Early morphogenesis of the sinuatrial region of the chick heart: a contribution to the understanding of the pathogenesis of direct pulmonary venous connections to the right atrium and atrial septal defects in hearts with right isomerism of the atrial appendages

The biodistribution of polystyrene nanoparticles administered intravenously in the chicken embryo

Nanoplastics can cause severe malformations in chicken embryos. To improve our understanding of the toxicity of nanoplastics to embryos, we have studied their biodistribution in living chicken embryos. We injected the embryos in the vitelline vein at stages 18-19. We injected polystyrene nanoparticles (PS-NPs) tagged with europium- or fluorescence. Their biodistribution was tracked using inductively-coupled plasma mass spectrometry on tissue lysates, paraffin histology, and vibratome sections analysed by machine learning algorithms. PS-NPs were found at high levels in the heart, liver and kidneys. Furthermore, PS-NPs crossed the endocardium of the heart at sites of epithelial-mesenchymal transformation; they also crossed the liver endothelium. Finally, we detected PS-NPs in the allantoic fluid, consistent with their being excreted by the kidneys. Our study shows the power of the chicken embryo model for analysing the biodistribution of nanoplastics in embryos. Such experiments are difficult or impossible in mammalian embryos. These findings are a major advance in our understanding of the biodistribution and tissue-specific accumulation of PS-NPs in developing animals.

TAPVR: Molecular Pathways and Animal Models

The venous pole of the heart where the pulmonary veins will develop encompasses the sinus venosus and the atrium. In the fourth week of development, the sinus venosus consists of a left and a right part receiving blood from the common cardinal vein, the omphalomesenteric and umbilical veins. Asymmetrical expansion of the common atrium corresponds with a rightward shift of the connection of the sinus to the atrium. The right-sided part of the sinus venosus including its tributing cardinal veins enlarges to form the right superior and inferior vena cava that will incorporate into the right atrium. The left-sided part in human development largely obliterates and remodels to form the coronary sinus in adults. In approximately the same time window (4th-fifth weeks), a splanchnic vascular plexus surrounds the developing lung buds (putative lungs) with a twofold connection. Of note, during early developmental stages, the primary route of drainage from the pulmonary plexus is toward the systemic veins and not to the heart. After lumenization of the so-called mid-pharyngeal endothelial strand (MPES), the first anlage of the pulmonary vein, the common pulmonary vein can be observed in the dorsal mesocardium, and the primary route of drainage will gradually change toward a cardiac drainage. The splanchnic pulmonary venous connections with the systemic cardinal veins will gradually disappear during normal development. In case of absence or atresia of the MPES, the pulmonary-to-systemic connections will persist, clinically resulting in total anomalous pulmonary venous return (TAPVR). This chapter describes the developmental processes and molecular pathways underlying anomalous pulmonary venous connections.

Heart morphology during the embryonic development of Podocnemis unifilis Trosquel 1948 (Testudines: Podocnemididae)

Cardiogenesis is similar in all vertebrates, but differences in the valvuloseptal morphogenesis among non-crocodilian reptiles, birds, and mammals are noted. The origin of mesenchymal structures such as valves that regulate the passage of blood and the formation of partial septa that prevent the complete mixing of oxygen-rich and low-oxygen blood present in adult chelonians are essential in the evolutionary understanding of complete septation, endothermy and malformations, even in mammals. In this context, this study analyzed the heart morphogenesis of Podocnemis unifilis (Testudines: Podocnemididae) from the 4th to the 60th day of incubation. We identified the tubular heart stage, folding of the cardiac tube and expansion of the atrial and ventricular compartments followed by atrial septation by the septum primum, ventricle septation by partial septa, outflow tract septation and the formation of bicuspid valves with cartilage differentiation at the base. The formation of the first atrial septum with the mesenchymal cap is noted during the development of the atrial septum, joining the atrioventricular cushion on the 17th day and completely dividing the atria. Small secondary perforations appeared in the mid-cranial part, observed up to the 45th day. Partial ventricle septation into the pulmonary, venous, and arterial subcompartments takes place by trabeculae carneae thickening and grouping on the 15th day. The outflow tract forms the aorticopulmonary and interaortic septa on the 16th day and the bicuspid valves, on the 20th day. Therefore, after the first 20 days, the heart exhibits a general anatomical conformation similar to that of adult turtles.

