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Fig. 1 osr1 expression during early gastrulation. (A,B) osr1 expression in the germ ring at 30% epiboly (A) and in the gastrulating cells at 75% epiboly (B). (C) Two-color in situ of osr1 (blue) and ntl (red) mRNA transcripts at 30% epiboly. (D) Cross-section of C at the level indicated shows that osr1 is not expressed in the ntl-positive mesodermal cells farther from the margin (red arrowhead) and is restricted to mesendoderm cells closest to the margin (blue arrowhead). (E) Colocalization of osr1 (blue) and sox32 (red) mRNA transcripts at 30% epiboly. (F) Cross-section of E at the level indicated shows osr1 expression in the mesendoderm cells with some overlap with sox32-positive endodermal cells (blue arrowhead) but the absence of osr1 expression in the sox32-positive YSL cells (red arrowhead). (G-O) Magnified lateral views of 60% epiboly embryos just above the blastoderm margin with dorsal on the right. (G-I) Double-fluorescent in situ hybridization of osr1 (red) and ntl (green) probes at 60% epiboly. The images represent a maximum intensity projection of a confocal z-series stack (four slices, 1.9 μm each). (I) Merge of G and H showing distinct expression of osr1 (G) in endoderm cells (red arrowhead, G,I) that do not express ntl (H). osr1 (G) is co-expressed with ntl (H) in mesendoderm cells (green arrowhead, H,I). (J-L) Double-fluorescent in situ hybridization of osr1 (red) and sox17 (green) at 60% epiboly. Images represent a single confocal slice of 1.9 μm. (L) Merge of J and L showing co-expression of osr1 and sox17 in endoderm cells. (M-O) Double-fluorescent in situ hybridization of osr1 (red) and sox32 (green) at 60% epiboly. Images represent a single confocal slice of 1.9 μm. (O) Merge of M and N showing co-expression of osr1 and sox32 in endoderm cells. Scale bars: in G,I,L, 10 μm for G-O.