CCM images were from fixed rodent tissue preparations acquired using established methods (doi: 10.1161/circimaging.112.000121). In short, rodent hearts were isolated and Langendorff-perfused with 4% paraformaldehyde. Tissue from the atrial working myocardium, sinoatrial node, and atrioventricular node were dissected from the hearts and fluorescently labeled with wheat germ agglutinin conjugated to CF488A (29022-1; Biotium, Hayward, CA; 1:25). Fluorescently labeled tissue preparations were imaged using a conventional laser-scanning confocal microscope (Zeiss LSM5 Duo; Zeiss, Jena, Germany) with a 40x oil immersion lens (NA 1.3). The fluorescent signal was captured using an Argon/2 ion laser (488 nm excitation) and a bandpass filter (505 to 555 nm emission). Three-dimensional image stacks were acquired at a spatial resolution (xyz dimensions), field of view (xy) and a z-scan range of 0.2x0.2x0.2 μm, 204.8x204.8 μm and 50 μm, respectively. Example cross-sections through the image stacks were stored as CCM images.