Enzymatically digested myocyte-depleted cardiac cell preparations resuspended in HBSS (supplemented with 2% FBS and 10 mΜ HEPES) were plated onto laminin-coated slides and incubated with Hoechst 33342 (H1399, Molecular probes). Thirty minutes later, the percentage of GFP+ cells was quantified using epifluorescence microscopy (Nikon, ECLIPSE TE2000). Enzymatically digested myocyte-depleted cardiac cell preparations (either freshly isolated or after FACS for the purification of GFP+ cells) were cytospun onto laminin-coated slides to undergo fluorescent immunocytochemistry. Fluorescent immunocytochemistry was performed with antibodies against GFP (ab13970, Abcam), α-sarcomeric actin (A7811, Sigma), α-myosin heavy chain (ab15, Abcam), NKX2-5 (ab35842, Abcam), GATA4 (ab84593, Abcam), MEF2C (ab64644, Abcam), TBX5 (ab101227, Abcam), Isl1 (ab20670, Abcam), Ki67 (RM-9106, Thermo Scientific), H3P (ab5176, Abcam), lacZ (ab9361, Abcam), Sca-1 (ab25195, Abcam), Nestin (ab6142, Abcam), c-kit (ab5506, Abcam), SSEA1 (ab16285, Abcam), CD34 (ab8158, Abcam), PDGFRα (ab61219, Abcam), CD31 (ab28364, Abcam), CD 45 (ab10558, Abcam), CXCR4 (ab2074, Abcam), VEGFR1 (ab32152, Abcam), VEGFR2 (ab2349, Abcam), and VEGFR3 (ab27278, Abcam). For histology, hearts were arrested with KCl solution, explanted, frozen in OCT compound, and sectioned in 6-μm sections on a cryostat. Cryosections were subsequently fixed with 4% paraformaldehyde. Quantitative morphometric analysis with Masson's trichrome staining was performed on 6-μm sections (four sections per heart at 500-μm intervals, starting at the level of LAD ligation). For each section, infarct size was determined as the percentage of LV area by manual tracing (ImageJ). Scar and viable myocardium volumes were calculated by multiplying scar and viable area for each slice with 500 μm (the sectioning interval). Scar and viable mass were quantified by multiplying total LV mass (measured at the time of heart explantation) with the volumetric fraction of scar and viable myocardium (expressed as percentage of total LV volume). Sections were stained with α-sarcomeric actinin (A7811, Sigma), GFP (ab13970, Abcam), lacZ (ab9361, Abcam), isolectin B4 (I21411, Molecular probes), connexin 43 (C6219, Sigma), CD45 (ab10558, Abcam), von Willebrand factor (ab11713, Abcam), α-smooth muscle actin (ab5694, Abcam), Ki67 (RM-9106, Thermo Scientific), NKX2-5 (ab35842, Abcam), GATA4 (ab84593, Abcam), and MEF2C (ab64644, Abcam). Border zone (Fig 8G) was defined as the area within one 20×-power field from the edges of the scar. In all fluorescent immunocytochemistry and immunohistochemistry experiments, Alexa Fluor-conjugated secondary antibodies (Molecular probes) were used and counterstaining with DAPI (P36931, Molecular probes) was performed. Sections were imaged using a confocal laser scan microscope (Leica Microsystems), and images were processed by Leica LAS software suite.