Shaping of the chick neuroepithelium during primary and secondary neurulation: Role of cell elongation

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Abstract
Cell elongation is a cardinal event in formation and shaping of the neuroepithelium during both primary and secondary neurulation. This study had three purposes. The first was to clarify the role of microtubules in maintaining the elongated configurations of chick neuroepithelial cells. Neuroepithelial cells of the neural plate (the rudiment of the primary neural tube) and medullary cord (the rudiment of the secondary neural tube) reduced their heights an average of approximately 25% when their microtubules were depolymerized, but most cells remained considerably elongated and columnar. Complete rounding up occurred only as cells entered metaphase where they arrested. These results suggest that microtubules as well as other factors are required to maintain the fully elongated configurations of cells composing epithelial sheets. The second purpose of this study was to evaluate the hypothesis that neuroepithelial cell elongation plays a major role in narrowing of the neural plate. To do this, the width of the neural plate was examined after microtubule depolymerization and repolymerization. As the heights of neuroepithelial cells decreased with loss of their microtubules, the width of the neuroepithelium increased roughly proportionately; subsequent repolymerization with concomitant cell elongation resulted again in neural plate narrowing. Thus, the hypothesis is supported. The third purpose of this study was to examine the roles of cell rearrangement and change in neuroepithelial cell or extracellular volume in neural plate narrowing and extension. Extensive cell rearrangement, resulting in net cell loss from the width of selected, representative levels of the neural plate, does not seem to play a major role in plate narrowing, but decreases in cell or extracellular volume are likely involved. Further studies are necessary to complete our understanding of the mechanisms driving neural plate shaping and bending.