Fig. 5. The earliest her5-positive domain at gastrulation contains an ordered distribution of MH precursors and prefigures the later MH domain. (A) Experimental approach. The earliest her5-positive domain (schematized in blue on a dorsal view of the neural plate at 70% epiboly, left panel) is reflected by GFP protein expression starting at 95% epiboly (green, right panel). Thus, the anterior and posterior extremities of the early her5-positive domain were fate mapped by laser activation of caged fluorescein within the most anterior or posterior GFP-positive cell rows at 95% epiboly (yellow and red dots, respectively). (B-E) Location of cells activated in A, revealed at 24 hpf by whole-mount anti-fluorescein immunocytochemistry (brown staining) (all embryos anterior leftwards, with black arrow to the midbrain-hindbrain boundary). (B,C) Anterior activations give rise to cell clones distributing within the anterior midbrain (two different embryos are shown, brackets to the midbrain, yellow arrows to delimit the cluster of uncaged cells). (D,E) Posterior activations produce cell clones located posterior to the midbrain-hindbrain boundary and populate r2 (two different embryos are shown, brackets to r1 and r2, red arrows to delimit the cluster of uncaged cells). mid, midbrain; r1-2, rhombomeres 1-2.

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