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Fig. 2. Characterisation of the embryonic/aVBI progenitor population. Blood and endothelial genes are co-expressed in the anterior ventral mesoderm of the mid-neurula stage embryo. Expression of blood and endothelial genes in the ventral mesoderm of the stage 17 embryo was analysed by whole-mount in situ hybridisation (A) and in situ hybridisation on sections (B). Lineage labelling experiments (C) indicate that the blastomeres of the 32-cell stage embryo, which give rise to the aVBI, also give rise to the anterior ventral progenitor population. (A) Single whole-mount in situ hybridisation analysis of the expression of GATA2 (a), Xfli1 (b), SCL (c), Xaml (d) and XHex (e); and double whole mount in situ hybridisation analysis of the expression of SCL + Xfli1 (f) and SCL+ XHex (g) in the stage 17 Xenopus embryo. Anterior views in all cases with dorsal towards the top. Black arrows indicate SCL+Xfli1- cells in f and SCL+XHex- cells in g. (B) Expression of Xfli1 (b), SCL (c), Xaml (d,h), XHex (e), GATA2 (f) and the cement gland marker XCG1 (CG-1, g) in the ventral mesoderm of the stage 17 Xenopus embryo. (a) A sagittal section of the stage 17 embryo, red square indicates the area shown in photographs. (i) Summary of gene expression analysis. In situ hybridisation was performed on 10 µm sequential sections. (b-f) Sections from a single embryo and (g,h) sections from a second embryo. Anterior is towards the left and ventral is towards the bottom in all cases. (C) Lineage trace showing the origins of the ventral progenitor population and its fate. (a) Representation of the 32-cell stage embryo with the D1 blastomere highlighted in blue. (b) Embryo showing localisation of D1 progeny at stage 17 of development, anterior view with dorsal towards the top. (c) Sagittal section (10 µm) of stage 17, D1 ß-gal injected embryo. Red arrow indicates ß-gal positive cells located in the anterior ventral mesoderm (blood and endothelial progenitors). (d,e) Transverse 10 µm wax sections showing localisation of D1 progeny at stage 41. Lineage trace is seen in a mature vessel, the ventral aorta (red arrowhead, d and e), and in circulating blood cells within the vessel (red arrow, e). Lineage label was also seen in the endocardium and vitelline veins in seven out of seven embryos. ARCH, archenteron; CG, cement gland; NF, neural fold.