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Fig. 7 A two-step model for zebrafish ectodermal patterning. Step 1 depicts the DV gradient present during gastrulation. High BMP activity is observed ventrally as generated by bmp2b/4/7a ligand expression and reinforced by Cvl2. BMP activity is inhibited dorsally by expression of the diffusible antagonists Chordin and Noggin. Prospective PPE and NC are formed at intermediate levels of BMP signalling. In step 2 the initial gradient has reshaped into distinct BMP activity domains during neurulation. The regulatory network is depicted. Components in black act positively, whereas those in red act negatively. NC, neural crest; PPE, preplacodal ectoderm.