BM transplantation and quantification of engraftment were performed (see supplemental Materials and methods) using Tie2/GFP mice as donors. Myocardial infarction was induced in recipient mice under artificial ventilation by permanent ligation of the middle of the left anterior descending (LAD) coronary artery. Mice were randomized to receive i.v. injection of either 200 μg of anti–α4 integrin Ab or control IgG, twice weekly starting on day 1. On day 14, the mice were injected with 50 μl of BS lectin I–Rhodamine (Vector Laboratories) at the apex of the left ventricle (LV), and after 5 min the cardiac vasculature was perfused with 4% PFA through the right carotid artery with distal aortic arch clamped. Cardiac tissue was fixed for 1 h in 4% PFA, incubated in 30% sucrose solution overnight, snap frozen in liquid nitrogen, and preserved at −80°C. Serial cryosectioning was performed starting at 1 mm below the suture (used to ligate the LAD) moving toward the apex, with three consecutive sections per 1 mm to allow for quantitative pathohistological analysis at each level (see next paragraph). Three sections per ischemic heart and 9 fields per section (6 fields in the infarct border zone, 3 fields in the infarct area) were examined with BS lectin I–Rhodamine+ to quantify total capillary density or with Rhodamine+GFP+ to determine BM EPC-derived capillary density. Masson's Trichrome staining was performed as previously described (64 (link)). The fibrosis area was calculated as the ratio of the length of fibrotic area to the length of LV inner circumference (Fig. 5 E, d/c), and the LV dimension was quantified histologically (Fig. 5 E, (a+b)/2). All surgical procedures and patho/histological analysis was performed by investigators blinded to treatment assignment.