Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. III. An immunohistochemical analysis of the distribution of the neural tissue antigen G1N2 in the embryonic heart; implications for the development of the atrioventricular conduction system

The Atrioventricular Conduction Axis Revisited for the 21st Century

Although first described in the final decade of the 19th century, the axis responsible for atrioventricular conduction has long been the source of multiple controversies. Some of these continue to reverberate. When first described by His, for example, many doubted the existence of the bundle we now name in his honour, while Kent suggested that multiple pathways crossed the atrioventricular junctions in the normal heart. It was Tawara who clarified the situation, although many of his key definitions have not universally been accepted. In key studies in the third decade of the 20th century, Mahaim then suggested the presence of ubiquitous connections that provided "paraspecific" pathways for atrioventricular conduction. In this review, we show the validity of these original investigations, based on our own experience with a large number of datasets from human hearts prepared by serial histological sectioning. Using our own reconstructions, we show how the atrioventricular conduction axis can be placed back within the heart. We emphasise that newly emerging techniques will be key in providing the resolution to map cellular detail to the gross evidence provided by the serial sections.

The Terminal End of Retro-aortic root branch ------An unrecognized Origin for " Proximal Left Anterior Fascicle" Premature Ventricular Complexes with narrow QRS duration

BACKGROUND

Premature ventricular complexes (PVCs) with narrow QRS duration, inferior frontal plane QRS axis and right bundle branch block(RBBB) pattern generally originate from the proximal segment of the left anterior fascicle(LAF).

OBJECTIVE

This study aimed to investigate the exact origin of this category of PVCs.

METHODS

22 patients with assumed proximal LAF-PVCs were enrolled in the present study. Detailed mapping of fascicular potentials (FPs) was performed during sinus rhythm (SR) and PVCs.

RESULTS

During SR, a cluster of FPs could be found at the most superior portion of the left ventricle (LV). These FPs represented the terminal end of a discrete branch of the left fascicular system which we named the "retro-aortic root branch"(RARB). The shortest distance between the proximal LAF and the terminal end of RARB was 13.5±4.2mm. The earliest activation site of PVCs in all patients were confirmed at the terminal end of RARB, where the FP-V interval was 35.1±4.3 ms during PVCs. The shortest distance from the RCC to the EAS was 5.3±3.5mm. PVCs could be eliminated by ablation from the RCC in 45.5%(10/22) cases, in the remaining cases, ablation at the EAS in the LV endocardium successfully abolished PVCs.

CONCLUSIONS

The terminal end of the retro-aortic root branch was the actual origin site for PVCs with inferior frontal plane axis, RBBB pattern and narrow QRS duration. Ablation in the right coronary cusp or at the earliest activation site in the LV could both eliminate PVCs safely with high efficacy.

Fetal Tricuspid Valve Agenesis/Atresia: Testing Predictions of the Embryonic Etiology

Tricuspid valve agenesis/atresia (TVA) is a congenital cardiac malformation where the tricuspid valve is not formed. It is hypothesized that TVA results from a failure of the normal rightward expansion of the atrioventricular canal (AVC). We tested predictions of this hypothesis by morphometric analyses of the AVC in fetal hearts. We used high-resolution MRI and ultrasonography on a post-mortem fetal heart with TVA and with tricuspid valve stenosis (TVS) to validate the position of measurement landmarks that were to be applied to clinical echocardiograms. This revealed a much deeper right atrioventricular sulcus in TVA than in TVS. Subsequently, serial echocardiograms of in utero fetuses between 12 and 38 weeks of gestation were included (n = 23 TVA, n = 16 TVS, and n = 74 controls) to establish changes in AVC width and ventricular dimensions over time. Ventricular length and width and estimated fetal weight all increased significantly with age, irrespective of diagnosis. Heart rate did not differ between groups. However, in the second trimester, in TVA, the ratio of AVC to ventricular width was significantly lower compared to TVS and controls. This finding supports the hypothesis that TVA is due to a failed rightward expansion of the AVC. Notably, we found in the third trimester that the AVC to ventricular width normalized in TVA fetuses as their mitral valve area was greater than in controls. Hence, TVA associates with a quantifiable under-development of the AVC. This under-development is obscured in the third trimester, likely because of adaptational growth that allows for increased stroke volume of the left ventricle.

A pictorial account of the human embryonic heart between 3.5 and 8 weeks of development

Heart development is topographically complex and requires visualization to understand its progression. No comprehensive 3-dimensional primer of human cardiac development is currently available. We prepared detailed reconstructions of 12 hearts between 3.5 and 8 weeks post fertilization, using Amira® 3D-reconstruction and Cinema4D®-remodeling software. The models were visualized as calibrated interactive 3D-PDFs. We describe the developmental appearance and subsequent remodeling of 70 different structures incrementally, using sequential segmental analysis. Pictorial timelines of structures highlight age-dependent events, while graphs visualize growth and spiraling of the wall of the heart tube. The basic cardiac layout is established between 3.5 and 4.5 weeks. Septation at the venous pole is completed at 6 weeks. Between 5.5 and 6.5 weeks, as the outflow tract becomes incorporated in the ventricles, the spiraling course of its subaortic and subpulmonary channels is transferred to the intrapericardial arterial trunks. The remodeling of the interventricular foramen is complete at 7 weeks.