Assembly of the Cardiac Pacemaking Complex: Electrogenic Principles of Sinoatrial Node Morphogenesis

Cardiac pacemaker cells located in the sinoatrial node initiate the electrical impulses that drive rhythmic contraction of the heart. The sinoatrial node accounts for only a small proportion of the total mass of the heart yet must produce a stimulus of sufficient strength to stimulate the entire volume of downstream cardiac tissue. This requires balancing a delicate set of electrical interactions both within the sinoatrial node and with the downstream working myocardium. Understanding the fundamental features of these interactions is critical for defining vulnerabilities that arise in human arrhythmic disease and may provide insight towards the design and implementation of the next generation of potential cellular-based cardiac therapeutics. Here, we discuss physiological conditions that influence electrical impulse generation and propagation in the sinoatrial node and describe developmental events that construct the tissue-level architecture that appears necessary for sinoatrial node function.

Spontaneous Left Cardiac Isomerism in Chick Embryos: Case Report, Review of the Literature, and Possible Significance for the Understanding of Ventricular Non-Compaction Cardiomyopathy in the Setting of Human Heterotaxy Syndromes

The outer shape of most vertebrates is normally characterized by bilateral symmetry. The inner organs, on the other hand, are normally arranged in bilaterally asymmetric patterns. Congenital deviations from the normal organ asymmetry can occur in the form of mirror imagery of the normal arrangement (situs inversus), or in the form of arrangements that have the tendency for the development of bilateral symmetry, either in a pattern of bilateral left-sidedness (left isomerism) or bilateral right-sidedness (right isomerism). The latter two forms of visceral situs anomalies are called “heterotaxy syndromes”. During the past 30 years, remarkable progress has been made in uncovering the genetic etiology of heterotaxy syndromes. However, the pathogenetic mechanisms causing the spectrum of cardiovascular defects found in these syndromes remain poorly understood. In the present report, a spontaneous case of left cardiac isomerism found in an HH-stage 23 chick embryo is described. The observations made in this case confirmed the existence of molecular isomerism in the ventricular chambers previously noted in mouse models. They, furthermore, suggest that hearts with left cardiac isomerism may have the tendency for the development of non-compaction cardiomyopathy caused by defective development of the proepicardium.

Embryonic Chicken (Gallus gallus domesticus) as a Model of Cardiac Biology and Development

Cardiovascular disease remains one of the top contributors to morbidity and mortality in the United States. Increasing evidence suggests that many processes, pathways, and programs observed during development and organogenesis are recapitulated in adults in the face of disease. Therefore, a heightened understanding of cardiac development and organogenesis will help increase our understanding of developmental defects and cardiovascular diseases in adults. Chicks have long served as a model system in which to study developmental problems. Detailed descriptions of morphogenesis, low cost, accessibility, ease of manipulation, and the optimization of genetic engineering techniques have made chicks a robust model for studying development and make it a powerful platform for cardiovascular research. This review summarizes the cardiac developmental milestones of embryonic chickens, practical considerations when working with chicken embryos, and techniques available for use in chicks (including tissue chimeras, genetic manipulations, and live imaging). In addition, this article highlights examples that accentuate the utility of the embryonic chicken as model system in which to study cardiac development, particularly epicardial development, and that underscore the importance of how studying development informs our understanding of disease.

