Arrested rotation of the outflow tract may explain double-outlet right ventricle

The developmental processes of ventricular septal defects with outflow tract malalignment

Ventricular septal defects (VSD) with outflow tract (OFT) malalignment are a common group of congenital heart diseases with varying severity. The developmental process of these defects is challenging to understand due to the complex nature of cardiac morphogenesis and the difficulties in visualizing the temporal and spatial changes that occur during pathogenesis. However, recent advancements in imaging techniques, such as high-resolution episcopic microscopy, have provided valuable insights into the normal septation of ventricular chambers and OFT alignment. Building upon this knowledge, we have utilized lightsheet microscopy, another innovative imaging method, to further investigate the developmental processes that lead to abnormal formation of the ventricular septum and the malalignment of arterial roots with the ventricular chambers. Our study highlights endocardial cushion hypoplasia and insufficient rotation of the outflow tract as two interrelated central factors contributing to the pathogenesis of these defects. This finding has the potential to enhance our understanding of the etiology of congenital heart diseases and may contribute to the development of improved diagnostic and therapeutic strategies in the future.

Molecular Pathways and Animal Models of Tetralogy of Fallot and Double Outlet Right Ventricle

Tetralogy of Fallot and double-outlet right ventricle are outflow tract (OFT) alignment defects situated on a continuous disease spectrum. A myriad of upstream causes can impact on ventriculoarterial alignment that can be summarized as defects in either i) OFT elongation during looping morphogenesis or ii) OFT remodeling during cardiac septation. Embryological processes underlying these two developmental steps include deployment of second heart field cardiac progenitor cells, establishment and transmission of embryonic left/right information driving OFT rotation and OFT cushion and valve morphogenesis. The formation and remodeling of pulmonary trunk infundibular myocardium is a critical component of both steps. Defects in myocardial, endocardial, or neural crest cell lineages can result in alignment defects, reflecting the complex intercellular signaling events that coordinate arterial pole development. Importantly, however, OFT alignment is mechanistically distinct from neural crest-driven OFT septation, although neural crest cells impact indirectly on alignment through their role in modulating signaling during SHF development. As yet poorly understood nongenetic causes of alignment defects that impact the above processes include hemodynamic changes, maternal exposure to environmental teratogens, and stochastic events. The heterogeneity of causes converging on alignment defects characterizes the OFT as a hotspot of congenital heart defects.

Early Embryonic Expression of AP-2α Is Critical for Cardiovascular Development

Congenital cardiovascular malformation is a common birth defect incorporating abnormalities of the outflow tract and aortic arch arteries, and mice deficient in the transcription factor AP-2α (Tcfap2a) present with complex defects affecting these structures. AP-2α is expressed in the pharyngeal surface ectoderm and neural crest at mid-embryogenesis in the mouse, but the precise tissue compartment in which AP-2α is required for cardiovascular development has not been identified. In this study we describe the fully penetrant AP-2α deficient cardiovascular phenotype on a C57Bl/6J genetic background and show that this is associated with increased apoptosis in the pharyngeal ectoderm. Neural crest cell migration into the pharyngeal arches was not affected. Cre-expressing transgenic mice were used in conjunction with an AP-2α conditional allele to examine the effect of deleting AP-2α from the pharyngeal surface ectoderm and the neural crest, either individually or in combination, as well as the second heart field. This, surprisingly, was unable to fully recapitulate the global AP-2α deficient cardiovascular phenotype. The outflow tract and arch artery phenotype was, however, recapitulated through early embryonic Cre-mediated recombination. These findings indicate that AP-2α has a complex influence on cardiovascular development either being required very early in embryogenesis and/or having a redundant function in many tissue layers.

