Definition and function of commissures, slits and scallops of the mitral valve: Analysis in 100 hearts

One hundred human hearts from autopsies were studied to clarify controversies in the literature about commissures, slits, chordae and leaflets of the mitral valve. This study revealed that no 2 hearts are alike in the morphology of commissures, slits and scallops. We have designated perpendiculars drawn from the annulus to the free edge at the shortest height of the mitral veil on either side of the aortic leaflet as the anterolateral (AL) and posteromedial (PM) commissural lines. These lines divide the mitral veil precisely into aortic and mural leaflets. Slits in the leaflets were easily identified by the dipping in of the free edge into the cusp tissue. There were no slits in the aortic leaflet. Ninety-eight hearts had slits in mural leaflet; in the AL halves in 81, PM halves in 76, and in the centre in one. The number of slits in the mural leaflet varied from one to five but was commonly two. Unlike the relatively static and straight annulus of the aortic leaflet, the curved annulus of the mural leaflet contracts and changes in contour during systole. Hence slits are necessary in the mural leaflet to help it fold and adapt to the reduced orifice during systole and unfold during diastole. In addition to this coarse adjustment of the mural leaflet, both leaflets are pleated due to “hooding up” of the leaflet tissue between chordal attachments during systole, providing the fine tuning to enable the leaflets to adapt themselves to the reduced systolic orifice. The nodular appearance (rather than a straight ridge) of the line of apposition of the leaflet is evidence of this pleating mechanism during valve closure. Slit lines, which we have designated as perpendiculars drawn from the summit of the slits to the annulus, were used arbitrarily to divide the mural leaflet into 2 to 6 scallops. When slits are absent there are no scallops. The slits and scallops are best serially numbered counterclockwise from the surgeon's view through the atrium. The scallops immediately behind the commissural lines, customarily labelled as the commissural scallops, are best defined as mural leaflet tissue between the AL or PM commissural line and the closest AL or PM slit line. AL commissural scallops were seen in 81 hearts and PM commissural scallops in 76 hearts. There is no consistent fan-shaped chordal pattern at the commissural lines or at the slits. However the chordae arising from each papillary muscle group, in toto, form a fan, reaching out to the corresponding adjacent halves of the 2 leaflets, restraining their splaying out in diastole and upward bulge during systole. These findings are relevant to the pathogenesis of mitral valvular disease and reparative procedures.