Cross-histological sections of the left (a), (b) and right (c), (d) pulmonary veins stained with Masson’s trichrome showing variations in circumferential thickness of the myocardial sleeves (arrows). Note the interpulmonary myocardial connections () between the superior and inferior veins and that the inferior veins tended to have less myocardial coverage than the superior veins. Longitudinal histological sections (e), (f) with Masson’s trichrome stain show the thicker atrial wall becoming thinner at the entrance of the left superior pulmonary vein to form the muscular sleeve, which tapers toward the lung. Note the presence of gaps of connective tissue bridges between the myocardial fibers (arrows) and small areas of myocardial degeneration with fibrous replacement (). LI = left inferior pulmonary vein, LS = left superior pulmonary vein, RI = right inferior pulmonary vein, and RS = right superior pulmonary vein.
