Three-dimensional and molecular analysis of the arterial pole of the developing human heart

Zebrafish arterial valve development occurs through direct differentiation of second heart field progenitors

AIMS

Bicuspid aortic valve (BAV) is the most common congenital heart defect, affecting at least 2% of the population. The embryonic origins of BAV remain poorly understood, with few assays for validating patient variants, limiting the identification of causative genes for BAV. In both human and mouse, the left and right leaflets of the arterial valves arise from the outflow tract cushions, with interstitial cells originating from neural crest cells and the overlying endocardium through endothelial-to-mesenchymal transition (EndoMT). In contrast, an EndoMT-independent mechanism of direct differentiation of cardiac progenitors from the second heart field (SHF) is responsible for the formation of the anterior and posterior leaflets. Defects in either of these developmental mechanisms can result in BAV. Although zebrafish have been suggested as a model for human variant testing, their naturally bicuspid arterial valve has not been considered suitable for understanding human arterial valve development. Here, we have set out to investigate to what extent the processes involved in arterial valve development are conserved in zebrafish and, ultimately, whether functional testing of BAV variants could be carried out.

METHODS AND RESULTS

Using a combination of live imaging, immunohistochemistry, and Cre-mediated lineage tracing, we show that the zebrafish arterial valve primordia develop directly from SHF progenitors with no contribution from EndoMT or neural crest, in keeping with the human and mouse anterior and posterior leaflets. Moreover, once formed, these primordia share common subsequent developmental events with all three aortic valve leaflets.

CONCLUSION

Our work highlights a conserved ancestral mechanism of arterial valve leaflet formation from the SHF and identifies that development of the arterial valve is distinct from that of the atrioventricular valve in zebrafish. Crucially, this confirms the utility of zebrafish for understanding the development of specific BAV subtypes and arterial valve dysplasia, offering potential for high-throughput variant testing.

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.

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.

Human Cardiac Development

Many aspects of heart development are topographically complex and require three-dimensional (3D) reconstruction to understand the pertinent morphology. We have recently completed a comprehensive primer of human cardiac development that is based on firsthand segmentation of structures of interest in histological sections. We visualized the hearts of 12 human embryos between their first appearance at 3.5 weeks and the end of the embryonic period at 8 weeks. The models were presented as calibrated, interactive, 3D portable document format (PDF) files. We used them to describe the appearance and the subsequent remodeling of around 70 different structures incrementally for each of the reconstructed stages. In this chapter, we begin our account by describing the formation of the single heart tube, which occurs at the end of the fourth week subsequent to conception. We describe its looping in the fifth week, the formation of the cardiac compartments in the sixth week, and, finally, the septation of these compartments into the physically separated left- and right-sided circulations in the seventh and eighth weeks. The phases are successive, albeit partially overlapping. Thus, the basic cardiac layout is established between 26 and 32 days after fertilization and is described as Carnegie stages (CSs) 9 through 14, with development in the outlet component trailing that in the inlet parts. Septation at the venous pole is completed at CS17, equivalent to almost 6 weeks of development. During Carnegie stages 17 and 18, in the seventh week, the outflow tract and arterial pole undergo major remodeling, including incorporation of the proximal portion of the outflow tract into the ventricles and transfer of the spiraling course of the subaortic and subpulmonary channels to the intrapericardial arterial trunks. Remodeling of the interventricular foramen, with its eventual closure, is complete at CS20, which occurs at the end of the seventh week. We provide quantitative correlations between the age of human and mouse embryos as well as the Carnegie stages of development. We have also set our descriptions in the context of variations in the timing of developmental features.

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.

Activation of AcvR1-Mediated Signaling Results in Semilunar Valve Defects

Calcific aortic valve disease (CAVD) is a common cardiac defect, particularly in the aging population. While several risk factors, such as bi-leaflet valve structure and old age, have been identified in CAVD pathogenesis, molecular mechanisms resulting in this condition are still under active investigation. Bone morphogenetic protein signaling via the activin type I receptor (AcvRI) plays an important role during physiological and pathological processes involving calcification, e.g., bone formation and heterotopic ossification. In addition, AcvRI is required for normal cardiac valve development, yet its role in aortic valve disease, if any, is currently unknown. Here, we induced the expression of constitutively active AcvRI in developing mouse embryos in the endocardium and in cells at the valve leaflet-wall junction that are not of endocardium origin using the Nfac1Cre transgene. The mutant mice were born alive, but showed thickened aortic and pulmonary valve leaflets during the early postnatal period. Adult mutant mice developed aortic stenosis with high frequency, sclerotic aortic valves, and displayed Alcian Blue-positive hypertrophic chondrocyte-like cells at the leaflet-wall junction. Calcification was only seen with low penetrance. In addition, we observed that the expression levels of gene sets associated with inflammation-related cytokine signaling, smooth muscle cell contraction, and cGMP signaling were altered in the mutants when compared with those of the controls. This work shows that, in a mouse model, such continuous AcvRI activity in the Nfatc1Cre recombination domain results in pathological changes in the aortic valve structure and function.

