Normal and abnormal development of the intrapericardial arterial trunks in humans and mice

Endocardial-to-mesenchymal transition underlies cardiac outflow tract septation and bicuspid aortic valve formation in the Syrian hamster model

Right-left bicuspid aortic valve (R-L BAV) is the most frequent phenotype of the most common congenital heart disease. Its etiology is based on two associated morphogenetic defects during cardiac outflow tract (OFT) septation: abnormal migration of cardiac neural crest (CNC) cells, and excessive fusion of the conal ridges (CRs). The aim of this study is to elucidate the mechanism involved in the fusion of the CRs responsible for normal and abnormal OFT septation and BAV formation. Two mechanisms have been proposed: endocardial apoptosis and endocardial-mesenchymal transition (EMT). The involvement of these mechanisms in the fusion event was tested in embryos of the hamster model with BAV. Apoptotic cells were absent in the fusion area of the CRs. However, we detected endocardial cells (CD34+;VE-Cadherin+) showing positive signals for migration markers (α-actin+) in the fusion area of the CRs of embryos developing both normal aortic valve and BAV. These cells showed an intermediate morphological phenotype between endocardial and mesenchymal cells. The findings clearly indicate that EMT, and not apoptosis, is the cellular mechanism underlying the normal and excessive fusion of CRs that give rise to tricuspid aortic valve and BAV, respectively. Furthermore, our results show that the fusion of CRs in embryos developing BAV continues after the OFT septation, suggesting over-induction of EMT by abnormally distributed CNC cells.

Cardiac development demystified by use of the HDBR atlas

Much has been learned over the last half century regarding the molecular and genetic changes that take place during cardiac development. As yet, however, these advances have not been translated into knowledge regarding the marked changes that take place in the anatomical arrangements of the different cardiac components. As such, therefore, many aspects of cardiac development are still described on the basis of speculation rather than evidence. In this review, we show how controversial aspects of development can readily be arbitrated by the interested spectator by taking advantage of the material now gathered together in the Human Developmental Biology Resource; HDBR. We use the material to demonstrate the changes taking place during the formation of the ventricular loop, the expansion of the atrioventricular canal, the incorporation of the systemic venous sinus, the formation of the pulmonary vein, the process of atrial septation, the remodelling of the pharyngeal arches, the major changes occurring during formation of the outflow tract, the closure of the embryonic interventricular communication, and the formation of the ventricular walls. We suggest that access to the resource makes it possible for the interested observer to arbitrate, for themselves, the ongoing controversies that continue to plague the understanding of cardiac development.

Surgical Repair of Common Arterial Trunk With Ventriculoarterial Septal Defect and Dual Orifice Truncal Valve

We present a case of a rare example of a ventriculo-arterial septal defect found in a patient with a common arterial trunk, with balanced aortic and pulmonary components, but with separate valvar orifices within the common truncal valve. We managed the lesion using a two-patch approach. Performing a palliative procedure to relieve the elevated right ventricular pressure aided in the preservation of the pulmonary component of the common valve. We validated the success of the technique using postoperative computerized tomography and four-dimensional flow magnetic resonance imaging.

Revisiting the anatomy of the left ventricle in the light of knowledge of its development

Despite centuries of investigation, certain aspects of left ventricular anatomy remain either controversial or uncertain. We make no claims to have resolved these issues, but our review, based on our current knowledge of development, hopefully identifies the issues requiring further investigation. When first formed, the left ventricle had only inlet and apical components. With the expansion of the atrioventricular canal, the developing ventricle cedes part of its inlet to the right ventricle whilst retaining the larger parts of the cushions dividing the atrioventricular canal. Further remodelling of the interventricular communication provides the ventricle with its outlet, with the aortic root being transferred to the left ventricle along with the newly formed myocardium supporting its leaflets. The definitive ventricle possesses inlet, apical and outlet parts. The inlet component is guarded by the mitral valve, with its leaflets, in the normal heart, supported by papillary muscles located infero-septally and supero-laterally. There is but a solitary zone of apposition between the leaflets, which we suggest are best described as being aortic and mural. The trabeculated component extends beyond the inlet to the apex and is confluent with the outlet part, which supports the aortic root. The leaflets of the aortic valve are supported in semilunar fashion within the root, with the ventricular cavity extending to the sinutubular junction. The myocardial-arterial junction, however, stops well short of the sinutubular junction, with myocardium found only at the bases of the sinuses, giving rise to the coronary arteries. We argue that the relationships between the various components should now be described using attitudinally appropriate terms rather than describing them as if the heart is removed from the body and positioned on its apex.

Development of the arterial roots and ventricular outflow tracts

The separation of the outflow tract of the developing heart into the systemic and pulmonary arterial channels remains controversial and poorly understood. The definitive outflow tracts have three components. The developing outflow tract, in contrast, has usually been described in two parts. When the tract has exclusively myocardial walls, such bipartite description is justified, with an obvious dogleg bend separating proximal and distal components. With the addition of non-myocardial walls distally, it becomes possible to recognise three parts. The middle part, which initially still has myocardial walls, contains within its lumen a pair of intercalated valvar swellings. The swellings interdigitate with the distal ends of major outflow cushions, formed by the remodelling of cardiac jelly, to form the primordiums of the arterial roots. The proximal parts of the major cushions, occupying the proximal part of the outflow tract, which also has myocardial walls, themselves fuse and muscularise. The myocardial shelf thus formed remodels to become the free-standing subpulmonary infundibulum. Details of all these processes are currently lacking. In this account, we describe the anatomical changes seen during the overall remodelling. Our interpretations are based on the interrogation of serially sectioned histological and high-resolution episcopic microscopy datasets prepared from developing human and mouse embryos, with some of the datasets processed and reconstructed to reveal the specific nature of the tissues contributing to the separation of the outflow channels. Our findings confirm that the tripartite postnatal arrangement can be correlated with the changes occurring during development.

