Fig. 3. Characterisation of the DLP progenitor population. (A) The dorsal aorta and intra aortic haematopoietic clusters derive from the C3 blastomere. Transverse 10 µm wax section through the trunk at the level of the pronephric duct of a stage 43 embryo that had been injected with 250 pg ß-gal RNA in the C3 blastomere at the 32-cell stage. At stage 43, ß-gal is located in the endothelial wall of the dorsal aorta (red arrowhead) and in clusters of blood stem cells on the floor of the aorta (red arrow). Dorsal is towards the top. Black arrowheads indicate the pronephric ducts; n, notochord. (B) Timing of gene expression in the DLP. Expression profiles were obtained by analysis of embryos subjected to whole-mount in situ hybridisation. Arrows indicate expression before stage 18 (Xlim1) or after stage 36 (Xlim1, Xfli1, GATA3, Xmsr and XHex). (C) Analysis of gene expression in the DLP of the stage 26 embryo. Embryos were subjected to whole-mount double in situ hybridisation and then 10 µm transverse wax sections cut at the level of the pronephric duct in order to analyse gene expression. (a-c) Xlim1+Xfli1; (d-f) Xlim1 +SCL; (g-i) Xlim1+XHex; (j-l) Xlim1+GATA3; (m-o) Xfli1+SCL; (p-r) GATA3+Xfli1; (s-u) GATA3+SCL; (v-x) GATA3+XHex; (y-aa) GATA3+GATA2; (bb-dd) Xlim1+Xmsr, (ee-gg) GATA3+Xmsr. Inset, summary of gene expression analysis in the DLP. Dorsal is towards the top in all cases; in a,d,g,j,m,p,s,v,y,bb,ee, anterior is towards the left and lines indicate the levels where sections shown were taken.

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