Evolution and Development of the Atrial Septum

The complete division of the atrial cavity by a septum, resulting in a left and right atrium, is found in many amphibians and all amniotes (reptiles, birds, and mammals). Surprisingly, it is only in eutherian, or placental, mammals that full atrial septation necessitates addition from a second septum. The high incidence of incomplete closure of the atrial septum in human, so-called probe patency, suggests this manner of closure is inefficient. We review the evolution and development of the atrial septum to understand the peculiar means of forming the atrial septum in eutherian mammals. The most primitive atrial septum is found in lungfishes and comprises a myocardial component with a mesenchymal cap on its leading edge, reminiscent to the primary atrial septum of embryonic mammals before closure of the primary foramen. In reptiles, birds, and mammals, the primary foramen is closed by the mesenchymal tissues of the atrioventricular cushions, the dorsal mesenchymal protrusion, and the mesenchymal cap. These tissues are also found in lungfishes. The closure of the primary foramen is preceded by the development of secondary perforations in the septal myocardium. In all amniotes, with the exception of eutherian mammals, the secondary perforations do not coalesce to a secondary foramen. Instead, the secondary perforations persist and are sealed by myocardial and endocardial growth after birth or hatching. We suggest that the error-prone secondary foramen allows large volumes of oxygen-rich blood to reach the cardiac left side, needed to sustain the growth of the extraordinary large offspring that characterizes eutherian mammals. Anat Rec, 302:32-48, 2019. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Comparative developmental biology of the cardiac inflow tract

The vertebrate heart receives the blood through the cardiac inflow tract. This area has experienced profound changes along the evolution of vertebrates; changes that have a reflection in the cardiac ontogeny. The development of the inflow tract involves dynamic changes due to the progressive addition of tissue derived from the secondary heart field. The inflow tract is the site where oxygenated blood coming from lungs is received separately from the systemic return, where the cardiac pacemaker is established and where the proepicardium develops. Differential cell migration towards the inflow tract breaks the symmetry of the primary heart tube and determines the direction of the cardiac looping. In air-breathing vertebrates, an inflow tract reorganization is essential to keep separate blood flows from systemic and pulmonary returns. Finally, the sinus venosus endocardium has recently been recognized as playing a role in the constitution of the coronary vasculature. Due to this developmental complexity, congenital anomalies of the inflow tract can cause severe cardiac diseases. We aimed to review the recent literature on the cellular and molecular mechanisms that regulate the morphogenesis of the cardiac inflow tract, together with comparative and evolutionary details, thus providing a basis for a better understanding of these mechanisms.

Left atrial ligation alters intracardiac flow patterns and the biomechanical landscape in the chick embryo

BACKGROUND

Hypoplastic left heart syndrome (HLHS) is a major human congenital heart defect that results in single ventricle physiology and high mortality. Clinical data indicate that intracardiac blood flow patterns during cardiac morphogenesis are a significant etiology. We used the left atrial ligation (LAL) model in the chick embryo to test the hypothesis that LAL immediately alters intracardiac flow streams and the biomechanical environment, preceding morphologic and structural defects observed in HLHS.

RESULTS

Using fluorescent dye injections, we found that intracardiac flow patterns from the right common cardinal vein, right vitelline vein, and left vitelline vein were altered immediately following LAL. Furthermore, we quantified a significant ventral shift of the right common cardinal and right vitelline vein flow streams. We developed an in silico model of LAL, which revealed that wall shear stress was reduced at the left atrioventricular canal and left side of the common ventricle.

CONCLUSIONS

Our results demonstrate that intracardiac flow patterns change immediately following LAL, supporting the role of hemodynamics in the progression of HLHS. Sites of reduced WSS revealed by computational modeling are commonly affected in HLHS, suggesting that changes in the biomechanical environment may lead to abnormal growth and remodeling of left heart structures.