Revisiting the anatomy of the right ventricle in the light of knowledge of its development

Controversies continue regarding several aspects of the anatomy of the morphologically right ventricle. There is disagreement as to whether the ventricle should be assessed in bipartite or tripartite fashion, and the number of leaflets to be found in the tricuspid valve. In particular, there is no agreement as to whether a muscular outlet septum is present in the normally constructed heart, nor how many septal components are to be found during normal development. Resolving these issues is of potential significance to those investigating and treating children with congenitally malformed hearts. With all these issues in mind, we have revisited our own experience in investigating the development and morphology of the normal right ventricle. To assess development, we have examined a large number of datasets, prepared by both standard and episcopic microscopy, from human and murine embryos. In terms of gross anatomy, we have compared dissections of normal autopsied hearts with virtual dissections of datasets prepared using computed tomography. Our developmental and postnatal studies, taken together, confirm that the ventricle is best assessed in tripartite fashion, with the three parts representing its inlet, apical trabecular, and outlet components. The ventricular septum, however, has only muscular and membranous components. The muscular part incorporates a small component derived from the muscularised fused proximal outflow cushions, but this part cannot be distinguished from the much larger part that is incorporated within the free-standing muscular infundibular sleeve. We confirm that the tricuspid valve itself has three components, which are located inferiorly, septally, and antero-superiorly.

Anatomy of the Ventricular Outflow Tracts: An Electrophysiology Perspective

Outflow tract ventricular arrhythmias are the most common type of idiopathic ventricular arrhythmia. A systematic understanding of the outflow tract anatomy improves procedural efficacy and enables electrophysiologists to anticipate and prevent complications. This review emphasizes the three-dimensional spatial relationships between the ventricular outflow tracts using seven anatomical principles. In turn, each principle is elaborated on from a clinical perspective relevant for the practicing electrophysiologist. The developmental anatomy of the outflow tracts is also discussed and reinforced with a clinical case.

Molecular Pathways and Animal Models of Semilunar Valve and Aortic Arch Anomalies

The great arteries of the vertebrate carry blood from the heart to the systemic circulation and are derived from the pharyngeal arch arteries. In higher vertebrates, the pharyngeal arch arteries are a symmetrical series of blood vessels that rapidly remodel during development to become the asymmetric aortic arch arteries carrying oxygenated blood from the left ventricle via the outflow tract. At the base of the aorta, as well as the pulmonary trunk, are the semilunar valves. These valves each have three leaflets and prevent the backflow of blood into the heart. During development, the process of aortic arch and valve formation may go wrong, resulting in cardiovascular defects, and these may, at least in part, be caused by genetic mutations. In this chapter, we will review models harboring genetic mutations that result in cardiovascular defects affecting the great arteries and the semilunar valves.

Molecular Pathways and Animal Models of Truncus Arteriosus

In normal cardiovascular development in birds and mammals, the outflow tract of the heart is divided into two distinct channels to separate the oxygenated systemic blood flow from the deoxygenated pulmonary circulation. When the process of outflow tract septation fails, a single common outflow vessel persists resulting in a serious clinical condition known as persistent truncus arteriosus or common arterial trunk. In this chapter, we will review molecular pathways and the cells that are known to play a role in the formation and development of the outflow tract and how genetic manipulation of these pathways in animal models can result in common arterial trunk.

The advantages of naming rather than numbering the arteries of the pharyngeal arches

Controversies continue as to how many pharyngeal arches, with their contained arteries, are to be found in the developing human. Resolving these controversies is of significance to paediatric cardiologists since many investigating abnormalities of the extrapericardial arterial pathways interpret their findings on the basis of persistence of a fifth set of such arteries within an overall complement of six sets. The evidence supporting such an interpretation is open to question. In this review, we present the history of the existence of six such arteries, emphasising that the initial accounts of human development had provided evidence for the existence of only five sets. We summarise the current evidence that substantiates these initial findings. We then show that the lesions interpreted on the basis of persistence of the non-existing fifth arch arteries are well described on the basis of the persistence of collateral channels, known to exist during normal development, or alternatively due to remodelling of the aortic sac.

Surgical Repair of a Rare Variant of Common Arterial Trunk, With Considerations of its Significance for Morphogenesis

We present a successful surgical repair of a rare variant of the common arterial trunk with unusual arrangement of the pulmonary arteries. The variant was not readily classified using either of the popular classifications for the common arterial trunk. It is appropriately described as a common arterial trunk showing aortic dominance, but with extrapericardial origin of the pulmonary arteries from the underside of the truncal arch. We also provide an account of cardiac development, which we suggest offers an accurate explanation for its morphogenesis.

Isolated Absent Aortic Valve: A Unique Fetal Case With Echocardiographic, Pathologic, and Genetic Correlation

We present a 22-week fetus with isolated absent aortic valve and inverse circular shunt. The pregnancy was interrupted. Here, echocardiography and pathology images demonstrate this rare entity. Whole genome sequencing revealed a potentially disease-causing variant in the APC gene. Whole genome sequencing should be considered in severe and rare fetal diseases. (Level of Difficulty: Advanced.).

Quantitative Image Processing for Three-Dimensional Episcopic Images of Biological Structures: Current State and Future Directions

Episcopic imaging using techniques such as High Resolution Episcopic Microscopy (HREM) and its variants, allows biological samples to be visualized in three dimensions over a large field of view. Quantitative analysis of episcopic image data is undertaken using a range of methods. In this systematic review, we look at trends in quantitative analysis of episcopic images and discuss avenues for further research. Papers published between 2011 and 2022 were analyzed for details about quantitative analysis approaches, methods of image annotation and choice of image processing software. It is shown that quantitative processing is becoming more common in episcopic microscopy and that manual annotation is the predominant method of image analysis. Our meta-analysis highlights where tools and methods require further development in this field, and we discuss what this means for the future of quantitative episcopic imaging, as well as how annotation and quantification may be automated and standardized across the field.