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.

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.

Dual role for CXCL12 signaling in semilunar valve development

Cxcl12-null embryos have dysplastic, misaligned, and hyperplastic semilunar valves (SLVs). In this study, we show that CXCL12 signaling via its receptor CXCR4 fulfills distinct roles at different stages of SLV development, acting initially as a guidance cue to pattern cellular distribution within the valve primordia during the endocardial-to-mesenchymal transition (endoMT) phase and later regulating mesenchymal cell proliferation during SLV remodeling. Transient, anteriorly localized puncta of internalized CXCR4 are observed in cells undergoing endoMT. In vitro, CXCR4+ cell orientation in response to CXCL12 requires phosphatidylinositol 3-kinase (PI3K) signaling and is inhibited by suppression of endocytosis. This dynamic intracellular localization of CXCR4 during SLV development is related to CXCL12 availability, potentially enabling activation of divergent downstream signaling pathways at key developmental stages. Importantly, Cxcr7-/- mutants display evidence of excessive CXCL12 signaling, indicating a likely role for atypical chemokine receptor CXCR7 in regulating ligand bioavailability and thus CXCR4 signaling output during SLV morphogenesis.

Heart Development and Congenital Structural Heart Defects

Congenital heart disease is the most frequent birth defect and the leading cause of death for the fetus and in the first year of life. The wide phenotypic diversity of congenital heart defects requires expert diagnosis and sophisticated repair surgery. Although these defects have been described since the seventeenth century, it was only in 2005 that a consensus international nomenclature was adopted, followed by an international classification in 2017 to help provide better management of patients. Advances in genetic engineering, imaging, and omics analyses have uncovered mechanisms of heart formation and malformation in animal models, but approximately 80% of congenital heart defects have an unknown genetic origin. Here, we summarize current knowledge of congenital structural heart defects, intertwining clinical and fundamental research perspectives, with the aim to foster interdisciplinary collaborations at the cutting edge of each field. We also discuss remaining challenges in better understanding congenital heart defects and providing benefits to patients.

Lack of morphometric evidence for ventricular compaction in humans

The remodeling of the compact wall by incorporation of trabecular myocardium, referred to as compaction, receives much attention because it is thought that its failure causes left ventricular non-compaction cardiomyopathy (LVNC). Although the notion of compaction is broadly accepted, the nature and strength of the evidence supporting this process is underexposed. Here, we review the literature that quantitatively investigated the development of the ventricular wall to understand the extent of compaction in humans, mice, and chickens. We queried PubMed using several search terms, screened 1127 records, and selected 56 publications containing quantitative data on ventricular growth. For humans, only 34 studies quantified wall development. The key premise of compaction, namely a reduction of the trabecular layer, was never documented. Instead, the trabecular layer grows slower than the compact wall in later development and this changes wall architecture. There were no reports of a sudden enlargement of the compact layer (from incorporated trabeculae), be it in thickness, area, or volume. Therefore, no evidence for compaction was found. Only in chickens, a sudden increase in compact myocardial thickness layer was reported coinciding with a decrease in trabecular thickness. In mice, morphometric and lineage tracing investigations have yielded conflicting results that allow for limited compaction to occur. In conclusion, compaction in human development is not supported while rapid intrinsic growth of the compact wall is supported in all species. If compaction takes place, it likely plays a much smaller role in determining wall architecture than intrinsic growth of the compact wall.

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.

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.

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.

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.

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.

Cells, Materials, and Fabrication Processes for Cardiac Tissue Engineering

Cardiovascular disease is the number one killer worldwide, with myocardial infarction (MI) responsible for approximately 1 in 6 deaths. The lack of endogenous regenerative capacity, added to the deleterious remodelling programme set into motion by myocardial necrosis, turns MI into a progressively debilitating disease, which current pharmacological therapy cannot halt. The advent of Regenerative Therapies over 2 decades ago kick-started a whole new scientific field whose aim was to prevent or even reverse the pathological processes of MI. As a highly dynamic organ, the heart displays a tight association between 3D structure and function, with the non-cellular components, mainly the cardiac extracellular matrix (ECM), playing both fundamental active and passive roles. Tissue engineering aims to reproduce this tissue architecture and function in order to fabricate replicas able to mimic or even substitute damaged organs. Recent advances in cell reprogramming and refinement of methods for additive manufacturing have played a critical role in the development of clinically relevant engineered cardiovascular tissues. This review focuses on the generation of human cardiac tissues for therapy, paying special attention to human pluripotent stem cells and their derivatives. We provide a perspective on progress in regenerative medicine from the early stages of cell therapy to the present day, as well as an overview of cellular processes, materials and fabrication strategies currently under investigation. Finally, we summarise current clinical applications and reflect on the most urgent needs and gaps to be filled for efficient translation to the clinical arena.

