Regulation of neuroepithelial cell shape during bending of the chick neural plate

The early chick neural plate is a flat epithelial sheet that appears to consist of a homogeneous population of cells. As the notochord forms and becomes anchored to the overlying neural plate, a median hinge point (MHP) for bending is generated, and neuroepithelial cells within the MHP become strikingly different from those in adjacent lateral areas of the neural plate (L). MHP cells become about one-half as tall as L cells; they come to line a furrow, whereas L cells do not; and most of them transform from spindle-shaped to wedge-shaped, whereas L cells remain predominantly spindle-shaped. The first part of this dissertation was undertaken to evaluate the hypothesis that 'wedging' of MHP cells involves basal cellular expansion owing to alteration of the cell cycle. Cell generation time in the MHP was found to be significantly longer than generation times in L and in the flat neural plate prior to bending. Furthermore, it was found that all MHP and L cells were proliferative during stages of bending, both the DNA-synthetic phase and non-DNA synthetic portion of the cell cycle were significantly longer in the MHP than in L, and some MHP nuclei were located basally during these phases. Collectively, these results provide a positive correlation between cell wedging, resulting from basal expansion, and cell cycle alteration in the MHP. The second part was undertaken to evaluate the hypothesis that MHP characteristics develop owing to inductive interactions between the notochord and overlying neuroepithelial cells. It was found that midline neuroepithelial cells failed to develop MHP characteristics in the absence of the notochord and that the notochord was capable of inducing MHP characteristics in L cells. This strongly suggests that the notochord plays an inductive role in the formation of MHP characteristics. This investigation further revealed that the neural plate can undergo bending in the absence of both the notochord and wedging of MHP cells, forming a neural tube with an abnormal morphology. Thus, cell wedging does not drive bending, as is generally believed, but is necessary to establish the normal morphology of the neural tube.