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Fig. 3 Ectopic endothelial cells can influence thyroid morphology cell non-autonomously. Lateral views (A,B) and cross-sections (C-I) through the head region. (A,C) In wild-type embryos, bilateral groups of kdr domains (arrowheads) mark part of the anterior (head) lateral plate mesoderm that give rise to vascular structures. (B,D) Co-injection of scl and lmo2 mRNA leads to massive upregulation of kdr in the head mesenchyme of zebrafish embryos. (E) Experimental procedure: donor cells from scl and lmo2 co-injected embryos were grafted into wild-type embryos. (F,G) Donor cells have all developed into endothelial cells when found in the head mesenchyme of host embryos. Arrows indicate kdr expression (blue) overlapping with the cell marker biotin (brown) present in grafted cells. Endogenous kdr expression (arrowhead in F) is variably visible on sections, depending on the exact level of sectioning (compare with A). (H) At 55 hpf, grafted scl+lmo2 cells (brown, arrows) have caused the thyroid primordium (blue) to expand laterally. One of the four embryos where grafted cells were detected adjacent to the thyroid primordium. (I) An embryo where gfp injected donor cells are found close to the thyroid, which is not expanded towards the grafted cell. In H and I, care was taken to chose the section through the median level of the thyroid, and diameters were measured on the base of these sections. In I, nuclei in the thyroid appear more prominent than in H, so that the cytoplasmic nk2.1a signal looks slightly weaker on this median section. Broken lines indicate the border of the neural tube (nt) in C,F,G, and the midline in H,I. 12 ss, 12 somite stage.