Inferior Extensions of the Atrioventricular Node

The pathways for excitation of the atrioventricular node enter either superiorly, as the so-called ‘fast’ pathway, or inferiorly as the ‘slow’ pathway. However, knowledge of the specific anatomical details of these pathways is limited. Most of the experimental studies that established the existence of these pathways were conducted in mammalian hearts, which have subtle differences to human hearts. In this review, the authors summarise their recent experiences investigating human cardiac development, correlating these results with the arrangement of the connections between the atrial myocardium and the compact atrioventricular node as revealed by serial sectioning of adult human hearts. They discuss the contributions made from the atrioventricular canal myocardium, as opposed to the primary ring. Both these rings are incorporated into the atrial vestibules, albeit with the primary ring contributing only to the tricuspid vestibule. The atrial septal cardiomyocytes are relatively late contributors to the nodal inputs. Finally, they relate our findings of human cardiac development to the postnatal arrangement.

New Insights into the Development and Morphogenesis of the Cardiac Purkinje Fiber Network: Linking Architecture and Function

The rapid propagation of electrical activity through the ventricular conduction system (VCS) controls spatiotemporal contraction of the ventricles. Cardiac conduction defects or arrhythmias in humans are often associated with mutations in key cardiac transcription factors that have been shown to play important roles in VCS morphogenesis in mice. Understanding of the mechanisms of VCS development is thus crucial to decipher the etiology of conduction disturbances in adults. During embryogenesis, the VCS, consisting of the His bundle, bundle branches, and the distal Purkinje network, originates from two independent progenitor populations in the primary ring and the ventricular trabeculae. Differentiation into fast-conducting cardiomyocytes occurs progressively as ventricles develop to form a unique electrical pathway at late fetal stages. The objectives of this review are to highlight the structure–function relationship between VCS morphogenesis and conduction defects and to discuss recent data on the origin and development of the VCS with a focus on the distal Purkinje fiber network.

Quantified growth of the human embryonic heart

The size and growth patterns of the components of the human embryonic heart have remained largely undefined. To provide these data, three-dimensional heart models were generated from immunohistochemically stained sections of ten human embryonic hearts ranging from Carnegie stage 10 to 23. Fifty-eight key structures were annotated and volumetrically assessed. Sizes of the septal foramina and atrioventricular canal opening were also measured. The heart grows exponentially throughout embryonic development. There was consistently less left than right atrial myocardium, and less right than left ventricular myocardium. We observed a later onset of trabeculation in the left atrium compared to the right. Morphometry showed that the rightward expansion of the atrioventricular canal starts in week 5. The septal foramina are less than 0.1 mm² and are, therefore, much smaller than postnatal septal defects. This chronological, graphical atlas of the growth patterns of cardiac components in the human embryo provides quantified references for normal heart development. Thereby, this atlas may support early detection of cardiac malformations in the foetus.This article has an associated First Person interview with the first author of the paper.

Development of the Cardiac Conduction System

The cardiac conduction system initiates and propagates each heartbeat. Specialized conducting cells are a well-conserved phenomenon across vertebrate evolution, although mammalian and avian species harbor specific components unique to organisms with four-chamber hearts. Early histological studies in mammals provided evidence for a dominant pacemaker within the right atrium and clarified the existence of the specialized muscular axis responsible for atrioventricular conduction. Building on these seminal observations, contemporary genetic techniques in a multitude of model organisms has characterized the developmental ontogeny, gene regulatory networks, and functional importance of individual anatomical compartments within the cardiac conduction system. This review describes in detail the transcriptional and regulatory networks that act during cardiac conduction system development and homeostasis with a particular emphasis on networks implicated in human electrical variation by large genome-wide association studies. We conclude with a discussion of the clinical implications of these studies and describe some future directions.

An Appreciation of Anatomy in the Molecular World

Robert H. Anderson is one of the most important and accomplished cardiac anatomists of the last decades, having made major contributions to our understanding of the anatomy of normal hearts and the pathologies of acquired and congenital heart diseases. While cardiac anatomy as a research discipline has become largely subservient to molecular biology, anatomists like Professor Anderson demonstrate anatomy has much to offer. Here, we provide cases of early anatomical insights on the heart that were rediscovered, and expanded on, by molecular techniques: migration of neural crest cells to the heart was deduced from histological observations (1908) and independently shown again with experimental interventions; pharyngeal mesoderm is added to the embryonic heart (1973) in what is now defined as the molecularly distinguishable second heart field; chambers develop from the heart tube as regional pouches in what is now considered the ballooning model by the molecular identification of regional differentiation and proliferation. The anatomical discovery of the conduction system by Purkinje, His, Tawara, Keith, and Flack is a special case because the main findings were never neglected in later molecular studies. Professor Anderson has successfully demonstrated that sound knowledge of anatomy is indispensable for proper understanding of cardiac development.

