Click on image to view larger version.

Figure 3 Longitudinal section of a human left superior pulmonary vein from a patient with atrial fibrillation, stained with Masson's trichrome, showing loosely coupled atrial cardiomyocytes (red/brown) with high degree (>50% of atrial myocardium) of intercellular fibrosis (blue; arrows). (Bottom side of figure represents luminal side of the vein.)