Pax9 is required for cardiovascular development and interacts with Tbx1 in the pharyngeal endoderm to control 4th pharyngeal arch artery morphogenesis

Developmental defects affecting the heart and aortic arch arteries are a significant phenotype observed in individuals with 22q11 deletion syndrome and are caused by a microdeletion on chromosome 22q11. TBX1, one of the deleted genes, is expressed throughout the pharyngeal arches and is considered a key gene, when mutated, for the arch artery defects. Pax9 is expressed in the pharyngeal endoderm and is downregulated in Tbx1 mutant mice. We show here that Pax9-deficient mice are born with complex cardiovascular malformations that affect the outflow tract and aortic arch arteries with failure of the 3rd and 4th pharyngeal arch arteries to form correctly. Transcriptome analysis indicated that Pax9 and Tbx1 may function together, and mice double heterozygous for Tbx1/Pax9 presented with a significantly increased incidence of interrupted aortic arch when compared with Tbx1 heterozygous mice. Using a novel Pax9Cre allele, we demonstrated that the site of this Tbx1-Pax9 genetic interaction is the pharyngeal endoderm, therefore revealing that a Tbx1-Pax9-controlled signalling mechanism emanating from the pharyngeal endoderm is required for crucial tissue interactions during normal morphogenesis of the pharyngeal arch artery system.

Summary: A strong genetic interaction between Tbx1 and Pax9 that leads to 4th PAA-derived defects in double heterozygous mice is cell-autonomous within the pharyngeal endoderm.

Mechanism responsible for D-transposition of the great arteries: Is this part of the spectrum of right isomerism?

D-transposition of the great arteries (TGA) is one of the most common conotruncal heart defects at birth and is characterized by a discordant ventriculoarterial connection with a concordant atrioventricular connection. The morphological etiology of TGA is an inverted or arrested rotation of the heart outflow tract (OFT, conotruncus), by which the aorta is transposed in the right ventral direction to the pulmonary trunk. The rotational defect of the OFT is thought to be attributed to hypoplasia of the subpulmonic conus, which originates from the left anterior heart field (AHF) residing in the mesodermal core of the first and second pharyngeal arches. AHF, especially on the left, at the early looped heart stage (corresponding to Carnegie stage 10-11 in the human embryo) is one of the regions responsible for the impediment that causes TGA morphology. In human or experimentally produced right isomerism, malposition of the great arteries including D-TGA is frequently associated. Mutations in genes involving left-right (L-R) asymmetry, such as NODAL, ACTRIIB and downstream target FOXH1, have been found in patients with right isomerism as well as in isolated TGA. The downstream pathways of Nodal-Foxh1 play a critical role not only in L-R determination in the lateral plate mesoderm but also in myocardial specification and differentiation in the AHF, suggesting that TGA is a phenotype in heterotaxia as well as the primary developmental defect of the AHF.

Impaired development of left anterior heart field by ectopic retinoic acid causes transposition of the great arteries

Background

Transposition of the great arteries is one of the most commonly diagnosed conotruncal heart defects at birth, but its etiology is largely unknown. The anterior heart field (AHF) that resides in the anterior pharyngeal arches contributes to conotruncal development, during which heart progenitors that originated from the left and right AHF migrate to form distinct conotruncal regions. The aim of this study is to identify abnormal AHF development that causes the morphology of transposition of the great arteries.

Methods and Results

We placed a retinoic acid–soaked bead on the left or the right or on both sides of the AHF of stage 12 to 14 chick embryos and examined the conotruncal heart defect at stage 34. Transposition of the great arteries was diagnosed at high incidence in embryos for which a retinoic acid–soaked bead had been placed in the left AHF at stage 12. Fluorescent dye tracing showed that AHF exposed to retinoic acid failed to contribute to conotruncus development. FGF8 and Isl1 expression were downregulated in retinoic acid–exposed AHF, and differentiation and expansion of cardiomyocytes were suppressed in cultured AHF in medium supplemented with retinoic acid.

Conclusions

The left AHF at the early looped heart stage, corresponding to Carnegie stages 10 to 11 (28 to 29 days after fertilization) in human embryos, is the region of the impediment that causes the morphology of transposition of the great arteries.

The trabecula septomarginalis (Leonardo's cord) in abnormal ventriculo-arterial connections: anatomic and morphogenetic implications

Background

The abnormal ventriculo-arterial connections in atrio-ventricular concordance and situs solitus with two well developed ventricles include the range from tetralogy of Fallot throughout the different forms of double outlet right ventricle to transposition of great arteries.

