The atrioventricular junction is guarded by two masses of endocardial cushions, a superior and inferior cushion. These two masses meet in the middle resulting in the formation of the mitral and tricuspid orifices. The atria and the inlet portion of the ventricle enlarge, while the atrioventricular junction (or canal) lag behind. Such a process will cause the sulcus tissue to invaginate into the ventricular cavity, forming a hanging flap.
Formation of atrioventricular valves.
The endocardial cushion tissue is located at the tip of this flap, which are formed of 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 become undermined forming the tethering chords holding the newly formed valve leaflets. The inner sulcus tissue will eventually become in contact with the cushion tissue at the tip of valve leaflets, thus interrupting the muscular continuity between the atria and ventricles.
More Details
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 [16,
43].
The atrioventricular valves from by the process of invagination of
sulcus tissue. Undermining of the inlet myocardium forms the papillary muscles which
anchor the atrioventricular valve leaflets.
The atrioventricular junction is guarded by two masses of endocardial cushions—a superior and inferior cushion. These two masses will 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 [18], and the role of apoptosis in this process is debatable. The fusion of the two endocardial cushions results in the formation of two atrioventricular orifices. 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 toward the atrioventricular endocardial cushion and fuses with it [41].
The formation of the atrioventricular valve starts when the atria and inlet portion of the ventricle enlarge; 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, which is formed from three layers—the outer layer from atrial tissue, the inner layer from ventricular tissue, and the middle layer from 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 [16]