Pulmonary ductal coarctation and left pulmonary artery interruption; pathology and role of neural crest and second heart field during development

Objectives

In congenital heart malformations with pulmonary stenosis to atresia an abnormal lateral ductus arteriosus to left pulmonary artery connection can lead to a localised narrowing (pulmonary ductal coarctation) or even interruption We investigated embryonic remodelling and pathogenesis of this area.

Material and methods

Normal development was studied in WntCre reporter mice (E10.0–12.5) for neural crest cells and Nkx2.5 immunostaining for second heart field cells. Data were compared to stage matched human embryos and a VEGF120/120 mutant mouse strain developing pulmonary atresia.

Results

Normal mouse and human embryos showed that the mid-pharyngeal endothelial plexus, connected side-ways to the 6^th pharyngeal arch artery. The ventral segment formed the proximal pulmonary artery. The dorsal segment (future DA) was solely surrounded by neural crest cells. The ventral segment had a dual outer lining with neural crest and second heart field cells, while the distal pulmonary artery was covered by none of these cells. The asymmetric contribution of second heart field to the future pulmonary trunk on the left side of the aortic sac (so-called pulmonary push) was evident. The ventral segment became incorporated into the pulmonary trunk leading to a separate connection of the left and right pulmonary arteries. The VEGF120/120 embryos showed a stunted pulmonary push and a variety of vascular anomalies.

Summary

Side-way connection of the DA to the left pulmonary artery is a congenital anomaly. The primary problem is a stunted development of the pulmonary push leading to pulmonary stenosis/atresia and a subsequent lack of proper incorporation of the ventral segment into the aortic sac. Clinically, the aberrant smooth muscle tissue of the ductus arteriosus should be addressed to prohibit development of severe pulmonary ductal coarctation or even interruption of the left pulmonary artery.

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.

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.

Three-Dimensional Portable Document Format (3D PDF) in Clinical Communication and Biomedical Sciences: Systematic Review of Applications, Tools, and Protocols

Background

The Portable Document Format (PDF) is the standard file format for the communication of biomedical information via the internet and for electronic scholarly publishing. Although PDF allows for the embedding of three-dimensional (3D) objects and although this technology has great potential for the communication of such data, it is not broadly used by the scientific community or by clinicians.

Objective

The objective of this review was to provide an overview of existing publications that apply 3D PDF technology and the protocols and tools for the creation of model files and 3D PDFs for scholarly purposes to demonstrate the possibilities and the ways to use this technology.

Methods

A systematic literature review was performed using PubMed and Google Scholar. Articles searched for were in English, peer-reviewed with biomedical reference, published since 2005 in a journal or presented at a conference or scientific meeting. Ineligible articles were removed after screening. The found literature was categorized into articles that (1) applied 3D PDF for visualization, (2) showed ways to use 3D PDF, and (3) provided tools or protocols for the creation of 3D PDFs or necessary models. Finally, the latter category was analyzed in detail to provide an overview of the state of the art.

Results

The search retrieved a total of 902 items. Screening identified 200 in-scope publications, 13 covering the use of 3D PDF for medical purposes. Only one article described a clinical routine use case; all others were pure research articles. The disciplines that were covered beside medicine were many. In most cases, either animal or human anatomies were visualized. A method, protocol, software, library, or other tool for the creation of 3D PDFs or model files was described in 19 articles. Most of these tools required advanced programming skills and/or the installation of further software packages. Only one software application presented an all-in-one solution with a graphical user interface.

Conclusions

The use of 3D PDF for visualization purposes in clinical communication and in biomedical publications is still not in common use, although both the necessary technique and suitable tools are available, and there are many arguments in favor of this technique. The potential of 3D PDF usage should be disseminated in the clinical and biomedical community. Furthermore, easy-to-use, standalone, and free-of-charge software tools for the creation of 3D PDFs should be developed.

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.

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.