Reptiles as a Model System to Study Heart Development

A chambered heart is common to all vertebrates, but reptiles show unparalleled variation in ventricular septation, ranging from almost absent in tuataras to full in crocodilians. Because mammals and birds evolved independently from reptile lineages, studies on reptile development may yield insight into the evolution and development of the full ventricular septum. Compared with reptiles, mammals and birds have evolved several other adaptations, including compact chamber walls and a specialized conduction system. These adaptations appear to have evolved from precursor structures that can be studied in present-day reptiles. The increase in the number of studies on reptile heart development has been greatly facilitated by sequencing of several genomes and the availability of good staging systems. Here, we place reptiles in their phylogenetic context with a focus on features that are primitive when compared with the homologous features of mammals. Further, an outline of major developmental events is given, and variation between reptile species is discussed.

The State of Cardiovascular Developmental Biology is Strong - Honoring Dr. Roger Markwald and his Seminal Contributions to the Field

In August 2017, the Cardiovascular Developmental Biology Center (CDBC), together with the "Department of Regenerative Medicine and Cell Biology (RMCB) at the Medical University of South Carolina (MUSC), organized their 13th Annual CDBC Symposium. During this special event, which was organized in collaboration with The Anatomical Record, the unique and important contributions of Dr. Roger Markwald (known to all of us as Roger) to the field of cardiovascular research were celebrated. Fifteen leading investigators in the field presented their ideas and reported results of their studies to an audience that included many familiar faces from Roger's past and present. This group consisted of established investigators from around the world as well as young and upcoming scientists from local institutions. In their presentations, the platform speakers emphasized the significance of Roger's scientific contributions and advice to their professional development and career. In this Special Issue of The Anatomical Record, we assembled a collection of invited papers written by several attendees of the symposium. The issue also contains a number of articles written by colleagues who, for one reason or the other, were not able to attend the meeting, but expressed their desire to contribute to this special "festschrift" of The Anatomical Record in honor and recognition of Roger's amazing career. Anat Rec, 302:14-18, 2019. © 2018 Wiley Periodicals, Inc.

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.

Expression patterns of intermediate filament proteins desmin and lamin A in the developing conduction system of early human embryonic hearts

Since embryonic heart development is a complex process and acquisition of human embryonic specimens is challenging, the mechanism by which the embryonic conduction system develops remains unclear. Herein, we attempt to gain insights into this developmental process through immunohistochemical staining and 3D reconstructions. Expression analysis of T-box transcription factor 3, cytoskeleton desmin, and nucleoskeleton lamin A protein in human embryos in Carnegie stages 11-20 showed that desmin is preferentially expressed in the myocardium of the central conduction system compared with the peripheral conduction system, and is co-expressed with T-box transcription factor 3 in the central conduction system. Further, lamin A was first expressed in the embryonic ventricular trabeculations, where the terminal ramifications of the peripheral conduction system develop, and extended progressively to all parts of the central conduction system. The uncoupled spatiotemporal distribution pattern of lamin A and desmin indicated that the association of cytoskeleton desmin and nucleoskeleton lamin A may be a late event in human embryonic heart development. Compared with model animals, our data provide a direct morphological basis for understanding the arrhythmogenesis caused by mutations in human DES and LMNA genes.

Transcriptional regulation of the cardiac conduction system

The rate and rhythm of heart muscle contractions are coordinated by the cardiac conduction system (CCS), a generic term for a collection of different specialized muscular tissues within the heart. The CCS components initiate the electrical impulse at the sinoatrial node, propagate it from atria to ventricles via the atrioventricular node and bundle branches, and distribute it to the ventricular muscle mass via the Purkinje fibre network. The CCS thereby controls the rate and rhythm of alternating contractions of the atria and ventricles. CCS function is well conserved across vertebrates from fish to mammals, although particular specialized aspects of CCS function are found only in endotherms (mammals and birds). The development and homeostasis of the CCS involves transcriptional and regulatory networks that act in an embryonic-stage-dependent, tissue-dependent, and dose-dependent manner. This Review describes emerging data from animal studies, stem cell models, and genome-wide association studies that have provided novel insights into the transcriptional networks underlying CCS formation and function. How these insights can be applied to develop disease models and therapies is also discussed.

Remodeling of the Embryonic Interventricular Communication in Regard to the Description and Classification of Ventricular Septal Defects

Ventricular septal defects are the commonest congenital cardiac malformations. Appropriate knowledge of the steps involved in completion of ventricular septation should provide clues as to the morphology of the different phenotypes. Currently, however, consensus is lacking regarding the components of the developing ventricular septum, and how best to describe the different phenotypes seen in postnatal life. We have reassessed the previous investigations devoted to closure of the embryonic interventricular communication. On this basis, we discuss how studies in the early part of the 20th century correctly identified the steps involved in the remodeling of the embryonic interventricular foramen subsequent to the stage at which the outflow tract arises entirely above the cavity of the developing right ventricle. There has, however, already been remodeling of the foramen from the stage at which the atrioventricular canal is supported exclusively by the developing left ventricle. We show how these temporal changes in morphology can provide explanations for the different ventricular septal defects seen in the clinical setting. Thus, muscular defects represent inappropriate coalescence of muscular ventricular septum. The channels that are perimembranous are due to failure of closure of the persisting embryonic interventricular foramen. Those that are doubly committed and juxta-arterial reflect failure of formation of the free-standing subpulmonary muscular infundibular sleeve. The findings also point to the importance of appropriate alignment, during development, between the developing atrial and ventricular septums, and between the apical component of the ventricular septum and the ventricular outlet components. Anat Rec, 302:19-31, 2019. © 2018 Wiley Periodicals, Inc.

