Human atrial fibroblasts and their contribution to supraventricular arrhythmia

The healthy heart is heavily populated with fibroblasts. The mouse heart contains 56% cardiomyocytes, 27% fibroblasts, 7% endothelial cells, and 10% vascular smooth muscle cells, based on a study where hearts were enzymatically digested and the cells immunolabeled and sorted according to fluorescence, FACS (Banerjee et al. 2007). Another study from rat heart reports only 30–35% cardiomyocytes and 65–70% nonmuscle cells (Nag 1980). Despite their high numbers, the fibroblasts constitute less than 25% of the mass of the heart, and the majority of the mass comprised cardiomyocytes (Vliegen et al. 1991). The relative size of the fibroblasts is small; cell sizes measured as electrical capacitance are approximately 15 pF for freshly isolated fibroblasts (Poulet et al. 2016) compared to 90 pF atrial cells and 190 pF of ventricular cells (Calloe et al. 2013). The cardiac fibroblasts secrete the majority of the extracellular matrix proteins, like collagens, laminin, proteoglycans, and fibronectin, and are pivotal for keeping the three‐dimensional structure of the heart