Hitherto unknown detailed muscle anatomy in an 8-week-old embryo

Congenital muscle diseases, such as myopathies or dystrophies, occur relatively frequently, with estimated incidences of up to 4.7 per 100 000 newborns. To diagnose congenital diseases in the early stages of pregnancy, and to interpret the results of increasingly advanced in utero imaging techniques, a profound knowledge of normal human morphological development of the locomotor system and the nervous system is necessary. Muscular development, however, is an often neglected topic or is only described in a general way in embryology textbooks and papers. To provide the required detailed and updated comprehensive picture of embryologic muscular anatomy, three‐dimensional (3D) reconstructions were created based on serial histological sections of a human embryo at Carnegie stage 23 (8 weeks of development, crown–rump length of 23.8 mm), using amira reconstruction software. Reconstructed muscles, tendons, bones and nerves were exported in a 3D‐PDF file to permit interactive viewing. Almost all adult skeletal muscles of the trunk and limbs could be individually identified in their relative adult position. The pectoralis major muscle was divided in three separate muscle heads. The reconstructions showed remarkable highly developed extraocular, infrahyoid and suprahyoid muscles at this age but surprisingly also absence of the facial muscles that have been described to be present at this stage of development. The overall stage of muscle development suggests heterochrony of skeletal muscle development. Several individual muscle groups were found to be developed earlier and in more detail than described in current literature.

Changes in topographical relation between the ductus arteriosus and left subclavian artery in human embryos: a study using serial sections

At birth, the ductus arteriosus (DA) merges with the aortic arch in the caudal side of the origin of the left subclavian artery (ltSCA). Since the SCA (seventh segmental arteries) were fixed on the levels of the seventh cervical-first thoracic vertebral bodies, the confluence of the DA should migrate caudally toward the lower level. We aimed to describe the changing topographical anatomy of the DA and SCA using serial sections. First, we examined serial sagittal sections of 11 embryos (Carnegie stage 15-18), but the specimens were clearly divided into 2 groups with and without the lower confluence of the DA. Next, we examined serial horizontal sections of 40 specimens (Carnegie stage 14-16) and we chose 5 specimens (CRL 11 mm, 3 specimen; 1, 14 mm; 1, 15 mm) including the DA near (within 1-vertebral segment from) the ltSCA. The final approach of the DA occurred during the heart descent in which the apex of the heart migrated from the level of the first to the fourth thoracic vertebral body. Thus, the DA reached the SCA level before establishment of the heart descent. The right aortic arch maintained its entire course in 2 of the 5 specimens. Therefore, the positioning of the DA along the left aortic arch might occur independently of degeneration of the right arch. Notably, the tracheal bifurcation level was higher when the DA-ltSCA distance was greater. A contribution of the increased pulmonary volume was suggested for the final approach of the DA.

The independence of the infundibular building blocks in the setting of double-outlet right ventricle

It has long been contentious as to whether the presence of bilateral infundibulums, or conuses, is a prerequisite for the diagnosis of double-outlet right ventricle. As the use of such a criterion would abrogate the so-called "morphological method", which correctly states that one variable entity should not be defined on the basis of another entity that is itself variable, it is now accepted that double outlet can exist in the setting of fibrous continuity between the leaflets of the atrioventricular and arterial valves. Although this debate has now been resolved, there are other contentious areas still requiring clarification in the setting of hearts unified because of the presence of this particular ventriculo-arterial connection - for example, it is questionable whether the channel between the ventricles should be described as a "ventricular septal defect", whereas it is equally arguable that the mere presence of fibrous continuity between the leaflets of the arterial valves does not necessarily place the channel in a doubly committed location. In this review, we describe a series of autopsied hearts in which the anatomical features serve to illuminate these various topics. We then discuss recent findings regarding cardiac development that point to the individuality of the building blocks of the ventricular outflow tracts, specifically the outlet septum, the inner heart curvature, or ventriculo-infundibular fold, and the septomarginal trabeculation, or septal band.

Development and Morphology of the Ventricular Outflow Tracts

It is customary, at the current time, to consider many, if not most, of the lesions involving the ventricular outflow tract in terms of conotruncal malformations. This reflects the introduction, in the early 1940s, of the terms conus and truncus to describe the components of the developing outflow tract. The definitive outflow tracts in the postnatal heart, however, possess three, rather than two, components. These are the intrapericardial arterial trunks, the arterial roots, and the subvalvar ventricular outflow tracts. Congenital lesions afflicting the arterial roots, however, are not currently considered to be conotruncal malformations. This suggests a lack of logic in the description of cardiac development and its use as a means of categorizing congenital malformations. It is our belief that the developing outflow tract, like the postnatal outflow tracts, can readily be described in tripartite fashion, with its distal, intermediate, and proximal components forming the primordiums of the postnatal parts. In this review, we present evidence obtained from developing mice and human hearts to substantiate this notion. We show that the outflow tract, initially with a common lumen, is divided into its aortic and pulmonary components by a combination of an aortopulmonary septum derived from the dorsal wall of the aortic sac and outflow tract cushions that spiral through its intermediate and proximal components. These embryonic septal structures, however, subsequently lose their septal functions as the outflow tracts develop their own discrete walls. We then compare the developmental findings with the anatomic arrangements seen postnatally in the normal human heart. We show how correlations with the embryologic findings permit logical analysis of the congenital lesions involving the outflow tracts.