Should Prenatal Chromosomal Microarray Analysis Be Offered for Pulmonary Atresia? A Single-Center Retrospective Study in China

(1) Objective: To evaluate the application of chromosomal microarray analysis (CMA) in fetuses with pulmonary atresia (PA) and to explore the risk factors for predicting chromosomal imbalances and adverse perinatal outcomes. (2) Methods: This study investigated 428 cases of PA singleton pregnancies that were tested using CMA and quantitative fluorescent polymerase chain reaction (QF-PCR) as first-line genetic testing. The PA cases were divided into two groups: an isolated group and a non-isolated group. (3) Results: CMA revealed clinically relevant copy number variations (CNVs) in 9/139 (6.47%) PA fetuses, i.e., pathogenic copy number variations (pCNVs) in 8/139 (5.76%) fetuses and likely pathogenic CNVs in 1/139 (0.72%) fetuses. Stratified analysis showed that the incidence of clinically significant variants was higher in non-isolated PA fetuses than in isolated PA fetuses (12.50%, 6/48 vs. 3.30%, 3/91, p = 0.036). Regression analysis showed that a combination of other structural abnormalities at diagnosis of PA represented the principal risk factor for chromosomal imbalances (OR = 2.672). A combination of other structural abnormalities and a high maternal age increased the risk of adverse pregnancy outcomes in PA cases, including intrauterine fetal death (IUFD), termination of pregnancy (TOP), and preterm delivery. (4) Conclusions: The value of CMA for locating imbalanced genetic variations in fetuses with PA was highlighted by this study, particularly when combined with additional structural abnormalities.

Timeline of Developmental Defects Generated upon Genetic Inhibition of the Retinoic Acid Receptor Signaling Pathway

It has been established for almost 30 years that the retinoic acid receptor (RAR) signalling pathway plays essential roles in the morphogenesis of a large variety of organs and systems. Here, we used a temporally controlled genetic ablation procedure to precisely determine the time windows requiring RAR functions. Our results indicate that from E8.5 to E9.5, RAR functions are critical for the axial rotation of the embryo, the appearance of the sinus venosus, the modelling of blood vessels, and the formation of forelimb buds, lung buds, dorsal pancreatic bud, lens, and otocyst. They also reveal that E9.5 to E10.5 spans a critical developmental period during which the RARs are required for trachea formation, lung branching morphogenesis, patterning of great arteries derived from aortic arches, closure of the optic fissure, and growth of inner ear structures and of facial processes. Comparing the phenotypes of mutants lacking the 3 RARs with that of mutants deprived of all-trans retinoic acid (ATRA) synthesising enzymes establishes that cardiac looping is the earliest known morphogenetic event requiring a functional ATRA-activated RAR signalling pathway.

PAX Genes in Cardiovascular Development

The mammalian heart is a four-chambered organ with systemic and pulmonary circulations to deliver oxygenated blood to the body, and a tightly regulated genetic network exists to shape normal development of the heart and its associated major arteries. A key process during cardiovascular morphogenesis is the septation of the outflow tract which initially forms as a single vessel before separating into the aorta and pulmonary trunk. The outflow tract connects to the aortic arch arteries which are derived from the pharyngeal arch arteries. Congenital heart defects are a major cause of death and morbidity and are frequently associated with a failure to deliver oxygenated blood to the body. The Pax transcription factor family is characterised through their highly conserved paired box and DNA binding domains and are crucial in organogenesis, regulating the development of a wide range of cells, organs and tissues including the cardiovascular system. Studies altering the expression of these genes in murine models, notably Pax3 and Pax9, have found a range of cardiovascular patterning abnormalities such as interruption of the aortic arch and common arterial trunk. This suggests that these Pax genes play a crucial role in the regulatory networks governing cardiovascular development.

Transposition physiology in the setting of concordant ventriculo-arterial connections

BACKGROUND AND AIM

To review the anatomical details, diagnostic challenges, associated cardiovascular anomalies, and techniques and outcomes of management, including re-interventions, for the rare instances of transposition physiology with concordant ventriculo-arterial connections.

METHODS

We reviewed clinical and necropsy studies on diagnosis and surgical treatment of individuals with transposition physiology and concordant ventriculo-arterial connections, analyzing also individuals with comparable flow patterns in the setting of isomerism.

RESULTS

Among reported cases, just over two-thirds were diagnosed during surgery, after initial palliation, or after necropsy. Of the patients, four-fifths presented in infancy with either cyanosis or congestive cardiac failure, with complex associated cardiac malformations. Nearly half had ventricular septal defects, and one-fifth had abnormalities of the tricuspid valve, including hypoplasia of the morphologically right ventricle. A small minority had common atrioventricular junctions We included cases reported with isomerism when the flow patterns were comparable, although the atrioventricular connections are mixed in this setting. Management mostly involved construction of intraatrial baffles, along with correction of coexisting anomalies, either together or multistaged. Overall mortality was 25%, with one-fifth of patients requiring pacemakers for surgically-induced heart block. The majority of survivors were in good functional state.

CONCLUSIONS

The flow patterns produced by discordant atrioventricular and concordant ventriculo-arterial connections remain an important, albeit rare, indication for atrial redirection or hemi-Mustard's procedure with bidirectional Glenn. The procedure recruits the morphologically left ventricle in the systemic circuit, producing good long-term functional results. The approach can also be used for those with isomeric atrial appendages and comparable hemodynamic circuits.

Tetralogy of Fallot with pulmonary atresia and aortopulmonary window may mimic common arterial trunk

Tetralogy of Fallot with pulmonary atresia is a group of congenital cardiac malformations, which is defined by the absence of luminal continuity between both ventricles and the pulmonary artery, and an interventricular communication. Pulmonary arterial supply in patients with tetralogy of Fallot with pulmonary atresia can be via the arterial duct or from collateral arteries arising directly or indirectly from the aorta (systemic-to-pulmonary artery collaterals), or rarely both. The rarest sources of pulmonary blood flow are aortopulmonary window and fistulous communication with the coronary artery.Herein, we describe an outflow tract malformation, tetralogy of Fallot with pulmonary atresia and aortopulmonary window, which was misdiagnosed as common arterial trunk. We emphasise the morphological differences.

Development of the Human Arterial Valves: Understanding Bicuspid Aortic Valve

Abnormalities in the arterial valves are some of the commonest congenital malformations, with bicuspid aortic valve (BAV) occurring in as many as 2% of the population. Despite this, most of what we understand about the development of the arterial (semilunar; aortic and pulmonary) valves is extrapolated from investigations of the atrioventricular valves in animal models, with surprisingly little specifically known about how the arterial valves develop in mouse, and even less in human. In this review, we summarise what is known about the development of the human arterial valve leaflets, comparing this to the mouse where appropriate.