The infundibular septum and the trabecula septomarginalis are the fundamental anatomical landmarks for the segmental analysis.

In these abnormalities there is a pathological progressive counter-clockwise rotation of the infundibular septum which divorces from the antero-superior limb of the trabecula septomarginalis and achieves his identity. Is there any anatomical evidence of a simultaneous abnormal counter-clockwise rotation of the trabecula septomarginalis?

Methods

Malposition of great arteries is a generic term since all relationships have to be expected.

We present specimens with anatomical evidence of a progressive counter-clockwise rotation from 0° to about 180°of the plane passing throughout the trabecula septomarginalis’s limbs.

Results

We can observe sequentially:

1. Malformations in which the posterior limb of the trabecula septomarginalis is committed to the ventriculo infundibular fold: (tetralogy of Fallot, double outlet right ventricle with sub-aortic ventricular septal defect, truncus arteriosus and doubly committed ventricular septal defect);

2. Malformations in which the posterior limb of the trabecula septomarginalis is committed to the infundibular septum (double outlet right ventricle with sub-pulmonary ventricular septal defect, transposition of great arteries).

Conclusions

1. The sequential-segmental analysis identify all the morphologies.

2. The trabecula septomarginalis plane presents a progressive counter-clockwise twist on the long axis.

3. Since the trabeculated portions of the ventricles are the oldest developmental components, our observations support the hypothesis that the abnormal ventriculo-arterial connections could be in relation with a pathological myocardial process during early cardio-genesis.

We are promoting new studies to investigate our anatomical observations.

Perioperative Management of DORV

The diagnosis of double-outlet right ventricle (DORV) characterizes a complex heterogeneous group of congenital cardiac malformations for which multiple classification schemes have been used. A clear understanding of the anatomy is critical to understanding the physiologic consequences of the specific type of DORV. Perioperative considerations include the medical management of the patient during the preoperative period, anesthetic and surgical management, and postoperative care. Both anesthetic and surgical management strategies are very different depending on the type of DORV. Key principles for anesthetic management include balancing the systemic and pulmonary circulations, optimizing systemic cardiac output, and closely monitoring for impaired oxygen delivery to the tissues. Depending on the specific anatomy the patient is usually placed on a 1- or 2-ventricle pathway, and initial palliation may involve placement of a systemic arterial to pulmonary artery shunt or pulmonary artery banding. In some cases the child may undergo a complete repair during the first few months of life. Surgical outcomes, both short and long-term, are dependent on the type of DORV and surgical procedure done. These patients require long-term follow up and may present for surgical or catheter-based interventions as adults.

Cardiac outflow tract anomalies

The mature outflow tract (OFT) is, in basic terms, a short conduit. It is a simple, although vital, connection situated between contracting muscular heart chambers and a vast embryonic vascular network. Unfortunately, it is also a focal point underlying many multifactorial congenital heart defects (CHDs). Through the use of various animal models combined with human genetic investigations, we are beginning to comprehend the molecular and cellular framework that controls OFT morphogenesis. Clear roles of neural crest cells (NCC) and second heart field (SHF) derivatives have been established during OFT formation and remodeling. The challenge now is to determine how the SHF and cardiac NCC interact, the complex reciprocal signaling that appears to be occurring at various stages of OFT morphogenesis, and finally how endocardial progenitors and primary heart field (PHF) communicate with both these colonizing extra-cardiac lineages. Although we are beginning to understand that this dance of progenitor populations is wonderfully intricate, the underlying pathogenesis and the spatiotemporal cell lineage interactions remain to be fully elucidated. What is now clear is that OFT alignment and septation are independent processes, invested via separate SHF and cardiac neural crest (CNC) lineages. This review will focus on our current understanding of the respective contributions of the SHF and CNC lineage during OFT development and pathogenesis.

The pattern of the coronary arterial orifices in hearts with congenital malformations of the outflow tracts: a marker of rotation of the outflow tract during cardiac development?

