Two main theories exist for the explanation of univentricular hearts. One states that the bulboventricular septum becomes realigned to form the interventricular septum, and that univentricular hearts are a consequence of failure of this realignment. The other states that bulboventricular and interventricular septa are different structures, and that the univentricular heart results from failure of formation of the posterior interventricular septum. Four hearts are described in which both the posterior septum and an anterior bulboventricular septum are present. In each heart, therefore, the right ventricular sinus is separated both from the left ventricular sinus and from a discrete outlet chamber which supports the pulmonary artery. It is argued that these findings militate strongly against theories proposing reorientation of the bulboventricular septum to form the definitive interventricular septum. They support strongly the concept that the definitive right ventricle is formed in part from the bulbus and in part from the primitive ventricle. On the basis of these findings, it is suggested that the distinctive feature of the univentricular heart is absence of the posterior septum. Such hearts can properly be termed 'primitive ventricle'. It is also suggested that hearts with atretic or straddling valves should be included within this category.
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