Crk and Crkl have shared functions in neural crest cells for cardiac outflow tract septation and vascular smooth muscle differentiation

CRK and CRKL encode cytoplasmic adaptors that contribute to the etiology of congenital heart disease. Neural crest cells (NCCs) are required for cardiac outflow tract (OFT) septation and aortic arch formation. The roles of Crk/Crkl in NCCs during mouse cardiovascular development remains unknown. To test this, we inactivated Crk and/or Crkl in NCCs. We found that the loss of Crk, rather than Crkl, in NCCs resulted in double outlet right ventricle, while loss of both Crk/Crkl in NCCs resulted in severe defects with earlier lethality due to failed OFT septation and severe dilation of the pharyngeal arch arteries (PAAs). We found that these defects are due to altered cell morphology resulting in reduced localization of NCCs to the OFT and failed integrity of the PAAs, along with reduced expression of Integrin signaling genes. Further, molecular studies identified reduced differentiation of vascular smooth muscle cells that may in part be due to altered Notch signaling. Additionally, there is increased cellular stress that leads to modest increase in apoptosis. Overall, this explains the mechanism for the Crk/Crkl phenotype.

Is Transposition of the Great Arteries Associated With Shortening of the Intrapericardial Portions of the Great Arterial Trunks? An Echocardiographic Analysis on Newborn Infants With Simple Transposition of the Great Arteries to Explore an Animal Model-Based Hypothesis on Human Beings

Background The pathogenesis of transposition of the great arteries (TGA) as a congenital heart defect of the outflow tract with discordant ventriculoarterial connections remains an enigma. TGA usually have parallel great arteries suggesting that deficient torsion of the embryonic arterial heart pole might cause discordant ventriculoarterial connections. It has been speculated that deficient elongation of the embryonic outflow tract might prevent its normal torsion resulting in TGA. The aim of our study was to clarify whether the intrapericardial portions of the great arteries in human patients with TGA might be indeed shorter than in normal hearts. Methods and Results Thirty-four newborns with simple TGA and 35 newborns with normal hearts were analyzed by using images of the outflow tract in their echocardiograms and the following defined lengths of the great arteries were measured: aortic length 1, (AoL-1) and aortic length 2 (AoL-2) = distance between left and right aortic valve level and origin of the brachiocephalic artery, respectively. Pulmonary trunk length 1 (PTL-1) and pulmonary trunk length 2 (PTL 2) = distance between left and right pulmonary valve level and origin of left and right pulmonary artery, respectively. All measurements of the AoL were significantly shorter in TGA compared to normal hearts (AoL-1: 1.6±0.2 versus 2.05±0.1; P<0.0001; AoL-2: 1.55±0.2 versus 2.13±0.1; P<0.0001). With regard to the pulmonary trunk (PT), PTL-1 and PTL-2 were found to be shorter and longer, respectively, in TGA compared with normal hearts, reflecting the differences in the spatial arrangement of the PT between the 2 groups as in TGA the PT is showing a mirror image of the normal anatomy. However, the overall length of the PT between the 2 groups did not differ. Conclusions Our data demonstrate that, compared with normal newborns, the ascending aorta is significantly shorter in newborns with TGA whereas the overall length of the PT does not differ between the 2 groups. This finding is in accord with the animal model-based hypothesis that TGA may result from a growth deficit at the arterial pole of the embryonic heart.

Cardiac Neural Crest Cells: Their Rhombomeric Specification, Migration, and Association with Heart and Great Vessel Anomalies

Outflow tract abnormalities are the most frequent congenital heart defects. These are due to the absence or dysfunction of the two main cell types, i.e., neural crest cells and secondary heart field cells that migrate in opposite directions at the same stage of development. These cells directly govern aortic arch patterning and development, ascending aorta dilatation, semi-valvular and coronary artery development, aortopulmonary septation abnormalities, persistence of the ductus arteriosus, trunk and proximal pulmonary arteries, sub-valvular conal ventricular septal/rotational defects, and non-compaction of the left ventricle. In some cases, depending on the functional defects of these cells, additional malformations are found in the expected spatial migratory area of the cells, namely in the pharyngeal arch derivatives and cervico-facial structures. Associated non-cardiovascular anomalies are often underestimated, since the multipotency and functional alteration of these cells can result in the modification of multiple neural, epidermal, and cervical structures at different levels. In most cases, patients do not display the full phenotype of abnormalities, but congenital cardiac defects involving the ventricular outflow tract, ascending aorta, aortic arch and supra-aortic trunks should be considered as markers for possible impaired function of these cells. Neural crest cells should not be considered as a unique cell population but on the basis of their cervical rhombomere origins R3-R5 or R6-R7-R8 and specific migration patterns: R3-R4 towards arch II, R5-R6 arch III and R7-R8 arch IV and VI. A better understanding of their development may lead to the discovery of unknown associated abnormalities, thereby enabling potential improvements to be made to the therapeutic approach.

Interruption of the Ascending Aorta-Biventricular Surgical Repair

Interruption of the ascending aorta is an extremely rare anomaly defined by a point of interruption between the intrapericardial and extrapericardial aorta and can be explained by developmental errors proximal to the embryologic right aortic sac. Herein, we present a case of interruption of the ascending aorta and describe a successful biventricular surgical repair of this unique anomaly.

The Role of Epigenetics in Congenital Heart Disease

Congenital heart disease (CHD) is the most common birth defect among newborns worldwide and contributes to significant infant morbidity and mortality. Owing to major advances in medical and surgical management, as well as improved prenatal diagnosis, the outcomes for these children with CHD have improved tremendously so much so that there are now more adults living with CHD than children. Advances in genomic technologies have discovered the genetic causes of a significant fraction of CHD, while at the same time pointing to remarkable complexity in CHD genetics. For this reason, the complex process of cardiogenesis, which is governed by multiple interlinked and dose-dependent pathways, is a well investigated process. In addition to the sequence of the genome, the contribution of epigenetics to cardiogenesis is increasingly recognized. Significant progress has been made dissecting the epigenome of the heart and identified associations with cardiovascular diseases. The role of epigenetic regulation in cardiac development/cardiogenesis, using tissue and animal models, has been well reviewed. Here, we curate the current literature based on studies in humans, which have revealed associated and/or causative epigenetic factors implicated in CHD. We sought to summarize the current knowledge on the functional role of epigenetics in cardiogenesis as well as in distinct CHDs, with an aim to provide scientists and clinicians an overview of the abnormal cardiogenic pathways affected by epigenetic mechanisms, for a better understanding of their impact on the developing fetal heart, particularly for readers interested in CHD research.