Outflow tract defects, including cardiac neural crest defects (so-called conotruncal defects) and transposition of the great arteries, are due to an abnormal rotation of the outflow tract during cardiac development. Coronary orifices are often abnormal in outflow tract defects, particularly in common arterial trunk (CAT). A recent study indicates that abnormal coronary artery pattern in a mouse model with common arterial outlet (Tbx1-/- mouse mutant) could be due to a reduced and malpositioned subpulmonary coronary-refractory myocardial domain. The aim of our study was to demonstrate the relation between coronary orifices pattern in outflow tract defects in human and the abnormal embryonic rotation of the outflow tract. We analyzed 101 heart specimens with outflow tract defects: 46 CAT, 15 tetralogy of Fallot (TOF), 29 TOF with pulmonary atresia (TOF-PA), 11 double-outlet right ventricle with subaortic ventricular septal defect (DORV) and 17 controls. The position of left and right coronary orifices (LCO, RCO) was measured in degrees on the aortic/truncal circumference. The anterior angle between LCO and RCO (α) was calculated. The LCO was more posterior in TOF (31 °), TOF-PA (47 °), DORV (44 °), CAT (63 °), compared with controls (0 °, P < 0.05), and more posterior in CAT than in other outflow tract defects (P < 0.05). The RCO was more anterior in TOF (242 °), TOF-PA (245 °) and DORV (271 °) than in controls (213 °, P < 0.05), but not in CAT (195 °). The α angle was similar in TOF, TOF-PA, DORV and controls (149 °, 162 °, 133 °, 147 °), but significantly larger in CAT (229 °, P < 0.0001). In all outflow tract defects but CAT, the displacement of LCO (anterior) and RCO (posterior), while the α angle remains constant, might be due to incomplete rotation of the myocardium at the base of the outflow tract, leading to an abnormally positioned subpulmonary coronary-refractory myocardial domain. The larger α angle in CAT could reflect its dual identity, aortic and pulmonary.

Computational hemodynamic optimization predicts dominant aortic arch selection is driven by embryonic outflow tract orientation in the chick embryo

In the early embryo, a series of symmetric, paired vessels, the aortic arches, surround the foregut and distribute cardiac output to the growing embryo and fetus. During embryonic development, the arch vessels undergo large-scale asymmetric morphogenesis to form species-specific adult great vessel patterns. These transformations occur within a dynamic biomechanical environment, which can play an important role in the development of normal arch configurations or the aberrant arch morphologies associated with congenital cardiac defects. Arrested migration and rotation of the embryonic outflow tract during late stages of cardiac looping has been shown to produce both outflow tract and several arch abnormalities. Here, we investigate how changes in flow distribution due to a perturbation in the angular orientation of the embryonic outflow tract impact the morphogenesis and growth of the aortic arches. Using a combination of in vivo arch morphometry with fluorescent dye injection and hemodynamics-driven bioengineering optimization-based vascular growth modeling, we demonstrate that outflow tract orientation significantly changes during development and that the associated changes in hemodynamic load can dramatically influence downstream aortic arch patterning. Optimization reveals that balancing energy expenditure with diffusive capacity leads to multiple arch vessel patterns as seen in the embryo, while minimizing energy alone led to the single arch configuration seen in the mature arch of aorta. Our model further shows the critical importance of the orientation of the outflow tract in dictating morphogenesis to the adult single arch and accurately predicts arch IV as the dominant mature arch of aorta. These results support the hypothesis that abnormal positioning of the outflow tract during early cardiac morphogenesis may lead to congenital defects of the great vessels due to altered hemodynamic loading.

Myocardial progenitors in the pharyngeal regions migrate to distinct conotruncal regions

BACKGROUND

The cardiac progenitor cells for the outflow tract (OFT) reside in the visceral mesoderm and mesodermal core of the pharyngeal region, which are defined as the secondary and anterior heart fields (SHF and AHF), respectively.

RESULTS

Using chick embryos, we injected fluorescent-dye into the SHF or AHF at stage 14, and the destinations of the labeled cells were examined at stage 31. Labeled cells from the right SHF were found in the myocardium on the left dorsal side of the OFT, and cells from the left SHF were detected on the right ventral side of the OFT. Labeled cells from the right and left AHF migrated to regions of the ventral wall of the OFT close to the aortic and pulmonary valves, respectively.

