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Fig. 2
drl transgenic reporters track cardiac development and distinguish FHF from SHF myocardium.
(a,b) Confocal projection of the anterior lateral plate mesoderm (ALPM) with hatching gland most anteriorly at 16 somite stage (ss) marked by drl:EGFP, dorsal view (a, green in b) and lmo2:dsRED2 (endothelial and haematopoietic precursors in red, b); asterisks in b indicate migrating myeloid precursors, solid arrowhead indicates merging endocardium precursors, and open arrowheads indicate positions of presumptive myocardium precursors; anterior to the top and posterior to the bottom; scale bar, 100 µm. (c–f) Confocal projections of the linear heart tube at 26 h.p.f., anterior to the left, arterial pole (AP) and venous pole (VP) indicated, (c,d) top-down 2-µm projection, (e,f) transversal optical 2-µm section through heart tube. drl expression (drl:EGFP counter-stained with anti-GFP) in both the central endocardium and the myocardium (counter stained with anti-myosin/MF20 in red, and 4,6-diamidino-2-phenylindole (DAPI) in blue for nuclei in d,f; scale bar, 10 µm. (g,h) 56 h.p.f. two-chambered hearts transgenic for drl:EGFP top-down 2-µm confocal projection counter stained for anti-GFP (g,h), anti-myosin/MF20 in red, DAPI in blue for nuclei in h. EGFP is expressed in outer ventricular curvature and part of atria, with dotted lines indicating the margins of expression domains, the solid arrowhead point to drl:EGFP-negative regions at sinoatrial node (SA) and distal ventricle including outflow tract in g; scale bar, 10 µm. (i–k) Ventral view of a two-chambered heart from a drl:EGFP; ltbp3:TagRFP2Acre double-transgenic embryo at 72 h.p.f.; at this stage, drl:EGFP-positive FHF-derived cardiac cells (i, green in k) comprise central portion of the cardiac tube, while the SHF-derived distal ventricular cardiomyocytes population (arrowhead in i) is labelled by the ltbp3:TagRFP2Acre transgene (j, red in k); scale bar, 50 µm. (l,m) drl:creERT2 × myl7:loxP-Amcyan-loxP-ZsYellow (schematic) induced at shield stage labels all myocardial descendants at 72 h.p.f. including the outflow tract cardiomyocytes in the distal ventricle (arrowhead, l); scale bar, 50 µm. Atrium (A) and ventricle (V) labelled in g,i,l.