Embryonic Tbx3+ cardiomyocytes form the mature cardiac conduction system by progressive fate restriction

A small network of spontaneously active Tbx3+ cardiomyocytes forms the cardiac conduction system (CCS) in adults. Understanding the origin and mechanism of development of the CCS network are important steps towards disease modeling and the development of biological pacemakers to treat arrhythmias. We found that Tbx3 expression in the embryonic mouse heart is associated with automaticity. Genetic inducible fate mapping revealed that Tbx3+ cells in the early heart tube are fated to form the definitive CCS components, except the Purkinje fiber network. At mid-fetal stages, contribution of Tbx3+ cells was restricted to the definitive CCS. We identified a Tbx3+ population in the outflow tract of the early heart tube that formed the atrioventricular bundle. Whereas Tbx3+ cardiomyocytes also contributed to the adjacent Gja5+ atrial and ventricular chamber myocardium, embryonic Gja5+ chamber cardiomyocytes did not contribute to the Tbx3+ sinus node or to atrioventricular ring bundles. In conclusion, the CCS is established by progressive fate restriction of a Tbx3+ cell population in the early developing heart, which implicates Tbx3 as a useful tool for developing strategies to study and treat CCS diseases.

The Anatomy, Development, and Evolution of the Atrioventricular Conduction Axis

It is now well over 100 years since Sunao Tawara clarified the location of the axis of the specialised myocardium responsible for producing coordinated ventricular activation. Prior to that stellar publication, controversies had raged as to how many bundles crossed the place of the atrioventricular insulation as found in mammalian hearts, as well as the very existence of the bundle initially described by Wilhelm His Junior. It is, perhaps surprising that controversies continue, despite the multiple investigations that have taken place since the publication of Tawara’s monograph. For example, we are still unsure as to the precise substrates for the so-called slow and fast pathways into the atrioventricular node. Much has been done, nonetheless, to characterise the molecular make-up of the specialised pathways, and to clarify their mechanisms of development. Of this work itself, a significant part has emanated from the laboratory coordinated for a quarter of a century by Antoon FM Moorman. In this review, which joins the others in recognising the value of his contributions and collaborations, we review our current understanding of the anatomy, development, and evolution of the atrioventricular conduction axis.

Developmental Origin of the Cardiac Conduction System: Insight from Lineage Tracing

The components of the cardiac conduction system (CCS) generate and propagate the electrical impulse that initiates cardiac contraction. These interconnected components share properties, such as automaticity, that set them apart from the working myocardium of the atria and ventricles. A variety of tools and approaches have been used to define the CCS lineages. These include genetic labeling of cells expressing lineage markers and fate mapping of dye labeled cells, which we will discuss in this review. We conclude that there is not a single CCS lineage, but instead early cell fate decisions segregate the lineages of the CCS components while they remain interconnected. The latter is relevant for development of therapies for conduction system disease that focus on reprogramming cardiomyocytes or instruction of pluripotent stem cells.

Development and Function of the Cardiac Conduction System in Health and Disease

The generation and propagation of the cardiac impulse is the central function of the cardiac conduction system (CCS). Impulse initiation occurs in nodal tissues that have high levels of automaticity, but slow conduction properties. Rapid impulse propagation is a feature of the ventricular conduction system, which is essential for synchronized contraction of the ventricular chambers. When functioning properly, the CCS produces ~2.4 billion heartbeats during a human lifetime and orchestrates the flow of cardiac impulses, designed to maximize cardiac output. Abnormal impulse initiation or propagation can result in brady- and tachy-arrhythmias, producing an array of symptoms, including syncope, heart failure or sudden cardiac death. Underlying the functional diversity of the CCS are gene regulatory networks that direct cell fate towards a nodal or a fast conduction gene program. In this review, we will discuss our current understanding of the transcriptional networks that dictate the components of the CCS, the growth factor-dependent signaling pathways that orchestrate some of these transcriptional hierarchies and the effect of aberrant transcription factor expression on mammalian conduction disease.

Lineages of the Cardiac Conduction System

The cardiac conduction system (CCS) initiates and coordinately propagates the electrical impulse to orchestrate the heartbeat. It consists of a set of interconnected components with shared properties. A better understanding of the origin and specification of CCS lineages has allowed us to better comprehend the etiology of CCS disease and has provided leads for development of therapies. A variety of technologies and approaches have been used to investigate CCS lineages, which will be summarized in this review. The findings imply that there is not a single CCS lineage. In contrast, early cell fate decisions segregate the lineages of the CCS components while they remain connected to each other.

