Septum Formation In The Truncus Arteriosus And Conus Cordis

During the fifth week, pairs of opposing ridges appear in the truncus. These ridges, the truncus swellings, or cushions, lie on the right superior wall (right

Truncus Arteriosus Spiral Septum

Figure 11.20 A. Persistent common atrioventricular canal. This abnormality is always accompanied by a septum defect in the atrial as well as in the ventricular portion of the cardiac partitions. B. Valves in the atrioventricular orifices under normal conditions. C. Split valves in a persistent atrioventricular canal. D and E. Ostium primum defect caused by incomplete fusion of the atrioventricular endocardial cushions.

Figure 11.20 A. Persistent common atrioventricular canal. This abnormality is always accompanied by a septum defect in the atrial as well as in the ventricular portion of the cardiac partitions. B. Valves in the atrioventricular orifices under normal conditions. C. Split valves in a persistent atrioventricular canal. D and E. Ostium primum defect caused by incomplete fusion of the atrioventricular endocardial cushions.

superior truncus swelling) and on the left inferior wall (left inferior truncus swelling) (Fig. 11.17). The right superior truncus swelling grows distally and to the left, and the left inferior truncus swelling grows distally and to the right. Hence, while growing toward the aortic sac, the swellings twist around each other, foreshadowing the spiral course of the future septum (Figs. 11.22 and 11.23). After complete fusion, the ridges form the aorticopulmonary septum, dividing the truncus into an aortic and a pulmonary channel.

Conus Cordis
Figure 11.21 A. Normal heart. B. Tricuspid atresia. Note the small right ventricle and the large left ventricle.

When the truncus swellings appear, similar swellings (cushions) develop along the right dorsal and left ventral walls of the conus cordis (Figs. 11.17 and 11.23). The conus swellings grow toward each other and distally to unite with the truncus septum. When the two conus swellings have fused, the septum divides the conus into an anterolateral portion (the ouflow tract of the right ventricle) (Fig. 11.24) and a posteromedial portion (the outflow tract of the left ventricle) (Fig. 11.25).

Neural crest cells, migrating from the edges of the neural folds in the hind-brain region, contribute to endocardial cushion formation in both the conus cordis and truncus arteriosus. Abnormal migration, proliferation, or differentiation of these cells results in congenital malformations in this region, such as tetralogy of Fallot (Fig 11.29), pulmonary stenoses, transposition of the great vessels and persistent truncus arteriosus (Fig 11.30). Since neural crest cells also contribute to craniofacial development, it is not uncommon to see facial and cardiac abnormalities in the same individual.

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