Anomalous origin of left subclavian artery from ascending aorta in a right aortic arch: A case report with review of literature

We present a case of an 11-year-old boy with a double-outlet right ventricle and a right-sided aortic arch showing anomalous origin of the left subclavian artery from the ascending aorta as the first branch. This case highlights the importance of knowledge of this aberrant anatomy before planning any surgical or endovascular intervention.

Muscularization of the Mesenchymal Outlet Septum during Cardiac Development

After the formation of the linear heart tube, it becomes divided into right and left components by the process of septation. Relatively late during this process, within the developing outflow tract, the initially mesenchymal outlet septum becomes muscularized as the result of myocardialization. Myocardialization is defined as the process in which existing cardiomyocytes migrate into flanking mesenchyme. Studies using genetically modified mice, as well as experimental approaches using in vitro models, demonstrate that Wnt and TGFβ signaling play an essential role in the regulation of myocardialization. They also show the significance of the interaction between cardiomyocytes, endocardial derived cells, neural crest cells, and the extracellular matrix. Interestingly, Wnt-mediated non-canonical planar cell polarity signaling was found to be a crucial regulator of myocardialization in the outlet septum and Wnt-mediated canonical β-catenin signaling is an essential regulator of the expansion of mesenchymal cells populating the outflow tract cushions.

Diagnosis of anomalous origin of the right subclavian artery from the right pulmonary artery in a patient with D-transposition of the great arteries utilizing transthoracic echocardiography

Diagnosis of anomalous origin of the right subclavian artery (AORSA) from the right pulmonary artery (RPA) is usually made using CT, MRI, or invasive angiography. We report a patient diagnosed using transthoracic echocardiography (TTE). A newborn girl prenatally known to have d-TGA presented with cyanosis sparing the right hemithorax and arm. Oxygen saturations on the right hand were persistently higher than on the right ear and other extremities. Repeat TTE using a modified echocardiographic imaging plane allowed for full visualization of the entire subclavian artery course, revealing AORSA from RPA. We discuss further the approach to echocardiographic diagnosis and surgical implications.

A Rare Presentation of Common Arterial Trunk with Intact Ventricular Septum

Common arterial trunk is a rare anomaly on its own, but with an intact ventricular septum it is extremely rare. An unexpected finding at autopsy prompted a review of the literature and a review of the developmental considerations associated with the outflow tracts. The case presented was an intrauterine fetal death at 37 weeks gestation. At autopsy, the only anatomic abnormalities were pulmonary dominant common arterial trunk with an intact ventricular septum, ventriculo-arterial septal defect, coarctation and widely patent arterial duct. A review of the literature and the developmental concepts related to the outflow tracts of the developing heart demonstrate the rare nature of this particular variation of common arterial trunk.

New Concepts in the Development and Malformation of the Arterial Valves

Although in many ways the arterial and atrioventricular valves are similar, both being derived for the most part from endocardial cushions, we now know that the arterial valves and their surrounding structures are uniquely dependent on progenitors from both the second heart field (SHF) and neural crest cells (NCC). Here, we will review aspects of arterial valve development, highlighting how our appreciation of NCC and the discovery of the SHF have altered our developmental models. We will highlight areas of research that have been particularly instructive for understanding how the leaflets form and remodel, as well as those with limited or conflicting results. With this background, we will explore how this developmental knowledge can help us to understand human valve malformations, particularly those of the bicuspid aortic valve (BAV). Controversies and the current state of valve genomics will be indicated.

BVES downregulation in non-syndromic tetralogy of fallot is associated with ventricular outflow tract stenosis

BVES is a transmembrane protein, our previous work demonstrated that single nucleotide mutations of BVES in tetralogy of fallot (TOF) patients cause a downregulation of BVES transcription. However, the relationship between BVES and the pathogenesis of TOF has not been determined. Here we reported our research results about the relationship between BVES and the right ventricular outflow tract (RVOT) stenosis. BVES expression was significantly downregulated in most TOF samples compared with controls. The expression of the second heart field (SHF) regulatory network genes, including NKX2.5, GATA4 and HAND2, was also decreased in the TOF samples. In zebrafish, bves knockdown resulted in looping defects and ventricular outflow tract (VOT) stenosis, which was mostly rescued by injecting bves mRNA. bves knockdown in zebrafish also decreased the expression of SHF genes, such as nkx2.5, gata4 and hand2, consistent with the TOF samples` results. The dual-fluorescence reporter system analysis showed that BVES positively regulated the transcriptional activity of GATA4, NKX2.5 and HAND2 promoters. In zebrafish, nkx2.5 mRNA partially rescued VOT stenosis caused by bves knockdown. These results indicate that BVES downregulation may be associated with RVOT stenosis of non-syndromic TOF, and bves is probably involved in the development of VOT in zebrafish.

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.

