Murine cardiac fibroblast stages and function. Cardiac fibroblasts are derived from epicardial and endocardial progenitors after embryonic day 12.5 (E12.5). Tcf21 and PDGFRα direct fibroblast development at this stage. Fibroblast progenitors enter the ventricles and proliferate in the first week after birth. It is at this time that fibroblasts begin to deposit and degrade extracellular matrix. In the uninjured adult heart, proposed roles for fibroblasts include secretion of trophic factors, ECM surveillance, conduction system insulation, cardiomyocyte electrical coupling, and vascular maintenance. During the injury phase, fibroblasts proliferate, deposit ECM, and recruit inflammatory cells. Recent data has shown that after the proliferative phase of injury, previously activated cardiac fibroblasts can revert to a resting fibroblast gene profile. EMT; epithelial-to-mesenchymal transition.