Fig. 7. The number of endothelial progenitors is increased in scl/tal1-lmo2-injected embryos. (A-F,J-M) flat mounts. Anterior, top in A-F, left in J-K. (G-I) Whole mounts; anterior, left. (N) Schematic of results. (A-F) In scl/tal1-injected (S inj.) 10 somite (14 hpf) embryos, the endothelial genes flk1 and flt4 were ectopically expressed only in the SPM (B,E; arrows). In scl/tal1-lmo2-injected (SL inj.) embryos, flk1 showed strong ectopic expression in the head (C, arrowhead) and in the heart region (C, bracket), while flt4 was expressed in a wider domain in the head (F, arrowhead) that was expanded posteriorly into the heart region, leaving a smaller gap between the anterior and posterior flt4⁺ domains (F, bracket). (G-I) At 22 hpf, the late endothelial marker tie2 was expressed in all ECs including the cells of the dorsal aorta (DA) and axial vein (AV, G). Tie2 was ectopically expressed only in the posterior of scl/tal1-injected (S) embryos (H, arrows), but all along the anteroposterior axis in scl/tal1-lmo2-injected embryos (I, arrows and arrowheads). (J-M) Most of the tie2⁺ cells were scl/tal1^end. negative, suggesting that they were differentiating ECs. tie2 (J) was normally expressed in all ECs of the axial vessels (DA and AV) and the head (black arrows), scl/tal1 (L) is expressed in erythrocytes (red arrow), blood and endothelial progenitors of the tail (purple arrow), ECs of the head (black arrows) and the cardinal veins (green arrows), as well as in ventral neurons of the spinal cord (blue arrow). Ectopic expression of tie2 was widespread anteriorly (K, arrowhead), but scl/tal1^end. expression was limited to its normal expression in bilateral patches in the head mesoderm (L,M, black arrows). In the trunk, scl/tal1^end. was expressed in ventrolateral cells (M, red arrows) similar to the ßE1⁺ blood cells. By contrast, tie2 expression was found medially (K, red arrow) as well as dorsally (I, black arrows). Only posteriorly, did co-expression mark early blood and endothelial progenitors (I,K,M, brackets). (N) In the absence of a blood-inducing intrinsic/extrinsic factor, Scl/Tal1- and Scl/Tal1-Lmo2-induced ectopic haemangioblasts in the head, heart and somitic mesoderm express flk1 and flt4 (blue) and develop into ECs, suggesting that endothelial development is the default state.