Experimental analyses of the rearrangement of ectodermal cells during gastrulation and neurulation in avian embryos

Abstract

The rearrangement of ectodermal cells was studied in chimeras in which grafts were transplanted during late gastrula and early neurula stages to heterotopic locations in avian embryos. Three types of experiments were done. In all experiments, Hensen's node was extirpated completely and replaced with an epithelial plug derived from 1 of 3 regions of the prospective ectoderm. In type-1 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the floor plate of the neural tube. In type-2 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the lateral wall of the neural tube. In type-3 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the epidermal ectoderm. In all experiments, the amount and direction of cell rearrangement that occurred in the transplanted ectodermal plug was essentially typical for prospective ectodermal cells normally residing within Hensen's node. That is, transplanted ectodermal cells underwent lateralto-medial cell-cell intercalation and contributed to the ventral midline of the neural tube along its entire rostrocaudal extent. In most embryos, a notochord was 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 notochordal gaps. In such essentially notochordless embryos, the ventral midline of the neural tube still derived from grafted cells, but it failed to 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 rearrangement of ectodermal cells does not require the presence of a “trail” of prospective floor plate cells (laid down by the regressing Hensen's node), or of a notochordal substrate, and that the continued presence of an organizer per se, 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 cells 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 regulated in a cell-autonomous fashion, but rather it is determined by the overall magnitude and vector of the displacement of the community of rearranging cells within a developmental field.