Abstract 174: The Origin Of Cardiac Fibroblasts During Normal Development And After Injury

Abstract

Fibroblasts are the most abundant cells in the heart. They play a critical role in maintaining normal myocardial function as well as mediating fibrosis that occurs upon pathological injury. However, the source of cardiac fibroblasts during development and after the injury in the adult heart remains poorly characterized. The lack of cardiac fibroblast-specific markers has limited our ability to isolate or track them during normal development and in response to cardiac injury. Here, we define cardiac fibroblasts as mesenchymal cells with a surface-marker profile of CD45-/CD31-/Ter119-/CD11b-/Thy1.1+ /Thy1.2+. We used lineage tracing, transplantation studies, and parabiosis surgeries to identify the cellular origins of cardiac fibroblasts and their contribution to fibrosis during pathological injury induced by pressure overload. We show that fibrosis is associated with proliferation of fibroblasts within 7 days after pressure overload injury with no evidence of bone marrow or hematopoietic stem cells contributing to the cardiac fibroblast population. We then used the transgenic mouse models Tie2cre;mTmG, Tbx18cre;mTmG, and Pax3cre;mTmG to lineage trace the origin of fibroblasts from the epicardium, endothelial and the neural crest, respectively. By using fluorescent activated cell sorting (FACS) and immunostaining, we demonstrate that the majority of cardiac fibroblasts are derived from the epicardium, a minority arises from endothelial cells, and a small fraction from the neural crest. Although we observed an increase in the total number of fibroblasts after pressure overload injury, this response was not restricted to any single developmental subset. In summary, our findings show that the majority of the cardiac fibroblasts, during normal development as well as in response to pathological injury, are derived from the epicardium via epithelial-to-mesenchymal transformation. Endothelial to mesenchymal transition and neural crest sources also contribute to fibroblast generation and the progression of pressure overload-induced cardiac fibrosis.