Disappearance of Idiopathic Outflow Tract Premature Ventricular Contractions After Catheter Ablation of Overt Accessory Pathways

BACKGROUND

Multiple mechanisms have been proposed for idiopathic premature ventricular contractions (PVCs) originating from the outflow tracts (OTs). Recent observations such as the coexistence of these arrhythmias with atrioventricular nodal reentrant tachycardias and the association between discrete prepotentials and successful ablation sites of ventricular arrhythmias (VAs) from the OTs suggest a common link.

OBJECTIVE

In this case series we draw attention to a unique association between accessory pathways (APs) and idiopathic PVCs from the OTs, disappearing after AP ablation.

METHODS

We identified 6 cases in collaboration with several international electrophysiology centers, which presented with pre-excitation in association with OT, and in 1 case inflow tract (IT), PVCs on 12-lead surface ECG.

RESULTS

Six cases displayed pre-excitation and PVCs, in 5 cases originating from the right ventricular outflow tract (RVOT) and in 1 case from the right ventricular inflow tract (RVIT). In all patients, PVCs were monomorphic and had fixed coupling intervals, in 3 cases presenting in bigeminy. Catheter ablation of the AP led to the simultaneous disappearance of PVCs in 5 of 6 cases. The sites of ablation were remote from the OTs in all these cases. In most cases, the occurrence of OT PVCs was closely associated with the presence of pre-excitation.

CONCLUSION

The coexistence of pre-excitation and PVCs from the OTs and the fact that in 5 of 6 cases PVCs disappeared after AP ablation suggests a common mechanism for arrhythmia genesis.

The formation and function of the cardiac conduction system

The cardiac conduction system (CCS) consists of distinctive components that initiate and conduct the electrical impulse required for the coordinated contraction of the cardiac chambers. CCS development involves complex regulatory networks that act in stage-, tissue- and dose-dependent manners, and recent findings indicate that the activity of these networks is sensitive to common genetic variants associated with cardiac arrhythmias. Here, we review how these findings have provided novel insights into the regulatory mechanisms and transcriptional networks underlying CCS formation and function.

Of Tracts, Rings, Nodes, Cusps, Sinuses, and Arrhythmias-A Comment on Szili-Torok et al.'s Paper Entitled "The 'Dead-End Tract' and Its Role in Arrhythmogenesis". J. Cardiovasc. Dev. Dis. 2016, 3, 11

In the review, now published as part of the special issue devoted to the development of the conduction tissues, de Vries and his colleagues discuss the potential role of the so-called "dead-end tract" as a substrate for arrhythmogenesis [1].[...].

The "Dead-End Tract" and Its Role in Arrhythmogenesis

Idiopathic outflow tract ventricular arrhythmias (VAs) represent a significant proportion of all VAs. The mechanism is thought to be catecholamine-mediated delayed after depolarizations and triggered activity, although other etiologies should be considered. In the adult cardiac conduction system it has been demonstrated that sometimes an embryonic branch, the so-called "dead-end tract", persists beyond the bifurcation of the right and left bundle branch (LBB). Several findings suggest an involvement of this tract in idiopathic VAs (IVAs). The aim of this review is to summarize our current knowledge and the possible clinical significance of this tract.

Segregation of Central Ventricular Conduction System Lineages in Early SMA+ Cardiomyocytes Occurs Prior to Heart Tube Formation

The cardiac conduction system (CCS) transmits electrical activity from the atria to the ventricles to coordinate heartbeats. Atrioventricular conduction diseases are often associated with defects in the central ventricular conduction system comprising the atrioventricular bundle (AVB) and right and left branches (BBs). Conducting and contractile working myocytes share common cardiomyogenic progenitors, however the time at which the CCS lineage becomes specified is unclear. In order to study the fate and the contribution to the CCS of cardiomyocytes during early heart tube formation, we performed a genetic lineage analysis using a Sma-CreERT2 mouse line. Lineage tracing experiments reveal a sequential contribution of early Sma expressing cardiomyocytes to different cardiac compartments, labeling at embryonic day (E) 7.5 giving rise to the interventricular septum and apical left ventricular myocardium. Early Sma expressing cardiomyocytes contribute to the AVB, BBs and left ventricular Purkinje fibers. Clonal analysis using the R26-confetti reporter mouse crossed with Sma-CreERT2 demonstrates that early Sma expressing cardiomyocytes include cells exclusively fated to give rise to the AVB. In contrast, lineage segregation is still ongoing for the BBs at E7.5. Overall this study highlights the early segregation of the central ventricular conduction system lineage within cardiomyocytes at the onset of heart tube formation.