Variations in structure of the outflow tract of the human embryonic heart: A new hypothesis for generating bicuspid aortic semilunar valves

Outflow tract development of the heart is complex. The presence, differential growth and interactions of the various tissues through space and time contribute to the final development of the tract. This paper presents a novel interpretation of observations of outflow tract development, in particular of the aortic and pulmonary semilunar valves in embryos from the Shaner Collection at the University of Alberta. Three-dimensional reconstructions assist in the visualization of the spatial relationships of the developing valve tissues. In some embryos the aortic intercalated valve swelling is displaced proximally, giving rise to a bicuspid aortic semilunar valve more distally. In addition, the developing valve tissue first appears external to the myocardial cuff. The pulmonary semilunar valve regions appear to be more normal. This paper thus proposes a novel mechanism for generating a bicuspid aortic valve and also supports the idea that there is some independence of the aortic and pulmonary regions from each other during development.

Wnt signaling in the heart fields: Variations on a common theme

Wnt signaling plays an essential role in development and differentiation. Heart development is initiated with the induction of precardiac mesoderm requiring the tightly and spatially controlled regulation of canonical and noncanonical Wnt signaling pathways. The role of Wnt signaling in subsequent development of the heart fields is to a large extent unclear. We will discuss the role of Wnt signaling in the development of the arterial and venous pole of the heart, highlighting the dual roles of Wnt signaling with respect to its time- and dosage-dependent effects and the balance between the canonical and noncanonical signaling. Canonical signaling appears to be involved in retaining the cardiac precursors in a proliferative and precursor state, whereas noncanonical signaling promotes their differentiation. Thereafter, both canonical and noncanonical signaling regulate specific steps in differentiation of the cardiac compartments. Because heart development is a contiguous, rather than a sequential, process, analyses tend only to show a single timeframe of development. The repetitive alternating and reciprocal effect of canonical and noncanonical signaling is lost when studied in homogenates. Without the simultaneous in vivo visualization of the different Wnt signaling pathways, the mechanism of Wnt signaling in heart development remains elusive.

Endocardial Brg1 disruption illustrates the developmental origins of semilunar valve disease

The formation of intricately organized aortic and pulmonic valves from primitive endocardial cushions of the outflow tract is a remarkable accomplishment of embryonic development. While not always initially pathologic, developmental semilunar valve (SLV) defects, including bicuspid aortic valve, frequently progress to a disease state in adults requiring valve replacement surgery. Disrupted embryonic growth, differentiation, and patterning events that "trigger" SLV disease are coordinated by gene expression changes in endocardial, myocardial, and cushion mesenchymal cells. We explored roles of chromatin regulation in valve gene regulatory networks by conditional inactivation of the Brg1-associated factor (BAF) chromatin remodeling complex in the endocardial lineage. Endocardial Brg1-deficient mouse embryos develop thickened and disorganized SLV cusps that frequently become bicuspid and myxomatous, including in surviving adults. These SLV disease-like phenotypes originate from deficient endocardial-to-mesenchymal transformation (EMT) in the proximal outflow tract (pOFT) cushions. The missing cells are replaced by compensating neural crest or other non-EMT-derived mesenchyme. However, these cells are incompetent to fully pattern the valve interstitium into distinct regions with specialized extracellular matrices. Transcriptomics reveal genes that may promote growth and patterning of SLVs and/or serve as disease-state biomarkers. Mechanistic studies of SLV disease genes should distinguish between disease origins and progression; the latter may reflect secondary responses to a disrupted developmental system.

How frequent is the fifth arch artery?

