Murine hearts were harvested three weeks after matrix implantation to assess the occurrence and, if detected, the frequency of in vivo fusion. Hearts were bisected longitudinally through the matrix. The tissues were immediately placed into 10% buffered formalin (pH = 7.2; Fisher Scientific) for 24 h followed by 24 h of fresh 10% buffered formalin, and a final 24 h of 70% ethanol. Samples were further processed for paraffin embedding and sectioning as previously described [49] . Fluorescent in situ hybridization (FISH) tissue digestion kit with all human centromere probe (red) and all mouse centromere probe (green) (Kreatech, Amsterdam, the Netherlands) was performed on sections to detect fusion events. Samples were processed by the Cytogenetics Laboratory (WiCell Research Institute, Madison, WI, USA) according to manufacturer's protocol. Briefly, slides with paraffin embedded sections were baked for 4 h at 56 • C. Specimens were incubated with pepsin for 70 min for tissue digestion prior to sequential hybridization of the human probe followed by mouse probe. Images were acquired with a 60X UPlanSApo (NA = 1.35 Oil), DAPI, Green, and Orange filters, on an Olympus BX41 Upright Fluorescence Microscope (Olympus Valley, PA, USA), and analyzed with FISHView Version 5.5 software (Applied Spectral Imaging, Vista CA). Fusion events were defined as nuclei with positive staining for both human centromeres (red) and mouse centromeres (green). The frequency of fusion events was reported as the number of fusion events per total nuclei for a given region of the heart tissue: myocardium (myo), myocardial infarct (MI), border region (border), within Tissuemend patch (TM), and in the healthy myocardium (healthy). Five to twelve fields were selected for each location and the number of hearts analyzed were, n = 1 for TM only and n = 3 for TM + vMSC.