(A) Anterior view of P6 Cx40CreER; Rosa26TdTomato heart. Tamoxifen at P0, MI at P2, and harvested 4 days post-MI. Arteries are black, ligation in blue. Asterisks, right coronary artery (RCA) from posterior heart wall. (B) Boxed region from A. Arrowheads, collateral artery spanning watershed region. (C) Boxed region from B with non-coalesced arterial ECs in space between ligated left coronary artery (lig LCA) and non-ligated RCA branches. Pattern seen in n=8 hearts. (D) Anterior view of P11 Cx40CreER; Rosa26TdTomato. Tamoxifen at P0, ligated at P7, and harvested 4 days post-MI. (E) Boxed region from D reveal few lineage labeled artery cells in watershed and no collateral arteries. (F) Quantification at 4 days post-MI. Hearts: P2, n=12; P7, n=6. (G) Lectin perfusion in healthy and infarcted heart regions injured at P2 or P7, 4 days post-MI. (H) Quantification of mean fluorescence intensity. Hearts: P7, n=6, P2 n=6. Brch, branch; P, postnatal; EC, endothelial cells; MI, myocardial infarction. Scale bars: A and D, 500μm; B and E, 200 μm; G, 100μm. Error bars are st dev: *, p≤0.05; ****, p≤0.0001.