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Fig. 2
(A) Prior to ventricle expansion cells at dorsal midline of the neural tube are irregular and overlap the midline, while slightly deeper columnar cells align their apical end feet to the midline. (B) Pard3-GFP (yellow) is unpolarised in irregular dorsal cells but already polarized to the apical pole of columnar neuroepithelial cells that lie just deep to the dorsal surface of the neural tube. Transverse plane view of neural tube indicates level of sections in (A) and (B, C) Schematic diagrams of neural tubes show location of cells illustrated in panels (D) to (G, D) Three frames from time-lapse starting at 18hpf show two superficial cells at the dorsal midline (dashed line in drawings) initially overlap but later share a contiguous border that maps to the geometric midline just prior to opening of the ventricle. (E) Three frames from time-lapse show two spikey midline cells at dorsalmost aspect of neural tube transform directly into squamous roof plate cells as the ventricle opens over 4 hr. Dotted lines in drawings represent borders of roof plate (see Figure 2—video 1). (F) Four frames from time-lapse starting at 21hpf show irregular dorsal midline cell (nucleus marked by yellow dot on left hand images and cytoplasm filled yellow in adjacent right hand images) migrating along the dorsal tube midline to seed squamous roof plate as ventricle opens. A second roof plate pioneer is marked with blue dot and blue cytoplasm. Dotted lines in images show borders of roof plate and lateral edges of neural tube (see Figure 2—video 2). (G) Pard3-GFP protein is progressively allocated to cell-cell junctions surrounding a squamous roof plate cell during the opening of the ventricle. Scale bar in (G): 20 μm.