EXPERIMENTAL ANALYSES OF THE REARRANGEMENT OF ECTODERMAL CELLS DURINGGASTRULATION AND NEURULATION IN AVIAN EMBRYOS

The rearrangement of ectodermal cells was studied in chimeras in which grafts were transplanted during late gastrula and early neurula stage s to heterotopic locations in avian embryos. Three types of experiment s were done. In all experiments, Hensen's node was extirpated complete ly and replaced with an epithelial plug derived from 1 of 3 regions of the prospective ectoderm. In type-1 experiments, Hensen's node was re placed with a plug consisting of precursor cells of the floor plate of the neural tube. In type-2 experiments, Hensen's node was replaced wi th a plug consisting of precursor cells of the lateral wall of the neu ral tube. In type-3 experiments, Hensen's node was replaced with a plu g consisting of precursor cells of the epidermal ectoderm. In all expe riments, the amount and direction of cell rearrangement that occurred in the transplanted ectodermal plug was essentially typical for prospe ctive ectodermal cells normally residing within Hensen's node. That is , transplanted ectodermal cells underwent lateral-to-medial cell-cell intercalation and contributed to the ventral midline of the neural tub e along its entire rostrocaudal extent. In most embryos, a notochord w as reconstituted from host cells, despite the fact that Hensen's node - the prime source of prospective notochordal cells in intact embryos - was extirpated completely; however, a few embryos had long notochord al gaps. In such essentially notochordless embryos, the ventral midlin e of the neural tube still derived from grafted cells, but it failed t o form a floor plate, providing further confirmation of the results of several previous studies that;the notochord is required to induce the floor plate. Collectively, our results provide evidence that the rear rangement of ectodermal cells does not require the presence of a ''tra il'' of prospective floor plate cells (laid down by. the regressing He nsen's node), or of a notochordal substrate, and that the continued pr esence of an organizer pere, ostensibly Hensen's node, is not required . In addition, our results demonstrate that the rearrangement of cells still occurs in the absence of ''boundaries'' between ectodermal cell s of different phenotypes (e.g., between cells of the floor plate and lateral walls of the neural tube). Finally, our results reveal further that the amount and direction of cellular rearrangement is not regula ted in a cell-autonomous fashion, but rather it is determined by the o verall magnitude and vector of the displacement of the community of re arranging cells within a developmental field.