Objective: The MRL mouse strain shows extraordinary wound healing capacities. Some years ago, Leferovich et al. (Proc Natl Acad Sci U. S. A. 2001;98:9830-35) have reported the absence of scar formation after cryogenically-induced right ventricular myocardial infarcts in adult MRL mice. An independent group (Oh et al., Cardiovasc Pathol 2004; 13:203-6) found that MRL mice repair left ventricular ischemic infarcts after coronary artery ligation with regular scar formation. Given the divergent outcomes in infarct healing in MRL mice reported by those two studies, we have investigated whether MRL mice heal myocardial infarcts, without scar both in the cryoinjury and in the coronary ligation model. Methods and Results: Four different protocols of cryogenically induced right and left ventricular injury, as well as permanent ligation of the left anterior descending coronary artery, were tested in adult MRL and control C57Bl/6 mice. At 60 days after experimental infarction, MRL mice showed pronounced scarring of the affected right and left ventricular areas, with no significant differences in infarct size and thickness between MRL and C57Bl/6 mice using any of the five experimental protocols. Analysis of cell proliferation by 5-bromo-2' -deoxyuridine (BrdU) incorporation into the DNA did not show any difference between the two strains of mice after infarction. Histological analysis of infarct areas using picrosirius red staining did not show differences in extent of collagen and distribution between the two mouse strains. Conclusions: NIRL mice heal myocardial infarcts with scar formation in response to ischemic as well as to cryogenic injuries. (c) 2008 Elsevier Inc. All rights reserved.