Embryonic development of bicuspid aortic valves

Bicuspid aortic valve (BAV) is the most common congenital cardiac malformation, frequently associated with aortopathies and valvulopathies. The congenital origin of BAV is suspected to impact the development of the disease in the adult life. During the last decade, a number of studies dealing with the embryonic development of congenital heart disease have significantly improved our knowledge on BAV etiology. They describe the developmental defects, at the molecular, cellular and morphological levels, leading to congenital cardiac malformations, including BAV, in animal models. These models consist of a spontaneous hamster and several mouse models with different genetic manipulations in genes belonging to a variety of pathways. In this review paper, we aim to gather information on the developmental defects leading to BAV formation in these animal models, in order to tentatively explain the morphogenetic origin of the spectrum of valve morphologies that characterizes human BAV. BAV may be the only defect resulting from gene manipulation in mice, but usually it appears as the less severe defect of a spectrum of malformations, most frequently affecting the cardiac outflow tract. The genes whose alterations cause BAV belong to different genetic pathways, but many of them are direct or indirectly associated with the NOTCH pathway. These molecular alterations affect three basic cellular mechanisms during heart development, i.e., endocardial-to-mesenchymal transformation, cardiac neural crest (CNC) cell behavior and valve cushion mesenchymal cell differentiation. The defective cellular functions affect three possible morphogenetic mechanisms, i.e., outflow tract endocardial cushion formation, outflow tract septation and valve cushion excavation. While endocardial cushion abnormalities usually lead to latero-lateral BAVs and septation defects to antero-posterior BAVs, alterations in cushion excavation may give rise to both BAV types. The severity of the original defect most probably determines the specific aortic valve phenotype, which includes commissural fusions and raphes. Based on current knowledge on the developmental mechanisms of the cardiac outflow tract, we propose a unified hypothesis of BAV formation, based on the inductive role of CNC cells in the three mechanisms of BAV development. Alterations of CNC cell behavior in three possible alternative key valvulogenic processes may lead to the whole spectrum of BAV.

Development of the human heart

In 2014, an extensive review discussing the major steps of cardiac development focusing on growth, formation of primary and chamber myocardium and the development of the cardiac electrical system, was published. Molecular genetic lineage analyses have since furthered our insight in the developmental origin of the various component parts of the heart, which currently can be unambiguously identified by their unique molecular phenotype. Moreover, genetic, molecular and cell biological analyses have driven insights into the mechanisms underlying the development of the different cardiac components. Here, we build on our previous review and provide an insight into the molecular mechanistic revelations that have forwarded the field of cardiac development. Despite the enormous advances in our knowledge over the last decade, the development of congenital cardiac malformations remains poorly understood. The challenge for the next decade will be to evaluate the different developmental processes using newly developed molecular genetic techniques to further unveil the gene regulatory networks operational during normal and abnormal cardiac development.

Anatomical variations of the aortic arch branches in a sample of Chinese cadavers: embryological basis and literature review

OBJECTIVES

The aim of this study is to determine the incidence and explore the types of aortic arch branch variations found in our cadavers.

METHODS

The types and incidence of aortic branch variations in 120 cadavers were analysed after careful dissection.

RESULTS

One hundred and six of 120 cadavers had normal aortic arch branches and gave rise to usual branches, namely the brachiocephalic trunk, the left common carotid artery and the left subclavian artery. The remaining 14 cadavers had 2 basic types of branch variations, thus accounting for an incidence of 11.67%. A total of 9 aortic arches emitted 4 branches; the brachiocephalic trunk, the left common carotid artery, the left vertebral artery and the left subclavian artery (incidence 7.5%). The second subgroup of 5 cadavers also emitted 4 aortic branches: the right common carotid artery, the left common carotid artery, the left subclavian artery and the right subclavian artery (incidence 4.16%). In this group, the right subclavian artery sprung as a distal branch of the aortic arch (descending), thus making a vascular ring that takes a superoposterior course round the back of the trachea and the oesophagus to reach the right side. There was a single cadaver, different from the other 4 aortic branches of the second group which had a common origin for the common carotid arteries, while the left subclavian artery and distally placed right subclavian artery were present. We did not observe any Kommerell's aortic diverticula.

CONCLUSIONS

The variations of aortic arch branching are complex and diverse due to varied possible alterations in the embryological processes. There is an imperative need for further research on these variations to elucidate the possible relationships with clinical diagnostic or surgical events.

Planar cell polarity signaling regulates polarized second heart field morphogenesis to promote both arterial and venous pole septation

The second heart field (SHF) harbors progenitors that are important for heart formation, but little is known about its morphogenesis. We show that SHF population in the mouse splanchnic mesoderm (SpM-SHF) undergoes polarized morphogenesis to preferentially elongate anteroposteriorly. Loss of Wnt5, a putative ligand of the planar cell polarity (PCP) pathway, causes the SpM-SHF to expand isotropically. Temporal tracking reveals that the Wnt5a lineage is a unique subpopulation specified as early as E7.5, and undergoes bi-directional deployment to form specifically the pulmonary trunk and the dorsal mesenchymal protrusion (DMP). In Wnt5a-/- mutants, Wnt5a lineage fails to extend into the arterial and venous poles, leading to both outflow tract and atrial septation defects that can be rescued by an activated form of PCP effector Daam1. We identify oriented actomyosin cables in the medial SpM-SHF as a potential Wnt5a-mediated mechanism that promotes SpM-SHF lengthening and restricts its widening. Finally, the Wnt5a lineage also contributes to the pulmonary mesenchyme, suggesting that Wnt5a/PCP is a molecular circuit recruited by the recently identified cardiopulmonary progenitors to coordinate morphogenesis of the pulmonary airways and the cardiac septations necessary for pulmonary circulation.This article has an associated 'The people behind the papers' interview.

High-Resolution Episcopic Microscopy (HREM): Looking Back on 13 Years of Successful Generation of Digital Volume Data of Organic Material for 3D Visualisation and 3D Display

High-resolution episcopic microscopy (HREM) is an imaging technique that permits the simple and rapid generation of three-dimensional (3D) digital volume data of histologically embedded and physically sectioned specimens. The data can be immediately used for high-detail 3D analysis of a broad variety of organic materials with all modern methods of 3D visualisation and display. Since its first description in 2006, HREM has been adopted as a method for exploring organic specimens in many fields of science, and it has recruited a slowly but steadily growing user community. This review aims to briefly introduce the basic principles of HREM data generation and to provide an overview of scientific publications that have been published in the last 13 years involving HREM imaging. The studies to which we refer describe technical details and specimen-specific protocols, and provide examples of the successful use of HREM in biological, biomedical and medical research. Finally, the limitations, potentials and anticipated further improvements are briefly outlined.