Although usually shown in embryology textbooks, the presence of the fifth pair of pharyngeal arch arteries has long been controversial. To the best of our knowledge, six pairs of bilaterally symmetrical arteries developing within the pharyngeal arches are yet to be found in any mammalian or avian species. Collateral channels between the distal ends of the fourth and sixth arch arteries, in contrast, have been found in up to half of all developing mouse embryos. In only one human embryo, again to the best of our knowledge, has a channel been found that extends from the aortic sac to the dorsal aorta, and hence qualifies as an arch artery. Despite these confounding factors in terms of the developmental heritage of the fifth arch arteries, the purported channels are invoked with increasing frequency to describe various lesions discovered in the setting of the congenitally malformed heart. Persistence of the artery of the fifth arch was initially proposed to explain double-barrelled aorta. It was subsequently proposed to account for various systemic-to-pulmonary channels feeding the pulmonary circulation in the setting of pulmonary atresia. It has also been claimed to persist so as to explain abnormal branching of the brachiocephalic arteries from the aortic arch. In the light of the ongoing doubts concerning the existence of the arteries of the fifth arch themselves, we have reviewed the various descriptions of purported fifth arch arteries within the world literature. We have then sought to validate the descriptions on the basis of our own understanding of development, for this purpose providing images of the remoulding arch arteries in the mouse so as to substantiate our conclusions. While accepting that our own interpretations are speculative, we suggest that more convincing alternative explanations can be advanced to account for the majority of lesions currently interpreted on the basis of persistence of the arteries of the fifth arches. Although the interpretations do not necessarily change the therapeutic approaches to the channels, appropriate description is important in terms of their classification.

Fibulin-1 suppresses endothelial to mesenchymal transition in the proximal outflow tract

Endothelial to mesenchymal transition (EMT) that occurs during cardiac outflow tract (OFT) development is critical for formation of the semilunar valves. Fibulin-1 (Fbln1) is an extracellular matrix protein that is present at several sites of EMT, including the OFT (i.e., E9.5-10.5). The aim of this study was to determine the role of Fbln1 in EMT during the earliest events of OFT development. Examination of proximal OFT cushions in Fbln1 null embryos detected hypercellularity at both E9.5 (93% increase; p = 0.002) and E10.5 (43% increase; p = 0.01) as compared to wild type, suggesting that Fbln1 normally suppresses OFT endocardial cushion EMT. This was supported by studies of proximal OFT cushion explants, which showed that explants from Fbln1 null embryos displayed a 58% increase in cells migrating from the explants as compared to wild type (p = 0.005). We next evaluated the effects of Fbln1 deficiency on the expression of factors that regulate proximal OFT EMT. At E9.5, Fbln1 null proximal OFT endocardium and EMT-derived mesenchyme showed increased TGFβ2 (58% increase; p = 0.01) and increased Snail1-positive nuclei (27% increase; p = 0.0003). Histological examination of OFT cushions in Fbln1 null embryos (E9.5) also detected cells present in the cushion that were determined to be erythrocytes based on round morphology, autofluorescence, and positive staining for hemoglobin. Erythrocytes were also detected in Fbln1 null OFT cushions at E10.5. Together, the findings indicate that Fbln1 normally suppresses proximal OFT EMT preventing proximal cushion hypercellularity and blood cell accumulation.

The problems that exist when considering the anatomic variability between the channels that permit interventricular shunting

Although steps are being taken to produce a universally acceptable coding system for categorisation of the congenitally malformed hearts, obstacles remain in the search for consensus. One of the groups of lesions continuing to produce the greatest problems is those that permit interventricular shunting. The difficulties relate partly to the words used to describe the group itself, as those using Germanic languages describe the holes as ventricular septal defects, whereas those using Romance languages consider them to represent interventricular communications. The two terms, however, are not necessarily synonymous. Further disagreements relate to whether the lesions placed within the group should be sub-categorised on the basis of their geographical location within the ventricular mass, as opposed to the anatomic nature of their borders. In reality, attention to both the features is necessary if we are to recognise the full extent of phenotypic variability. In this review, we first review the evolution and theories of analysis naming the channels that permit interventricular shunting. We then demonstrate that embryologic techniques provide evidence that the changing morphology of the developing murine heart parallels the anatomy of the different lesions encountered in the congenitally malformed human heart. We suggest that, with attention paid to the temporal development of the normal murine heart, combined with a strict definition of the plane of separation between the right and left ventricular cavities, it will be feasible to produce a categorisation that is acceptable to all.

A detailed comparison of mouse and human cardiac development

BACKGROUND

Mouse mutants are used to model human congenital cardiovascular disease. Few studies exist comparing normal cardiovascular development in mice vs. humans. We carried out a systematic comparative analysis of mouse and human fetal cardiovascular development.

METHODS

Episcopic fluorescence image capture (EFIC) was performed on 66 wild-type mouse embryos from embryonic day (E) 9.5 to birth; 2-dimensional and 3-dimensional datasets were compared with EFIC and magnetic resonance images from a study of 52 human fetuses (Carnegie stage 13-23).

