeled in r3 move within and/or along the dorsal neural tube surface, either rostrally toward the r2/3 border or caudally toward the r3/4 border. The labeled cells exit the surface of the neural tube near these borders and migrate toward the first or second branchial arches several hours after initial labeling. Focal DiI injections into r5 resulted in neural crest cell contributions to both the second and third branchial arches, again via rostrocaudal movements of the cells before migration into the periphery. These results demonstrate conclusively that all rhombomeres give rise to neural crest cells, and that rostrocaudal rearrangement of the cells contributes to the segmental migration of neural crest cells adjacent to r2, r4, and r6. Furthermore, it appears that there are consistent exit points of neural crest cell emigration; for example, cells arising from r3 emigrate almost exclusively from the rostral or caudal borders of that rhombomere.