Figure 1.

(a) Diagrammatic transverse sections to illustrate cell organization and behaviour that transform the neural plate through the stages of neural keel, rod and then neural tube. At all stages the neural primordium is covered by the overlying simple squamous epithelium of the enveloping layer (EVL, shown in yellow). Cells derived from the left hand side of the neural plate are shown in outline and cells derived from the right hand side are shown filled. The superficial pole of cells in the plate is marked with a red dot and the deep pole of these cells is marked with a black dot. Following the red dots through the process of the midline crossing division reveals that the superficial pole of plate cells is destined to become the basal extremity of the daughter that crosses the midline, and the black dots reveal that the deep pole of plate cells will become the basal extremity of the ipsilateral daughter. (b) The distribution of the apical protein Pard3-GFP (shown in green) has been added to illustrate the development of mirror-image apical polarity. In the late keel and rod stages Pard3-GFP is found accumulating at the cleavage furrow and close to the midbody of cells undergoing the C-division. Pard3-GFP remains around this location as the cells complete cytokinesis and is thus mirror-symmetrically inherited into the prospective apical ends of the daughter cells that go on to form the endfeet that line the lumenal surface of the neural tube.