After myocardial infarction (MI), which causes a substantial loss of cardiomyocytes, fibroblasts are activated, proliferate, and differentiate into extracellular matrix (ECM)-producing myofibroblasts that contribute to the formation of a fibrotic scar in the wounded area. In the non-regenerative heart, the scar persists, stabilized by fibroblasts that revert to a quiescent state. A new study provides further insights into the phenotypic changes of cardiac fibroblasts during scar formation." Fibroblasts are absolutely critical for acute healing of the heart after injury," explains lead study investigator Jeffery Molkentin," yet at the same time, overzealous long-term activation of fibroblasts in more chronic heart disease states can hasten the demise of the heart." Molkentin remarks that studies into heart diseases have now started to focus on fibroblasts, with the recent advent of new genetic tools to investigate this cell type.

In this study, Molkentin and colleagues used three inducible, lineage-tracing mouse models, Tcf21-MerCreMer, Postn-MerCreMer, and Acta2-CreER, to reliably identify fibroblast dynamics and differentiated stages in the heart after MI. Analysis of the Tcf21-MerCreMer reporter system, which targets the majority of the tissue-resident cardiac fibroblasts, showed that fibroblast proliferation occurred in the first week after MI in the infarct and border regions, reaching a peak of 3.5-fold expansion at 2-4 days compared with uninjured areas. The fibroblast content in injured regions persisted for weeks, suggesting that these new fibroblasts are robust and do not undergo cell turnover. Activated fibroblasts, labelled using the Postn-MerCreMer system, were shown to derive from Tcf21-traced cells 4 days after MI. Postn-traced cells had similar proliferation dynamics as Tcf21-labelled cells, expressed smooth muscle [alpha]-actin ([alpha] SMA)--a myofibroblast marker encoded by Acta2--at 4-7 days after MI, and showed no significant proliferation rates or [alpha] SMA expression 2 weeks after MI. Acta2-lineage tracing confirmed that myofibroblasts appear at day 3-4 after MI, and lose [alpha] SMA expression by 10 days after MI. Altogether, these findings demonstrate that myofibroblasts are not a long-lasting differentiated state of fibroblasts, and that fibroblasts transition to an alternate stage refractory to proliferation by 10-14 days after MI. Histological analysis of the scar also revealed that the loss of [alpha] SMA expression and myofibroblast phenotype preceded the reorganization of the ECM associated with scar maturation, suggesting that myofibroblasts are a transient cellular support network that preserves ventricular wall integrity before collagen accumulation.