The Epicardium and the Development of the Atrioventricular Junction in the Murine Heart

Insight into the role of the epicardium in cardiac development and regeneration has significantly improved over the past ten years. This is mainly due to the increasing availability of new mouse models for the study of the epicardial lineage. Here we focus on the growing understanding of the significance of the epicardium and epicardially-derived cells in the formation of the atrioventricular (AV) junction. First, through the process of epicardial epithelial-to-mesenchymal transformation (epiEMT), the subepicardial AV mesenchyme is formed. Subsequently, the AV-epicardium and epicardially-derived cells (EPDCs) form the annulus fibrosus, a structure important for the electrical separation of atrial and ventricular myocardium. Finally, the AV-EPDCs preferentially migrate into the parietal AV valve leaflets, largely replacing the endocardially-derived cell population. In this review, we provide an overview of what is currently known about the regulation of the events involved in this process.

Resolving cell lineage contributions to the ventricular conduction system with a Cx40-GFP allele: a dual contribution of the first and second heart fields

BACKGROUND

The ventricular conduction system (VCS) coordinates the heartbeat and is composed of central components (the atrioventricular node, bundle, and right and left bundle branches) and a peripheral Purkinje fiber network. Conductive myocytes develop from common progenitor cells with working myocytes in a bimodal process of lineage restriction followed by limited outgrowth. The lineage relationship between progenitor cells giving rise to different components of the VCS is unclear.

RESULTS

Cell lineage contributions to different components of the VCS were analysed by a combination of retrospective clonal analysis, regionalized transgene expression studies, and genetic tracing experiments using Connexin40-GFP mice that precisely delineate the VCS. Analysis of a library of hearts containing rare large clusters of clonally related myocytes identifies two VCS lineages encompassing either the right Purkinje fiber network or left bundle branch. Both lineages contribute to the atrioventricular bundle and right bundle branch that segregate early from working myocytes. Right and left VCS lineages share the transcriptional program of the respective ventricular working myocytes and genetic tracing experiments discount alternate progenitor cell contributions to the VCS.

CONCLUSIONS

The mammalian VCS is comprised of cells derived from two lineages, supporting a dual contribution of first and second heart field progenitor cells.

The effect of connexin40 deficiency on ventricular conduction system function during development

AIMS

The aim of this study was to characterize ventricular activation patterns in normal and connexin40-deficient mice in order to dissect the role of connexin40 in developing the conduction system.

METHODS AND RESULTS

We performed optical mapping of epicardial activation between ED9.5-18.5 and analysed ventricular activation patterns and times of left ventricular activation. Mouse embryos deficient for connexin40 were compared with normal and heterozygous littermates. Morphology of the primary interventricular ring (PIR) was delineated with the help of T3-LacZ transgene. Four major types of ventricular activation patterns characterized by primary breakthrough in different parts of the heart were detected during development: PIR, left ventricular apex, right ventricular apex, and dual right and left ventricular apices. Activation through PIR was frequently present at the early stages until ED12.5. From ED14.5, the majority of hearts showed dual left and right apical breakthrough, suggesting functionality of both bundle branches. Connexin40-deficient embryos showed initially a delay in left bundle branch function, but the right bundle branch block, previously described in the adults, was not detected in ED14.5 embryos and appeared only gradually with 80% penetrance at ED18.5.

CONCLUSION

The switch of function from the early PIR conduction pathway to the mature apex to base activation is dependent upon upregulation of connexin40 expression in the ventricular trabeculae. The early function of right bundle branch does not depend on connexin40. Quantitative analysis of normal mouse embryonic ventricular conduction patterns will be useful for interpretation of effects of mutations affecting the function of the cardiac conduction system.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Epicardially derived fibroblasts preferentially contribute to the parietal leaflets of the atrioventricular valves in the murine heart

The importance of the epicardium for myocardial and valvuloseptal development has been well established; perturbation of epicardial development results in cardiac abnormalities, including thinning of the ventricular myocardial wall and malformations of the atrioventricular valvuloseptal complex. To determine the spatiotemporal contribution of epicardially derived cells to the developing fibroblast population in the heart, we have used a mWt1/IRES/GFP-Cre mouse to trace the fate of EPDCs from embryonic day (ED)10 until birth. EPDCs begin to populate the compact ventricular myocardium around ED12. The migration of epicardially derived fibroblasts toward the interface between compact and trabecular myocardium is completed around ED14. Remarkably, epicardially derived fibroblasts do not migrate into the trabecular myocardium until after ED17. Migration of EPDCs into the atrioventricular cushion mesenchyme commences around ED12. As development progresses, the number of EPDCs increases significantly, specifically in the leaflets which derive from the lateral atrioventricular cushions. In these developing leaflets the epicardially derived fibroblasts eventually largely replace the endocardially derived cells. Importantly, the contribution of EPDCs to the leaflets derived from the major AV cushions is very limited. The differential contribution of EPDCs to the various leaflets of the atrioventricular valves provides a new paradigm in valve development and could lead to new insights into the pathogenesis of abnormalities that preferentially affect individual components of this region of the heart. The notion that there is a significant difference in the contribution of epicardially and endocardially derived cells to the individual leaflets of the atrioventricular valves has also important pragmatic consequences for the use of endocardial and epicardial cre-mouse models in studies of heart development.

