Septation and Valve Formation
Proper positioning and function of the valves is critical for chamber formation and proper blood flow. The endocardial cushion serves as a makeshift valve until then.
The truncus arteriosus and the adjacent bulbus cordis partition by means of cells from the neural crest. Once the cells from the truncal ridge meet with the cells from the bulbar ridge they twist around each other in a spiral orientation as they fuse and form the aorticopulmonary septum. This will end dividing the aorta from the pulmonary trunk. Defects in this process produces persistent truncus arteriosus, unequal division of the truncus arteriosus, transposition of the great arteries, aortic and pulmonary valve stenosis or tetralogy of fallot.
The primitive atrium is divided in two by joining of several structures. From the roof of the primitive atrium descends the septum primum, which grows towards the endocardial cushions within the atrial canal. Right before the septum primum fuses with the endocardial cushions there's a temporary space called the foramen primum. Once they fuse a new opening forms in the middle of the septum primum called the ostium secundum or foramen secundum. To the right of the septum primum and also coming down from the roof of the primitive atrium, descends a semilunar-shaped partition called the septum secundum. The free edges of the septum secundum produce and orifice called foramen ovale, which closes after birth when the septum primum and secundum fuse to each other completing the formation of the atrial septum. Defects of this process produces various atrial septal defects.
The atrial canal is in turn divided into a right and left side by the atrioventricular septum, which originates from the union of the dorsal and ventral endocardial cushion. The right side of the atrial canal will become the tricuspid valve and the left will become the bicuspid valve. Defects in producing the AV septum produces atrioventricular septal defects, including a persistent AV canal and tricuspid atresia.
The floor at the midline of the primitive ventricle produces the interventricular septum, separating the chamber in two. The IV septum grows upward towards the endocardial cushion. As it grows, a foramen appears, the interventricular foramen, which later is closed by the non-muscular IV septum. Defects in producing the IV septum causes ventricular septal defects, which communicate both ventricles.
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