CONCLUSION

These observations indicate that myocardial progenitors from the SHF and AHF contribute to distinct conotruncal regions and that cells from the SHF migrate rotationally while cells from the AHF migrate in a non-rotational manner.

Msx1 and Msx2 regulate survival of secondary heart field precursors and post-migratory proliferation of cardiac neural crest in the outflow tract

Msx1 and Msx2 are highly conserved, Nk-related homeodomain transcription factors that are essential for a variety of tissue-tissue interactions during vertebrate organogenesis. Here we show that combined deficiencies of Msx1 and Msx2 cause conotruncal anomalies associated with malalignment of the cardiac outflow tract (OFT). Msx1 and Msx2 play dual roles in outflow tract morphogenesis by both protecting secondary heart field (SHF) precursors against apoptosis and inhibiting excessive proliferation of cardiac neural crest, endothelial and myocardial cells in the conotruncal cushions. During incorporation of SHF precursors into the OFT myocardium, ectopic apoptosis in the Msx1-/-; Msx2-/- mutant SHF is associated with reduced expression of Hand1 and Hand2, which from work on Hand1 and Hand2 mutants may be functionally important in the inhibition of apoptosis in Msx1/2 mutants. Later during aorticopulmonary septation, excessive proliferation in the OFT cushion mesenchyme and myocardium of Msx1-/-; Msx2-/- mutants is associated with premature down-regulation of p27(KIP1), an inhibitor of cyclin-dependent kinases. Diminished accretion of SHF precursors to the elongating OFT myocardium and excessive accumulation of mesenchymal cells in the conotruncal cushions may work together to perturb the rotation of the truncus arteriosus, leading to OFT malalignment defects including double-outlet right ventricle, overriding aorta and pulmonary stenosis.

Pitx2 regulates cardiac left-right asymmetry by patterning second cardiac lineage-derived myocardium

Current models of left-right asymmetry hold that an early asymmetric signal is generated at the node and transduced to lateral plate mesoderm in a linear signal transduction cascade through the function of the Nodal signaling molecule. The Pitx2 homeobox gene functions at the final stages of this cascade to direct asymmetric morphogenesis of selected organs including the heart. We previously showed that Pitx2 regulated an asymmetric pathway that was independent of cardiac looping suggesting a second asymmetric cardiac pathway. It has been proposed that in the cardiac outflow tract Pitx2 functions in both cardiac neural crest, as a target of canonical Wnt-signaling, and in the mesoderm-derived cardiac second lineage. We used fate mapping, conditional loss of function, and chimera analysis in mice to investigate the role of Pitx2 in outflow tract morphogenesis. Our findings reveal that Pitx2 is dispensable in the cardiac neural crest but functions in second lineage myocardium revealing that this cardiac progenitor field is patterned asymmetrically.

Cardiac outflow tract septation process in the mouse model of transposition of the great arteries

It has been reported that all-trans retinoic acid induces transposition of the great arteries (TGA) at 80-90% in ICR mice. The authors revealed that retinoic acid affects the initial formation of the conus cushions leading to a loss of spirality in the cardiac outflow tract. However, the aberrant process of septation has not been precisely defined. In this study, we observed the hearts of live embryos using a video system followed by scanning electron microscopic examination. First, we found that, in the retinoic acid-treated embryos, the proximal outflow tract cushions, in addition to hypoplasia and dysplasia, did not establish the continuity with the distal outflow tract cushions and could not contribute to the outflow septation. Second, the distal outflow tract did not rotate counter-clockwise, retaining the outflow septum anlage in the superoinferior position. Third, a tongue-like mesenchymal tissue had developed on the right anterior rim of the muscular interventricular septum and was incorporated into the interventricular septum. Altogether, these processes contributed to establishing a reversed relationship between the outflow septum anlage and the ventricular septum anlage. On the other hand, right-ward deviation of one or both of the distal outflow tract cushions, relative to the mesenchymal tissue, gave rise to variable degrees of overriding of the pulmonary artery orifice. We conclude that, due to hypoplasia and dysplasia of the proximal outflow tract cushions and lack of distal outflow tract rotation, the outflow septum anlage took an inverted relationship with the ventricular septum anlage. Various types of rightward shift of the outflow tract cushions produced a morphological spectrum of TGA-type cono-truncal anomalies.

