Scar formation after ischemic myocardial injury in MRL mice.

INTRODUCTION MRL is a strain of mice with the unusual ability to recover from injuries without forming scar tissue. It was recently reported that the MRL mice can recover with little or no scar formation following a cryogenically induced myocardial infarction of the right ventricle. These findings suggest that scarless recovery from myocardial injury is possible in mammalian ventricles. If applicable to humans, these results could have significant clinical implications in the management of patients with myocardial infarction. However, myocardial infarction in human patients usually results from ischemic injury instead of cryoinjury. Myocardial infarction usually occurs in the left ventricle. METHODS We performed a prospective study creating left ventricular myocardial infarction in 32 MRL mice using coronary ligation. RESULTS Among them, 21 mice survived surgery and had myocardial infarction documented by delayed contrast-enhanced magnetic resonance imaging (MRI) 35+/-10 days after surgery. All 21 mice had evidence of myocardial infarction in follow-up MRI performed 116+/-21 days after surgery. The infarct size was 27+/-12% of the area of visualized myocardium during the first MRI study and was 29+/-11% during the second MRI study (P=NS). Large myocardial infarction with aneurysm formation was documented by histological examinations in all 21 mice. The ventricular aneurysm showed transmural infarction with replacement fibrosis and mummified necrotic myocardium surrounded by collagenous scar tissue. Occasional surviving myocytes are present within the aneurysm. CONCLUSION Ischemic myocardial injury in MRL mice is not followed by scarless recovery.