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Fig. 1
The anterior borders of the neural plate, the prechordal plate and the opl expression domain colocalize at 80% epiboly. (A-C) Dorsal views, anterior to the top, 80% epiboly, white arrowheads indicate midline; (A-B), Nomarski optics. (A) Dorsal view of live embryo focused at the level of the ectoderm. The anterior border of the neural plate is apparent as a thickening of the ectoderm (black arrowhead). (B) A deeper focal plane view of the embryo pictured in A. Cells with filopodia (white arrows) are apparently migrating at the anterior edge (black arrowhead) of the axial mesendoderm. (C) An ectodermal cell was labeled with lineage tracer (arrow, red cell) superficial to the anterior edge of the migrating axial mesendoderm (black arrowhead), and the embryo was immediately fixed and hybridized with RNA probes for opl (black expression domain) and otx2 (red expression domain). This demonstrated that at 80% epiboly, the anterior edges of the axial mesendoderm, the neural plate, and the opl expression domain coincide. (D) Distance between the anterior edge of the axial mesendoderm and the anterior edge of the neural plate between the 80% epiboly and 1-somite stages. The distance between these two morphological landmarks can be used for precise staging of the embryos as the axial mesendoderm advances relative to the anterior edge of the neural plate and the tailbud. 80% epiboly, n=9; tailbud stage, n=14; 1-somite stage, n=10. Distance = average ± s.d. in μm. 1 Box, the length of a box in the 10x10 reticule used for the fate map (see also Fig. 2B). Time scale is approximately linear with developmental time. Scale bars, 50 μm (A-B), 200 μm (C).