Spectrum of looping disturbances in stage 34 chicken hearts after retinoic acid treatment

BACKGROUND

In a recently developed chick model the teratogen retinoic acid has appeared to induce a spectrum of double outlet right ventricle, which needs further detailed evaluation. It is known that retinoic acid is able to induce cardiac malformations. Although the exact mechanism is not known, an interaction with neural crest cell function is thought to exist.

METHODS

After treatment with 1 microgram all-trans retinoic acid at Hamburger and Hamilton stage 15 and reincubation until stage 34 of development 41 chicken embryos were evaluated macroscopically and microscopically, supported by graphic reconstructions. These retinoic acid treated embryos were compared with a control group (n = 8).

RESULTS

The retinoic acid treated embryos could be divided in three groups. Group 1 (23/41) had an intact septum, group 2 (11/41) had an isolated ventricular septal defect (VSD), and group 3 (7/41) had a double outlet right ventricle (DORV). Besides, in the group with an intact septum 11 hearts showed an abnormal course of the subaortic outflow tract. In the group with DORV a straddling tricuspid orifice (7/8) and a double inlet left ventricle (1/8) could be distinguished. Considering the external contour, the hearts in the DORV group all showed a dextroposed arterial pole. Malformed pharyngeal arch arteries were found in all three groups (11/41) and with a great diversity.

CONCLUSIONS

The present cardiac malformations in the chicken as a result of retinoic acid treatment are part of a continuous spectrum, varying from hearts with an intact ventricular septum and a normal course of the subaortic outflow tract to a double outlet right ventricle with a straddling tricuspid orifice or even a double inlet left ventricle. A remarkable observation in this spectrum concerns the correlation of malformations of the inflow and outflow tracts, which is explained as a cardiac looping disturbance. The disturbance of the looping process seems to lead to malalignment of septal components, although, in the chick, retinoic acid does not in general interfere with the formation of these septal components themselves.

Outflow tract abnormalities in atrioventricular canal malformations

Atrioventricular canal malformations are commonly regarded as being arrests of development of normal cardiac septation since their morphology strongly resembles the normal heart of Carnegie stages 14 through 18. Similarly, the spectrum of outflow tract abnormalities, which includes truncus arteriosus, transposition of the great arteries, double outlet right ventricle, and tetralogy of Fallot, has been interpreted as a developmental arrest because these malformations resemble the normal embryonic outflow tract in Carnegie stages 14 through 18, respectively. The causes of the developmental arrests in these two classes of malformations are unknown. Observation of an unusual autopsy case with features of both atrioventricular canal defect and tetralogy of Fallot prompted us to review the outflow tract morphology in 38 hearts with an atrioventricular canal malformation and two semilunar valves to determine if there was evidence to support the idea that both conditions may have the same pathogenesis. In all 38 hearts there was fibrous continuity between an anterior mitral leaflet and an aortic valve, which by anatomic definition means that no case had truncus arteriosus, transposition of the great arteries, or double outlet right ventricle. There were three cases with coarctation of the aorta, a lesion which arises when blood flow in the pulmonary trunk exceeds aortic flow during early development. There were four cases with pulmonary to aortic valve caliber ratios similar to those found in tetralogy of Fallot but the aortic-to-pulmonary valve angle relative to the heart base ranged over the full spectrum of angles seen in normal hearts and those with tetralogy of Fallot. Furthermore, this angle showed the same range of values for partial and complete canal defects, and did not correlate with patient age or ratio of pulmonary to aortic valve caliber. We conclude that the pathogenesis of the spectrum of outflow tract developmental arrests is different from that of atrioventricular canal malformations.

