Neurulation in the anterior trunk region of the zebrafish Brachydanio rerio.

We have studied the process of neurulation within the anterior trunk region of the zebrafish by means of serial sectioning of staged embryos and labelling cells by applications of the dye Dil and intracellular injections of fluoresceine dextran amine. The first morphological manifestation of the prospective neural plate is a dorsomedial ectodermal thickening which becomes visible immediately after gastrulation. Within 1-2 h, by the time somatogenesis begins, two bilaterally symmetrical thickenings have appeared more laterally, which eventually fuse with the medial thickening to form the neural keel. The central canal forms next by separation of the cells on either side of the midline of the neural keel, beginning ventrally at the 17-somite stage and progressing towards dorsal levels. By means of fluorescent dye labelling in the late gastrula, we found that both the medial and lateral thickenings contribute to the nerve cord. The medial thickening was found to contain, exclusively, neural progenitor cells from the 90-100% epiboly stage on, whereas the adjacent regions contained a mixture of neural and epidermal progenitor cells, as well as prospective neural crest cells. Between the 90-100% epiboly and 2-somite stages, this heterogeneity of developmental capabilities is resolved into territories, with epidermogenic and neurogenic cells clearly separated from each other. To achieve this segregation into neural and epidermal anlagen, cells from the lateral thickenings have to move over a distance of roughly 400 μm within 1-2 h. Epidermal overgrowth of the nerve cord occurs during the morphogenetic movements that accompany nerve cord formation.