Molecular analysis of patterning of conduction tissues in the developing human heart

BACKGROUND

Recent studies in experimental animals have revealed some molecular mechanisms underlying the differentiation of the myocardium making up the conduction system. To date, lack of gene expression data for the developing human conduction system has precluded valid extrapolations from experimental studies to the human situation.

METHODS AND RESULTS

We performed immunohistochemical analyses of the expression of key transcription factors, such as ISL1, TBX3, TBX18, and NKX2-5, ion channel HCN4, and connexins in the human embryonic heart. We supplemented our molecular analyses with 3-dimensional reconstructions of myocardial TBX3 expression. TBX3 is expressed in the developing conduction system and in the right venous valve, atrioventricular ring bundles, and retro-aortic nodal region. TBX3-positive myocardium, with exception of the top of the ventricular septum, is devoid of fast-conducting connexin40 and connexin43 and hence identifies slowly conducting pathways. In the early embryonic heart, we found wide expression of the pacemaker channel HCN4 at the venous pole, including the atrial chambers. HCN4 expression becomes confined during later developmental stages to the components of the conduction system. Patterns of expression of transcription factors, known from experimental studies to regulate the development of the sinus node and atrioventricular conduction system, are similar in the human and mouse developing hearts.

CONCLUSIONS

Our findings point to the comparability of mechanisms governing the development of the cardiac conduction patterning in human and mouse, which provide a molecular basis for understanding the functioning of the human developing heart before formation of a discrete conduction system.

Immunohistochemical distribution of desmin in the human fetal heart

Desmin is a member of the intermediate filaments, which play crucial roles in the maturation, maintenance and recovery of muscle fibers. Its expression has been examined in human cardiac muscle, rat and chicken, but its spatial distribution in the human fetal heart has not been described. The present study investigated desmin expression in the human fetal heart and associated great vessels in 14 mid-term fetuses from 9 to 18 weeks of gestation. Immunoreactivity for myosin heavy chain (MHC) and alpha smooth muscle actin (α-SMA), as well as neuron-specific enolase (NSE), was also examined. Increased expression of desmin from 9 to 18 weeks was clearly localized in the atrial wall, the proximal portions of the pulmonary vein and vena cava, and around the atrioventricular node. Desmin-positive structures were also positive for MHC. Meanwhile, the great vessels were also positive for α-SMA. The distribution of desmin exhibited a pattern quite different from that described in previous studies of rat and chicken. Thus, desmin in the human fetal heart does not seem to play a general role in myocardial differentiation but rather a specific role closely related to the maturation of the α-isozyme of MHC. Desmin expression in the developing fetal heart also appeared to be induced by mechanical stress due to the involvement of venous walls against the atrium.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

Sox4 mediates Tbx3 transcriptional regulation of the gap junction protein Cx43

Tbx3, a T-box transcription factor, regulates key steps in development of the heart and other organ systems. Here, we identify Sox4 as an interacting partner of Tbx3. Pull-down and nuclear retention assays verify this interaction and in situ hybridization reveals Tbx3 and Sox4 to co-localize extensively in the embryo including the atrioventricular and outflow tract cushion mesenchyme and a small area of interventricular myocardium. Tbx3, SOX4, and SOX2 ChIP data, identify a region in intron 1 of Gja1 bound by all tree proteins and subsequent ChIP experiments verify that this sequence is bound, in vivo, in the developing heart. In a luciferase reporter assay, this element displays a synergistic antagonistic response to co-transfection of Tbx3 and Sox4 and in vivo, in zebrafish, drives expression of a reporter in the heart, confirming its function as a cardiac enhancer. Mechanistically, we postulate that Sox4 is a mediator of Tbx3 transcriptional activity.

Posterior his bundle electrogram location in a patient with atrioventricular nodal reentrant tachycardia and structurally normal heart

Atrioventricular (AV) nodal reentrant tachycardia is the most common form of paroxysmal supraventricular tachycardia in adults, and slow AV nodal pathway ablation has evolved into first-line therapy. Variations in conduction system anatomy are occasionally found at electrophysiological study and may make this ablation procedure very challenging. Here, we present the case of a 69-year-old man with a structurally normal heart and posterior displaced His bundle who underwent successful slow pathway ablation. Demonstration of the characteristic slow pathway recording and His bundle electrogram is strongly recommended prior to radiofrequency energy application in the posterior septal region.

Biphasic development of the mammalian ventricular conduction system

RATIONALE

The ventricular conduction system controls the propagation of electric activity through the heart to coordinate cardiac contraction. This system is composed of specialized cardiomyocytes organized in defined structures including central components and a peripheral Purkinje fiber network. How the mammalian ventricular conduction system is established during development remains controversial.

OBJECTIVE

To define the lineage relationship between cells of the murine ventricular conduction system and surrounding working myocytes.

