Revisiting Cardiac Cellular Composition

Rationale: Accurate knowledge of the cellular composition of the heart is essential to fully understand the changes that occur during pathogenesis and to devise strategies for tissue engineering and regeneration. Objective: To examine the relative frequency of cardiac endothelial cells, hematopoietic-derived cells, and fibroblasts in the mouse and human heart. Methods and Results: Using a combination of genetic tools and cellular markers, we examined the occurrence of the most prominent cell types in the adult mouse heart. Immunohistochemistry revealed that endothelial cells constitute > 60%, hematopoietic-derived cells 5% to 10%, and fibroblasts < 20% of the nonmyocytes in the heart. A refined cell isolation protocol and an improved flow cytometry approach provided an independent means of determining the relative abundance of nonmyocytes. High-dimensional analysis and unsupervised clustering of cell populations confirmed that endothelial cells are the most abundant cell population. Interestingly, fibroblast numbers are smaller than previously estimated, and 2 commonly assigned fibroblast markers, Sca-1 and CD90, under-represent fibroblast numbers. We also describe an alternative fibroblast surface marker that more accurately identifies the resident cardiac fibroblast population. Conclusions: This new perspective on the abundance of different cell types in the heart demonstrates that fibroblasts comprise a relatively minor population. By contrast, endothelial cells constitute the majority of noncardiomyocytes and are likely to play a greater role in physiological function and response to injury than previously appreciated.