Elastin and fibronectin distribution in the SAN relative to LV.

<p>Right atrium containing SAN and LV were immunostained for elastin (<i>top</i>) or fibronectin (<i>bottom</i>) and hyperpolarization-activated cyclic nucleotide-activated (HCN)4 channels for identification of SAN region with pacemaking cardiomyocytes. The images in the top row for each protein show the SAN staining in a single channel for the specified ECM protein only. Fluorescence intensity for region marked by a dotted white line in the 60X SAN and LV images were plotted against HCN4 fluorescence in binary scale to indicate 1 or 0 for pacemaking cardiomyocytes (PCMs) or no pacemaking cells, respectively. Elastin was predominantly in the perimysium between clusters of pacemaking cardiomyocytes in the SAN. Fibronectin was in similarly distributed between the perimysium and endomysium surrounding the pacemaking cardiomyocytes. Scale bars: 400 μm (20X) and 50 μm (60X).