METHODS AND RESULTS

A retrospective clonal analysis using the alpha-cardiac actin(nlaacZ/+) mouse line was carried out in three week old hearts. Clusters of clonally related myocytes were screened for conductive cells using connexin40-driven enhanced green fluorescent protein expression. Two classes of clusters containing conductive cells were obtained. Mixed clusters, composed of conductive and working myocytes, reveal that both cell types develop from common progenitor cells, whereas smaller unmixed clusters, composed exclusively of conductive cells, show that proliferation continues after lineage restriction to the conduction system lineage. Differences in the working component of mixed clusters between the right and left ventricles reveal distinct progenitor cell histories in these cardiac compartments. These results are supported by genetic fate mapping using Cre recombinase revealing progressive restriction of connexin40-positive myocytes to a conductive fate.

CONCLUSIONS

A biphasic mode of development, lineage restriction followed by limited outgrowth, underlies establishment of the mammalian ventricular conduction system.

Effects of mechanical loading on early conduction system differentiation in the chick

The primary ring, a horseshoe-shaped structure situated between the left and right ventricle and connected superiorly to the atrioventricular canal, is the first specialized fast ventricular conduction pathway in the embryonic heart. It has been first defined immunohistochemically and is characterized as a region of slow myocyte proliferation. Recent studies have shown that it participates in spreading the ventricular electrical activation during stages preceding ventricular septation in the mouse, chick, and rat. Here we demonstrate its presence using optical mapping in chicks between embryonic days (ED) 3–5. We then tested the effects of hemodynamic unloading in the organ culture system upon its functionality. In ED3 hearts cultured without hemodynamic loading for 24 h, we observed a significant decrease in the percentage activated through the primary ring conduction pathway. A morphological examination revealed arrested growth, collapse, and elongation of the outflow tract and disorganized trabeculation. A similar reversal toward more primitive activation patterns was observed with culture between ED4 and ED5. This phenotype was completely rescued with the artificial loading of the ventricles with a droplet of silicone oil. We conclude that an appropriate loading is required during the early phases of the conduction system formation and maturation.

The Tbx2+ primary myocardium of the atrioventricular canal forms the atrioventricular node and the base of the left ventricle

The primary myocardium of the embryonic heart, including the atrioventricular canal and outflow tract, is essential for septation and valve formation. In the chamber-forming heart, the expression of the T-box transcription factor Tbx2 is restricted to the primary myocardium. To gain insight into the cellular contributions of the Tbx2+ primary myocardium to the components of the definitive heart, genetic lineage tracing was performed using a novel Tbx2Cre allele. These analyses revealed that progeny of Tbx2+ cells provide an unexpectedly large contribution to the Tbx2-negative ventricles. Contrary to common assumption, we found that the embryonic left ventricle only forms the left part of the definitive ventricular septum and the apex. The atrioventricular node, but not the atrioventricular bundle, was found to derive from Tbx2+ cells. The Tbx2+ outflow tract formed the right ventricle and right part of the ventricular septum. In Tbx2-deficient embryos, the left-sided atrioventricular canal was found to prematurely differentiate to chamber myocardium and to proliferate at increased rates similar to those of chamber myocardium. As a result, the atrioventricular junction and base of the left ventricle were malformed. Together, these observations indicate that Tbx2 temporally suppresses differentiation and proliferation of primary myocardial cells. A subset of these Tbx2Cre-marked cells switch off expression of Tbx2, which allows them to differentiate into chamber myocardium, to initiate proliferation, and to provide a large contribution to the ventricles. These findings imply that errors in the development of the early atrioventricular canal may affect a much larger region than previously anticipated, including the ventricular base.

Origin and fate of cardiac mesenchyme

The development of the embryonic heart is dependent upon the generation and incorporation of different mesenchymal subpopulations that derive from intra- and extra-cardiac sources, including the endocardium, epicardium, neural crest, and second heart field. Each of these populations plays a crucial role in cardiovascular development, in particular in the formation of the valvuloseptal apparatus. Notwithstanding shared mechanisms by which these cells are generated, their fate and function differ profoundly by their originating source. While most of our early insights into the origin and fate of the cardiac mesenchyme has come from experimental studies in avian model systems, recent advances in transgenic mouse technology has enhanced our ability to study these cell populations in the mammalian heart. In this article, we will review the current understanding of the role of cardiac mesenchyme in cardiac morphogenesis and discuss several new paradigms based on recent studies in the mouse.

Development of the cardiac conduction system

The cardiac conduction system (CCS) is a specialized tissue network that initiates and maintains a rhythmic heartbeat. The CCS consists of several functional subcomponents responsible for producing a pacemaking impulse and distributing action potentials across the heart in a coordinated manner. The formation of the distinct subcomponents of the CCS occurs within a precise temporal and spatial framework; thereby assuring that as the system matures from a tubular to a complex chambered organ, a rhythmic heartbeat is always maintained. Therefore, a defect in differentiation of any CCS component would lead to severe rhythm disturbances. Recent molecular, cell biological and physiological approaches have provided fresh and unexpected perspectives of the relationships between cell fate, gene expression and differentiation of specialized function within the developing myocardium. In particular, biomechanical forces created by the heartbeat itself have important roles in the inductive patterning and functional integration of the developing conduction system. This new understanding of the cellular origin and molecular induction of CCS tissues during embryogenesis may provide the foundation for tissue engineering, replacement and repair of these essential cardiac tissues in the future.