Isl-1 positive pharyngeal mesenchyme subpopulation and its role in the separation and remodeling of the aortic sac in embryonic mouse heart

BACKGROUND

Second heart field cells and neural crest cells have been reported to participate in the morphogenesis of the pharyngeal arch arteries (PAAs); however, how the PAAs grow out and are separated from the aortic sac into left and right sections is unknown.

RESULTS

An Isl-1 positive pharyngeal mesenchyme protrusion in the aortic sac ventrally extends and fuses with the aortic sac wall to form a midsagittal septum that divides the aortic sac. The aortic sac division separates the left and right PAAs to form independent arteries. The midsagittal septum dividing the aortic sac has a different expression pattern from the aortic-pulmonary (AP) septum in which Isl-1 positive cells are absent. At 11 days post-conception (dpc) in a mouse embryo, the Isl-1 positive mesenchyme protrusion appears as a heart-shaped structure, in which subpopulations with Isl-1+ Tbx3+ and Isl-1+ Nkx2.5+ cells are included.

CONCLUSIONS

The aortic sac is a dynamic structure that is continuously divided during the migration from the pharyngeal mesenchyme to the pericardial cavity. The separation of the aortic sac is not complete until the AP septum divides the aortic sac into the ascending aorta and pulmonary trunk. Moreover, the midsagittal septum and the AP septum are distinct structures.

Development and evolution of the metazoan heart

The mechanisms of the evolution and development of the heart in metazoans are highlighted, starting with the evolutionary origin of the contractile cell, supposedly the precursor of cardiomyocytes. The last eukaryotic common ancestor is likely a combination of several cellular organisms containing their specific metabolic pathways and genetic signaling networks. During evolution, these tool kits diversified. Shared parts of these conserved tool kits act in the development and functioning of pumping hearts and open or closed circulations in such diverse species as arthropods, mollusks, and chordates. The genetic tool kits became more complex by gene duplications, addition of epigenetic modifications, influence of environmental factors, incorporation of viral genomes, cardiac changes necessitated by air‐breathing, and many others. We evaluate mechanisms involved in mollusks in the formation of three separate hearts and in arthropods in the formation of a tubular heart. A tubular heart is also present in embryonic stages of chordates, providing the septated four‐chambered heart, in birds and mammals passing through stages with first and second heart fields. The four‐chambered heart permits the formation of high‐pressure systemic and low‐pressure pulmonary circulation in birds and mammals, allowing for high metabolic rates and maintenance of body temperature. Crocodiles also have a (nearly) separated circulation, but their resting temperature conforms with the environment. We argue that endothermic ancestors lost the capacity to elevate their body temperature during evolution, resulting in ectothermic modern crocodilians. Finally, a clinically relevant paragraph reviews the occurrence of congenital cardiac malformations in humans as derailments of signaling pathways during embryonic development.

The cardiac regulatory toolkit contains many factors including epigenetic, genetic, viral, hemodynamic, and environmental factors, but also transcriptional activators, repressors, duplicated genes, redundancies and dose‐dependancies.

Numerous toolkits regulate mechanisms including cell‐cell interactions, EMT, mitosis patterns, cell migration and differentiation and left/right sidedness involved in the development of endocardial cushions, looping, septum complexes, pharyngeal arch arteries, chamber and valve formation and conduction system.

Evolutionary development of the yolk sac circulation likely preceded the advent of endothermy in amniotes.

Parallel evolutionary traits regulate the development of contractile pumps in various taxa often in conjunction with the gut, lungs and excretory organs.

Common Arterial Trunk: Physiology, Imaging, and Management

Common arterial trunk (CAT), or truncus arteriosus, is a rare form of cyanotic congenital heart disease and is highly associated with DiGeorge syndrome (microdeletion 22q11.2). Prenatal diagnosis is highly feasible, allowing proper delivery planning and postnatal management. The clinical presentation is highly variable depending on the anatomical variation; however, most commonly presenting with mild cyanosis and significant tachypnea, although these patients can often go undetected in the immediate newborn period. Transthoracic echocardiography is adequate for diagnosis and detailed anatomical delineation in the majority. Additional imaging modalities such as cardiac catheterization, computed tomography angiography, or cardiac magnetic resonance imaging can be helpful in those with more complex pulmonary artery (PA) or aortic anatomy, or in the older repaired. The surgical management of CAT is complete repair in the neonatal period with resection of branch PAs from the CAT with placement of a right ventricular (RV)-to-PA conduit and patch closure of the ventricular septal defect. Overall surgical outcomes are excellent in most centers, with the expectation that the child will eventually outgrow the RV-to-PA conduit and require reoperation. Other potential reoperations or postsurgical interventions in addition to the RV-to-PA conduit may involve the truncal valve or branch PAs.

A new anatomic approach of the ventricular septal defect in the interruption of the aortic arch

The aim of this study was to analyse the anatomy of the ventricular septal defect (VSD) in heart specimens with interruption of the aortic arch (IAA) in order to explore the hypothesis of different embryologic mechanisms for the different anatomic types of IAA. We examined 42 human heart specimens, 25 with IAA as the main disease with concordant atrioventricular and ventriculo-arterial connections and two distinct great arteries, and 17 hearts with IAA associated with other malformations [six common arterial trunk (CAT), five double-outlet right ventricle (DORV), three transposition of the great arteries (TGA), three atrioventricular septal defect (AVSD)]. The interruption was classified according to Celoria and Patton. We focused on the anatomy of the VSD viewed from the right ventricular side. There were 15 IAA type A, 27 type B, no type C. The VSD in IAA type B was always an outlet VSD, located between the two limbs of the septal band, with posterior malalignment of the outlet septum in hearts with concordant ventriculo-arterial connections, without any fibrous tricuspid-aortic continuity. In addition, the aortic arch was always completely absent. Conversely, the VSD in IAA type A could be of any type (outlet in six, muscular in four, central perimembranous in two, inlet in three) and the aortic arch was either atretic or absent. In addition, IAA type B, when found in the setting of another anomaly, was always associated with neural crest-related anomalies (CAT and DORV), whereas IAA type A was found in association with anomalies not related to the neural crest (TGA and AVSD). These results reinforce the hypothesis that different pathogenic mechanisms are responsible for the two types of IAA, and the inclusion of IAA type B in the group of neural crest defects. Conversely, IAA type A could be due to overlapping mechanisms: flow-related defect (coarctation-like) and neural crest contribution.

