Ventricular Inversion with Transposition of the Great Arteries
If the cardiac tube loops to the left and anterior (L-loop) rather than to the right and anterior, most of the structures adjacent to and including ventricular segments of the heart tube (the AV valves, the ventricles, and the arterial roots) will develop in an inverted position. Subsequently, the right atrium is connected via a morphologic mitral valve to a morphologic left ventricle and the left atrium is connected via a morphologic tricuspid valve to a morphologic right ventricle. Within the aortic sac, the aorticopulmonary septum develops in a normal fashion. However, as partitioning of the inverted conotruncus (outflow track) takes place in mirror image, the end result is L-transposition of the great arteries, with the aorta arising anteriorly from a left-sided, morphologically right (systemic) ventricle and the pulmonary trunk arising posteriorly from a right-sided, morphologically left (venous) ventricle. Because systemic and pulmonary venous return are still routed to the pulmonary and systemic arterial circulations, respectively, this anomaly commonly is referred to as "corrected" transposition.
DORV is due to a failure in the leftward repositioning of the aortic portion of the outflow tract, resulting in the persistence of the more "primitive" embryonic morphology in which the entire outflow tract originates from the right ventricle. One morphologic hallmark of the failure of completion of the leftward shift of the aorta is the presence of myocardial tissue between the left AV valve and the aorta (mitral-aortic separation). This anomaly is found after a wide range of hemodynamic, metabolic, and genetic insults to the embryo, suggesting that the phenotype of the DORV may be a final common expression of a range of primary abnormalities that result in the persistence of the embryonic configuration.427
The processes of primary myocardial trabeculation, expansion of secondary and tertiary myocardial trabeculae, and myocardial compaction are critical to the structural maturation of the ventricular chambers. This process results in the transformation of the smooth-walled endocardial lining into complex three-dimensional structure of the right and left ventricular myocardium. Rapid cell division and interposition of endothelial cells along the right and left ventrolateral borders of the endocardial tube is associated with a rapid resorption of cardiac jelly, resulting in myocardial ridges and trabeculae lined with single layers of endocardial cells.428 The initial number and orientation of the myocardial ridges differ between species.429 In general, myocardial trabeculation begins at the ventricular outer curvature (future apex) and then extends proximally and distally. The intersection between the outer, compact myocardium and the base of the trabeculae probably is a site of peak wall stress, and myocyte division is most active at this site.130,131 Retroviral marker studies also have shown that ventricular myocardial growth is associated with a transmural distribution of clonally related myocardial cells extending from the epi- to the endocardium.79 80 Of note, these cells reside in muscle bundles that are oriented at an angle to the longitudinal axis of the heart, consistent with the adult myocardial architecture that results in efficient twist and contraction.79,80 However, the mechanisms that regulate clonal myocardial expansion and compaction have not been defined.
With the onset of myocardial trabeculation, diverticula first appear as two sharply defined areas along the right and left ventrolateral borders of the endocardial tube Fig. 9-8) .Li2 These diverticula develop initially at the expense of the cardiac jelly and later penetrate the myocardium as it increases in thickness, producing a spongy mass of trabeculae.128 The filling capacity of the heart is increased by the added intertrabecular spaces. The trabeculating embryonic heart now can be divided into primitive right and left ventricles as there are distinct morphologic differences between the trabecular architectures of the developing ventricular chambers. The developing LV is trabeculated along the majority of its greater curvature, while the developing RV has a significant portion of the greater curvature that is smooth-walled.!33 At this stage of development, the embryo is approximately 3 mm long and has an ovulation age of about 25 days.!07 The common outflow tract of the developing heart can be classified as having a proximal (conus) segment and a distal (truncus) segment. The conus eventually septates into the outflow portions of each ventricle with the incorporation of migrating neural crest cells, while the truncus contributes to the formation of the semilunar valves and the development of the aortic and pulmonary roots.
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