RESULTS

Time course of atrial, ventricular, and outflow septation were outlined and followed a similar sequence in both species. Bilateral venae cavae and prominent atrial appendages were seen in the mouse fetus; in human fetuses, atrial appendages were small, and a single right superior vena cava was present. In contrast to humans with separate pulmonary vein orifices, a pulmonary venous confluence with one orifice enters the left atrium in mice.

CONCLUSION

The cardiac developmental sequences observed in mouse and human fetuses are comparable, with minor differences in atrial and venous morphology. These comparisons of mouse and human cardiac development strongly support that mouse morphogenesis is a good model for human development.

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

Background

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

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

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

Methods

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

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

Results

We can observe sequentially:

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

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

Conclusions

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

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

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

We are promoting new studies to investigate our anatomical observations.

Pulmonary endoderm, second heart field and the morphogenesis of distal outflow tract in mouse embryonic heart

The second heart field (SHF), foregut endoderm and sonic hedgehog (SHH) signaling pathway are all reported to associate with normal morphogenesis and septation of outflow tract (OFT). However, the morphological relationships of the development of foregut endoderm and expression of SHH signaling pathway members with the development of surrounding SHF and OFT are seldom described. In this study, serial sections of mouse embryos from ED9 to ED13 (midgestation) were stained with a series of marker antibodies for specifically highlighting SHF (Isl-1), endoderm (Foxa2), basement membrane (Laminin), myocardium (MHC) and smooth muscle (α-SMA) respectively, or SHH receptors antibodies including patched1 (Ptc1), patched2 (Ptc2) and smoothened, to observe the spatiotemporal relationship between them and their contributions to OFT morphogenesis. Our results demonstrated that the development of an Isl-1 positive field in the splanchnic mesoderm ventral to foregut, a subset of SHF, is closely coupled with pulmonary endoderm or tracheal groove, the Isl-1 positive cells surrounding pulmonary endoderm are distributed in a special cone-shaped pattern and take part in the formation of the lateral walls of the intrapericardial aorta and pulmonary trunk and the transient aortic-pulmonary septum, and Ptc1 and Ptc2 are exclusively expressed in pulmonary endoderm during this Isl-l positive field development, suggesting special roles played in inducing the Isl-l positive field formation by pulmonary endoderm. It is indicated that pulmonary endoderm plays a role in the development and specification of SHF in midgestation, and that pulmonary endoderm-associated Isl-l positive field is involved in patterning the morphogenesis and septation of the intrapericardial arterial trunks.

AcvR1-mediated BMP signaling in second heart field is required for arterial pole development: implications for myocardial differentiation and regional identity

BMP signaling plays an essential role in second heart field-derived heart and arterial trunk development, including myocardial differentiation, right ventricular growth, and interventricular, outflow tract and aortico-pulmonary septation. It is mediated by a number of different BMP ligands, and receptors, many of which are present simultaneously. The mechanisms by which they regulate morphogenetic events and degree of redundancy amongst them have still to be elucidated. We therefore assessed the role of BMP Type I receptor AcvR1 in anterior second heart field-derived cell development, and compared it with that of BmpR1a. By removing Acvr1 using the driver Mef2c[AHF]-Cre, we show that AcvR1 plays an essential role in arterial pole morphogenesis, identifying defects in outflow tract wall and cushion morphology that preceded a spectrum of septation defects from double outlet right ventricle to common arterial trunk in mutants. Its absence caused dysregulation in gene expression important for myocardial differentiation (Isl1, Fgf8) and regional identity (Tbx2, Tbx3, Tbx20, Tgfb2). Although these defects resemble to some degree those in the equivalent Bmpr1a mutant, a novel gene knock-in model in which Bmpr1a was expressed in the Acvr1 locus only partially restored septation in Acvr1 mutants. These data show that both BmpR1a and AcvR1 are needed for normal heart development, in which they play some non-redundant roles, and refine our understanding of the genetic and morphogenetic processes underlying Bmp-mediated heart development important in human congenital heart disease.

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.

