The identity and function of the fibroblast, a highly prevalent cell type in the heart, have remained poorly defined. Recent faithful genetic lineage–tracing studies revealed that during development, the cardiac fibroblasts are derived from the epicardium and the endothelium, whereas in the adult heart, they constitute the cardiac injury–responsive resident fibroblast. In the current issue of the JCI, Molkentin and colleagues decipher the time course and mechanism of the fibroblast in response to myocardial infarction (MI). The model they propose is surprisingly simple and clear. It consists of three major phases. First, fibroblasts in the ischemic area die. Second, surrounding fibroblasts proliferate and migrate into the spaces created by dying cardiomyocytes over a few days. The new fibroblasts in the scar are activated and adopt a smooth muscle actin– and periostin-positive “myofibroblast” phenotype, which appears to last as long as the scar is not mature (~10 days after MI). In the third phase, initially proliferating myofibroblasts lose smooth muscle actin expression and convert to a nonproliferating, matrix-producing phenotype with a newly acquired tendon gene signature. Interestingly, this state appears to differ from that of quiescent fibroblasts in the uninjured heart, as it is resistant to proliferative stimuli. These cells are therefore termed “matrifibrocytes,” a novel category whose study will certainly further advance the field.
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