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Fig. 1 Pioneer and MFF precursors develop in close proximity to Laminin-rich extracellular matrices and Engrailed expression is absent from pioneers and MFFs in sly. (A,B) Transverse sections of 20 s stage wild-type (A) and sly embryos (B) at the level of early adaxial cell stacking. Dorsal is upwards. F59 (in green) labels adaxial cells and Laminin is in red; nuclei are counterstained with DAPI (in blue). (C-F) Transverse sections of 26 hpf wild-type (C,E) and sly (D,F) embryos at the level of the anterior tail (C,D) and the anterior trunk (E,F). (C,D) Laminin is in red. Owing to the somite chevron shape, vertical myosepta of the neighbour somite (blue arrows) are visible on these transverse sections. (C-F) 4D9 (in green) labels pioneers (arrowheads, C,E) and MFFs (arrows, E) in wild-type embryos and is absent in sly embryos (D,F). n, notochord; nt, neural tube. Scale bar: 50 μm.