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Figure 1 Whole-mount confocal imaging of non-cleared (A-D, F-I) and cleared (E,J) adult (A-F) and juvenile (G-J) transgenic zebrafish hearts expressing the pan-endothelial marker fli1a:GFP (Green), venous/lymphatic endothelial marker flt4:mCitrine (blue: A-F) and lymphatic endothelial marker prox1:Gal4-UAS:RFP (red: E-J). (A) Through juvenile and young adult stages, flt4:mCitrine-expressing endothelial cells are restricted to the surface of the BA. (B) At 26 mm standard body length, sprouts from this BA population are first observed on the ventricle surface (arrowhead and insert). (C) These sprouts then extend down the ventricle and start to form a cardiac lymphatic vessel that eventually extends the entire length of the ventricle to the apex (D). The cardiac lymphatic vessel (D, inset, blue arrowhead) bridges the blood vessel (D, inset, green arrowhead) it accompanies. (E) Distinct from the venous endothelium of the SV, the BA population and the sprouts extending from it also express prox1:Gal4-UAS:RFP. (F) Both flt4:mCitrine and prox1:Gal4-UAS:RFP are maintained in the formed cardiac lymphatic vessel (arrowhead). (G) Larval and early juvenile zebrafish lack the prox1:Gal4-UAS:RFP expressing lymphatic endothelium on the BA. (H) This population is observed in juvenile zebrafish prior to the onset of and during (I) coronary vessel development. (J) In early adult stages, alignment of single cell sprouts off the lymphatic endothelium (red arrowhead) with blood vasculature (green arrowhead) is observed within the cleft between the BA and ventricle. Scale bars 200 μm (A-J) and 50 μm (insets, B, D, F), n ≥ 3 (A-J).