Driving vascular endothelial cell fate of human multipotent Isl1+ heart progenitors with VEGF modified mRNA

Distinct families of multipotent heart progenitors play a central role in the generation of diverse cardiac, smooth muscle and endothelial cell lineages during mammalian cardiogenesis. The identification of precise paracrine signals that drive the cell-fate decision of these multipotent progenitors, and the development of novel approaches to deliver these signals in vivo, are critical steps towards unlocking their regenerative therapeutic potential. Herein, we have identified a family of human cardiac endothelial intermediates located in outflow tract of the early human fetal hearts (OFT-ECs), characterized by coexpression of Isl1 and CD144/vWF. By comparing angiocrine factors expressed by the human OFT-ECs and non-cardiac ECs, vascular endothelial growth factor (VEGF)-A was identified as the most abundantly expressed factor, and clonal assays documented its ability to drive endothelial specification of human embryonic stem cell (ESC)-derived Isl1⁺ progenitors in a VEGF receptor-dependent manner. Human Isl1-ECs (endothelial cells differentiated from hESC-derived ISL1⁺ progenitors) resemble OFT-ECs in terms of expression of the cardiac endothelial progenitor- and endocardial cell-specific genes, confirming their organ specificity. To determine whether VEGF-A might serve as an in vivo cell-fate switch for human ESC-derived Isl1-ECs, we established a novel approach using chemically modified mRNA as a platform for transient, yet highly efficient expression of paracrine factors in cardiovascular progenitors. Overexpression of VEGF-A promotes not only the endothelial specification but also engraftment, proliferation and survival (reduced apoptosis) of the human Isl1⁺ progenitors in vivo. The large-scale derivation of cardiac-specific human Isl1-ECs from human pluripotent stem cells, coupled with the ability to drive endothelial specification, engraftment, and survival following transplantation, suggest a novel strategy for vascular regeneration in the heart.

Development of the hearts of lizards and snakes and perspectives to cardiac evolution

Birds and mammals both developed high performance hearts from a heart that must have been reptile-like and the hearts of extant reptiles have an unmatched variability in design. Yet, studies on cardiac development in reptiles are largely old and further studies are much needed as reptiles are starting to become used in molecular studies. We studied the growth of cardiac compartments and changes in morphology principally in the model organism corn snake (Pantherophis guttatus), but also in the genotyped anole (Anolis carolinenis and A. sagrei) and the Philippine sailfin lizard (Hydrosaurus pustulatus). Structures and chambers of the formed heart were traced back in development and annotated in interactive 3D pdfs. In the corn snake, we found that the ventricle and atria grow exponentially, whereas the myocardial volumes of the atrioventricular canal and the muscular outflow tract are stable. Ventricular development occurs, as in other amniotes, by an early growth at the outer curvature and later, and in parallel, by incorporation of the muscular outflow tract. With the exception of the late completion of the atrial septum, the adult design of the squamate heart is essentially reached halfway through development. This design strongly resembles the developing hearts of human, mouse and chicken around the time of initial ventricular septation. Subsequent to this stage, and in contrast to the squamates, hearts of endothermic vertebrates completely septate their ventricles, develop an insulating atrioventricular plane, shift and expand their atrioventricular canal toward the right and incorporate the systemic and pulmonary venous myocardium into the atria.

Semaphorin 3d signaling defects are associated with anomalous pulmonary venous connections

Total anomalous pulmonary venous connection (TAPVC) is a potentially lethal congenital disorder that occurs when the pulmonary veins do not connect normally to the left atrium, allowing mixing of pulmonary and systemic blood. In contrast to the extensive knowledge of arterial vascular patterning, little is known about the patterning of veins. Here we show that the secreted guidance molecule semaphorin 3d (Sema3d) is crucial for the normal patterning of pulmonary veins. Prevailing models suggest that TAPVC occurs when the midpharyngeal endothelial strand (MES), the precursor of the common pulmonary vein, does not form at the proper location on the dorsal surface of the embryonic common atrium. However, we found that TAPVC occurs in Sema3d mutant mice despite normal formation of the MES. In these embryos, the maturing pulmonary venous plexus does not anastomose uniquely with the properly formed MES. In the absence of Sema3d, endothelial tubes form in a region that is normally avascular, resulting in aberrant connections. Normally, Sema3d provides a repulsive cue to endothelial cells in this area, establishing a boundary. Sequencing of SEMA3D in individuals with anomalous pulmonary veins identified a phenylalanine-to-leucine substitution that adversely affects SEMA3D function. These results identify Sema3d as a crucial pulmonary venous patterning cue and provide experimental evidence for an alternate developmental model to explain abnormal pulmonary venous connections.

Extrapericardial origin of the entirety of the left pulmonary artery in the absence of a pulmonary arterial sling

We report the findings at autopsy for an infant with respiratory distress and hypoplastic left lung. The left pulmonary artery, in its entirety, took origin extrapericardially from the right pulmonary artery in the absence of tracheobronchial encirclement. We speculate that it was abnormal course of the left pulmonary artery that produced the hypoplasia of the left lung due to compression of the main left bronchus.