(A) Experimental design where Cx40CreER-labeled arteries are red.(B) Confocal image of a watershed area from P2 control heart. Capillary bed (cyan) is fed by left (LCA) and right (RCA) coronary artery branches. (C and D) Confocal images (anterior views) of control (C) and myocardial infarction (MI) (D) hearts. Arterial ECs are black. MI induced collateral arteries that connect ligated (lig) branches (brchs) of LCA with RCA branches across the watershed area (boxed regions). Asterisks are RCA showing through from the posterior heart wall. (E) Schematic of how collateral arteries restore blood flow (arrows) to injured myocardium. Arteries, red; ligation, blue. (F) Some Cx40CreER-labeled collateral arteries contain smooth muscle. (G and H) Quantification of Cx40CreER-labeled (G) and smooth muscle covered (H) collateral arteries. Hearts: n=8 control, n=12 MI. (I) Collateral arteries were similar in diameter to quaternary (4֯) branches . Arteries: N=24 CX40+, n=9 αSMA+. (J) Collateral arteries were perfused. Cap, capillaries; P, postnatal; Tam, Tamoxifen; EC, endothelial cells; TdTom, TdTomato. Scale bars: B, 200μm; C, D low mag, 500μm; C, D boxed region, 200μm; F and J, 200μm. Error bars are st dev: ***, p≤0.001; ****, p≤0.0001.