Hemodynamics in Cardiac Development

The beating heart is subject to intrinsic mechanical factors, exerted by contraction of the myocardium (stretch and strain) and fluid forces of the enclosed blood (wall shear stress). The earliest contractions of the heart occur already in the 10-somite stage in the tubular as yet unsegmented heart. With development, the looping heart becomes asymmetric providing varying diameters and curvatures resulting in unequal flow profiles. These flow profiles exert various wall shear stresses and as a consequence different expression patterns of shear responsive genes. In this paper we investigate the morphological alterations of the heart after changing the blood flow by ligation of the right vitelline vein in a model chicken embryo and analyze the extended expression in the endocardial cushions of the shear responsive gene Tgfbeta receptor III. A major phenomenon is the diminished endocardial-mesenchymal transition resulting in hypoplastic (even absence of) atrioventricular and outflow tract endocardial cushions, which might be lethal in early phases. The surviving embryos exhibit several cardiac malformations including ventricular septal defects and malformed semilunar valves related to abnormal development of the aortopulmonary septal complex and the enclosed neural crest cells. We discuss the results in the light of the interactions between several shear stress responsive signaling pathways including an extended review of the involved Vegf, Notch, Pdgf, Klf2, eNos, Endothelin and Tgfβ/Bmp/Smad networks.

A novel source of arterial valve cells linked to bicuspid aortic valve without raphe in mice

Abnormalities of the arterial valve leaflets, predominantly bicuspid aortic valve, are the commonest congenital malformations. Although many studies have investigated the development of the arterial valves, it has been assumed that, as with the atrioventricular valves, endocardial to mesenchymal transition (EndMT) is the predominant mechanism. We show that arterial is distinctly different from atrioventricular valve formation. Whilst the four septal valve leaflets are dominated by NCC and EndMT-derived cells, the intercalated leaflets differentiate directly from Tnnt2-Cre+/Isl1+ progenitors in the outflow wall, via a Notch-Jag dependent mechanism. Further, when this novel group of progenitors are disrupted, development of the intercalated leaflets is disrupted, resulting in leaflet dysplasia and bicuspid valves without raphe, most commonly affecting the aortic valve. This study thus overturns the dogma that heart valves are formed principally by EndMT, identifies a new source of valve interstitial cells, and provides a novel mechanism for causation of bicuspid aortic valves without raphe.

The E3 ubiquitin ligase SMURF1 regulates cell-fate specification and outflow tract septation during mammalian heart development

Smad ubiquitin regulatory factor 1 (SMURF1) is a HECT-type E3 ubiquitin ligase that plays a critical role in vertebrate development by regulating planar cell polarity (PCP) signaling and convergent extension (CE). Here we show that SMURF1 is involved in mammalian heart development. We find that SMURF1 is highly expressed in outflow tract cushion mesenchyme and Smurf1^−/− mouse embryos show delayed outflow tract septation. SMURF1 is expressed in smooth muscle cells of the coronary arteries and great vessels. Thickness of the aortic smooth muscle cell layer is reduced in Smurf1^−/− mouse embryos. We show that SMURF1 is a negative regulator of cardiomyogenesis and a positive regulator of smooth muscle cell and cardiac fibroblast differentiation, indicating that SMURF1 is important for cell-type specification during heart development. Finally, we provide evidence that SMURF1 localizes at the primary cilium where it may regulate bone morphogenetic protein (BMP) signaling, which controls the initial phase of cardiomyocyte differentiation. In summary, our results demonstrate that SMURF1 is a critical regulator of outflow tract septation and cell-type specification during heart development, and that these effects may in part be mediated via control of cilium-associated BMP signaling.

Septation of the Intrapericardial Arterial Trunks in the Early Human Embryonic Heart

Background:

Outflow tract (OFT) septation defects are a common cause of congenital heart disease. Numerous studies have focused on the septation mechanism of the OFT, but have reported inconsistent conclusions. This study, therefore, aimed to investigate the septation of the aortic sac and the OFT in the early embryonic human heart.

Methods:

Serial sections of 27 human embryonic hearts from Carnegie stage (CS) 10 to CS19 were immunohistochemically stained with antibodies against α-smooth muscle actin (α-SMA) and myosin heavy chain.

Results:

At CS10–CS11, the OFT wall was an exclusively myocardial structure that was continuous with the aortic sac at the margin of the pericardial cavity. From CS13 onward, the OFT was divided into nonmyocardial and myocardial portions. The cushion formed gradually, and its distal border with the OFT myocardium was consistently maintained. The aortic sac between the fourth and sixth aortic arch arteries was degenerated. At CS16, the α-SMA-positive aortopulmonary septum formed and fused with the two OFT cushions, thus septating the nonmyocardial portion of the OFT into two arteries. At this stage, the cushions were not fused. At CS19, the bilateral cushions were fused to septate the myocardial portion of the OFT.

Conclusions:

Data suggest that the OFT cushion is formed before the aortopulmonary septum is formed. Thus, the OFT cushion is not derived from the aortopulmonary septum. In addition, the nonmyocardial part of the OFT is septated into the aorta and pulmonary trunk by the aortopulmonary septum, while the main part of the cushion fuses and septates the myocardial portion of the OFT.

Origins and consequences of congenital heart defects affecting the right ventricle

Congenital heart disease is a major health issue, accounting for a third of all congenital defects. Improved early surgical management has led to a growing population of adults with congenital heart disease, including patients with defects affecting the right ventricle, which are often classified as severe. Defects affecting the right ventricle often cause right ventricular volume or pressure overload and affected patients are at high risk for complications such as heart failure and sudden death. Recent insights into the developmental mechanisms and distinct developmental origins of the left ventricle, right ventricle, and the outflow tract have shed light on the common features and distinct problems arising in specific defects. Here, we provide a comprehensive overview of the current knowledge on the development into the normal and congenitally malformed right heart and the clinical consequences of several congenital heart defects affecting the right ventricle.