Rush Center for Congenital
and Structural Heart Disease




Atrioventricular canal

The atrioventricular valves form during the fifth to eighth week of development [26].  Initially, endocardial cushion tissue forms bulges at the atrioventricular junction.  These bulges have the appearance of valves, and although such tissue may play an important role in the eventual formation of the atrioventricular valves, endocardial cushion tissues are not the precursors of the mitral and tricuspid valves’ tissues [362, 377].

The atrioventricular junction is guarded by two masses of endocardial cushions, superior and inferior cushions.  These two masses meet in the middle, thus dividing the common atrioventricular canal into right and left atrioventricular orifices.  The process through which these two cushions fuse is not clear and the role of apoptosis in this process is debatable.   In addition, the atrioventricular cushion appears to play a role in the closure of the interatrial communication at the edge of the primum atrial septum.  This septum grows towards the atrioventricular endocardial cushion and fuse with it. [366]

The formation of the atrioventricular valves starts when the atria and inlet portion of the ventricle enlarge, while the atrioventricular junction (or canal) lags behind.  Such a process causes the sulcus tissue to invaginate into the ventricular cavity, forming a hanging flap.  The endocardial cushion tissue is located at the tip of this flap.  The hanging flap is formed from three layers at this point:  The outer layer from atrial tissue, the inner layer formed by ventricular tissue and the inner layer by invaginated sulcus tissue.  The inlet portion of the ventricles then becomes undermined forming the tethering cords holding the newly formed valve leaflets.  The inner sulcus tissue will eventually come in contact with the cushion tissue at the tip of valve leaflets, thus interrupting the muscular continuity between the atria and ventricles (Figure 8) [362]. 



Figure 8

Formation of atrio-ventricular valves:  The endocardial cushions initially serve as valves; however, the mitral and tricuspid valves’ tissue are derived from invagination of sulcus tissue into the atrioventricular junction.  The valve leaflets are eventually formed through a process of undermining of ventricular inlet myocardium to separate and form leaflet, chordal and papillary muscle tissues.