Morphogenesis The findings regarding cardiac development now help in understanding the development of the hearts having univentricular atrioventricular connections As described in Chapter 3, during early development, the orifice of the atrioventricular canal is supported almost exclusively above the inlet part of the primary heart tube, with the presumptive arterial pedicles supported by the outlet component During this early stage, furthermore, the apical component of the developing left ventricle balloons from the inlet component of the ventricular loop, whereas that of the developing right ventricle balloons from the outlet part (Fig 69.23) The ballooning of the two apical ventricular components occurs concomitant with development of the apical muscular septum If the two pouches did not grow separately but rather there was formation of a general apical component from the primary heart tube, the end result would be to produce a solitary ventricle of indeterminate morphology Whether the atrioventricular junctions are guarded by two valves or a common atrioventricular valve depends on the partitioning of the atrioventricular junction Should the pouches form in normal fashion but the atrioventricular junction remains connected only to the inlet part of the ventricular loop, the end result would be double-inlet left ventricle The apical component derived from the outlet part of the loop forms the basis of the incomplete right ventricle (Fig 69.24) Again, the arrangement of the atrioventricular valves would depend on the mode of development of the atrioventricular junction The ventriculoarterial connections present would depend on the development of the outlet portions The position of the incomplete right ventricle would probably be determined by the initial looping of the primary tube, but equally it could be influenced by rotation of the entire heart FIG 69.23 The developing heart (middle) can develop abnormally so as to produce a solitary and indeterminate ventricle (left) or a heart having dominant right or left ventricle (RV/LV) (right) FIG 69.24 Image prepared from an episcopic dataset made from a human embryo at Carnegie stage 14 The apical component of the right ventricle is beginning to balloon from the outlet component of the ventricular loop, but at this early stage, the atrioventricular canal opens exclusively to the developing right ventricle The only entrance to the developing right ventricle is the embryonic interventricular communication The comparison between the developing right ventricle and the incomplete right ventricle found in tricuspid atresia or double-inlet left ventricle (see Fig 69.2) is striking Double-inlet right ventricle results from transfer of the entire inlet part of the primary heart tube to the apical component derived from the outlet, this process occurring subsequent to formation of the apical component of the left ventricle (see Fig 69.23) This apical component will form the basis of the posteroinferiorly located incomplete left ventricle Valvar morphology and ventriculoarterial connections will again depend on the development of the other parts of the heart tube The position of the incomplete ventricle will depend on the initial direction of ventricular looping Rightward looping will give a leftsided rudimentary left ventricle, whereas leftward looping will result in a rightsided left ventricle Absence of an atrioventricular connection is much harder to explain Any offered hypothesis must account for the known anatomic facts, namely that in the setting of absence of the right connection, the left atrium can be connected to a morphologically left, right, or indeterminate ventricle In each of these circumstances, the right atrial anatomy is indistinguishable, as is the left atrial anatomy in the setting of absence of the left connection irrespective of whether the right atrium is connected to a morphologically right, left, or indeterminate ventricle The most likely explanation, therefore, is that the atrioventricular junctions developed as though to produce a double-inlet connection, but for some reason the right, or left, atrioventricular orifice failed to develop Indeed, examination of the developing human heart has shown that a stage exists early in its formation that is remarkably similar to absence of the right atrioventricular connection, albeit prior to formation of the atrial septum (see Fig 69.24) The findings in the hearts with univentricular atrioventricular connections all point to the developmental insult as having occurred at an early stage of formation of the heart, prior to the completion of cardiac septation In the human, this means prior to the end of the eighth week of development In the hearts with biventricular atrioventricular connections that are functionally univentricular, however, the abnormal development must have taken place much later, since in the setting of hypoplastic left heart syndrome or pulmonary atresia with intact ventricular septum, the integrity of the ventricular septum indicates