Pathogenesis of various forms of double outlet right ventricle in mouse fetal trisomy 13

The pathogenesis of double outlet right ventricle with or without pulmonary infundibular atresia in mouse fetal trisomy 13 was studied at the organ level using microdissection and scanning electron microscopy. Altogether, 394 karyotyped trisomic embryos were collected between 11 days and 16 hours of gestation (presence of a vaginal plug = day 1) and 15 days of gestation at intervals of 8 hours, and at 16 days of gestation. The hearts were perfusion-fixed, microdissected, and prepared to be observed in scanning electron microscope in the following standardized orientations: frontal, right or left profile, septal and parietal halves of the right ventricle and outflow tract (conotruncus). Comparison of 276 trisomic hearts with their normal counterparts described previously has shown that: the first pathognomonic feature is the abnormal anterior position of the proximal part of the parietal outflow tract ridge or of both ridges (at 12 days and 16 hours of gestation); the abnormal anterior fusion of these ridges ("coalescence") results in a mesenchymal mass behind which is deviated the pulmonary part of the outflow tract lumen; from 14 days and 16 hours of gestation on, this lumen is either obstructed, resulting in a supravalvar stenosis of the pulmonary trunk and subsequently evolving into double outlet right ventricle with pulmonary infundibular atresia; or, in a minority of cases, this lumen is not obstructed and the heart develops into double outlet right ventricle without pulmonary infundibular atresia. The pathogenesis of these malformations differs from most of the known hypotheses based on deductions from human malformed hearts, as well as from observations of the pathogenesis of similar outflow tract malformations, such as those found in the Keeshond dog or rats treated with trimethadione.

Morphogenetic considerations on congenital malformations of the outflow tract. Part 2: Complete transposition of the great arteries and double outlet right ventricle

On the basis of our recent embryologic work concerning the separation process of the outflow tract, our study of the morphology of specimens from the Leiden Collection of malformed hearts, and in conjunction with our review of the literature, we have reconsidered the morphogenesis of complete transposition and double outlet right ventricle. In complete transposition, a mirror-image arrangement of the columns of the aorto-pulmonary septum could explain the discordant ventriculo-arterial connexion and might thus play a role in its development. Dedicating a cardinal role to the aorto-pulmonary septum in the morphogenesis of complete transposition does not, however, seem justified. Double outlet right ventricle is not an embryologic entity. From the stance of the embryologist, we prefer to consider double outlet right ventricle in terms of a feature that may occur in specimens related to hearts with either a concordant ventriculo-arterial connexion (such as tetralogy of Fallot, or the so-called Eisenmenger ventricular septal defect), or a discordant ventriculo-arterial connexion (complete transposition).

Pathogenesis of persistent left superior vena cava with a coronary sinus connection

The basis for persistence of the left superior vena cava (LSVC), usually associated with cardiac malformations, is poorly understood. We examined 351 staged, serially sectioned human embryos in the Carnegie Embryological Collection and 1208 specimens with congenital cardiovascular malformations in the Pathology Collection of the Johns Hopkins Hospital. A standardized questionnaire was answered for each embryo and autopsy case and a computer program was employed to tabulate concurrent anatomic features. In the normal embryos a symmetric venous system appeared with the heart tube at Carnegie stage 9; the sinoatrial junction translocated to the right and the relationship of the coronary sinus to the LSVC was established by stage 12. The LSVC was patent through stage 20 and subsequently underwent luminal obliteration by compression between the left atrium and the hilum of the left lung. Among the 1208 hearts with a congenital abnormality, 104 (9%) had a persistent LSVC with a coronary sinus connection. Statistically, significantly more frequent associations were found between persistent LSVC and atrioventricular canal defects, cor triatriatum, and mitral atresia and a significantly less frequent association was observed between persistent LSVC and atrial septal defect or patent foramen ovale as a primary defect. The normally late embryonic obliteration of the LSVC suggests that its persistence would be secondary to reduce cardiac compression or to blood flow redistribution at an early stage, and the malformations associated with persistent LSVC support that view. Identification of a persistent left superior vena cava with coronary sinus connection should suggest an associated malformation, especially atrioventricular canal, cor triatriatum, or mitral atresia.