Figure 1. Complexity of Left Atrial Appendage Morphology Shown are endocardial views of the left atrial appendage from post-mortem specimens of human hearts. Left panel: The left atrial appendage ostia have more of an elliptical, rather than round shape. Middle panel: An endocardial view of that shows the relationship of the left atrial appendage ostia, neck, pectinate muscles, and outpouching structure is shown. The smooth left atrium and orifices of pulmonary veins are seen in the figure in relation to the appendage. RA; right atrium. LA; left atrium. White arrows; outpouching structure of the appendage where the neck extends outwards to a 3D body. Right panel: An endocardial view of the left atrial appendage showing the complexity of a multi-lobed structure, 3D shape, and prominent pectinate ridges. LAA; left atrial appendage. White arrows; point towards multiple lobes and pectinate muscles.