Chronological and morphological study of heart development in the rat

Adult and embryonic laboratory rats have been used as a mammalian model organism in biomedical research, descriptive and experimental cardiac embryology, and experimental teratology. There have been, however, considerable variations and discrepancies concerning the developmental staging of the rat embryo in the reported literature, which have resulted in several controversies and inconsistencies. Therefore, we carried out a careful anatomical and histological study of rat cardiac morphogenesis from the premorphogenetic period to the mature heart in a newborn pup. A correlation between the chronology and morphological features of the heart and embryo or newborn was made. We provide a simple and comprehensive guide relating the developmental timing and fate of the embryonic components of the heart and their morphological changes in the rat based on in vivo labeling studies in the chick. We also compare the timing of heart development in rats, humans, and mice.

Development of the pulmonary vein and the systemic venous sinus: an interactive 3D overview

Knowledge of the normal formation of the heart is crucial for the understanding of cardiac pathologies and congenital malformations. The understanding of early cardiac development, however, is complicated because it is inseparably associated with other developmental processes such as embryonic folding, formation of the coelomic cavity, and vascular development. Because of this, it is necessary to integrate morphological and experimental analyses. Morphological insights, however, are limited by the difficulty in communication of complex 3D-processes. Most controversies, in consequence, result from differences in interpretation, rather than observation. An example of such a continuing debate is the development of the pulmonary vein and the systemic venous sinus, or “sinus venosus”. To facilitate understanding, we present a 3D study of the developing venous pole in the chicken embryo, showing our results in a novel interactive fashion, which permits the reader to form an independent opinion. We clarify how the pulmonary vein separates from a greater vascular plexus within the splanchnic mesoderm. The systemic venous sinus, in contrast, develops at the junction between the splanchnic and somatic mesoderm. We discuss our model with respect to normal formation of the heart, congenital cardiac malformations, and the phylogeny of the venous tributaries.

Development of the venous pole of the heart in the frog Xenopus laevis: a morphological study with special focus on the development of the venoatrial connections

The heart of lung-breathing vertebrates normally shows an asymmetric arrangement of its venoatrial connections along the left-right (L-R) body axis. The systemic venous tributaries empty into the right atrium while the pulmonary venous tributaries empty into the left atrium. The ways by which this asymmetry evolves from the originally symmetrically arranged embryonic venous heart pole are poorly defined. Here we document the development of the venous heart pole in Xenopus laevis (stages 40-46). We show that, prior to the appearance of the mouth of the common pulmonary vein (MCPV), the systemic venous tributaries empty into a bilaterally symmetric chamber (sinus venosus) that is demarcated from the developing atriums by a circular ridge of tissue (sinu-atrial ridge). A solitary MCPV appears during stage 41. From the time point of its first appearance onwards, the MCPV lies cranial to the sinu-atrial ridge and to the left of the developing interatrial septum and body midline. L-R lineage analysis shows that the interatrial septum and MCPV both derive from the left body half. The CPV, therefore, opens from the beginning into the future left atrium. The definitive venoatrial connections are established by the formation of a septal complex that divides the lumen of the venous heart pole into systemic and pulmonary venous flow pathways. This complex arises from the anlage of the interatrial septum and the left half of the sinu-atrial ridge.

Dysregulation of the PDGFRA gene causes inflow tract anomalies including TAPVR: integrating evidence from human genetics and model organisms

Total anomalous pulmonary venous return (TAPVR) is a congenital heart defect inherited via complex genetic and/or environmental factors. We report detailed mapping in extended TAPVR kindreds and mutation analysis in TAPVR patients that implicate the PDGFRA gene in the development of TAPVR. Gene expression studies in mouse and chick embryos for both the Pdgfra receptor and its ligand Pdgf-a show temporal and spatial patterns consistent with a role in pulmonary vein (PV) development. We used an in ovo function blocking assay in chick and a conditional knockout approach in mouse to knock down Pdgfra expression in the developing venous pole during the period of PV formation. We observed that loss of PDGFRA function in both organisms causes TAPVR with low penetrance (approximately 7%) reminiscent of that observed in our human TAPVR kindreds. Intermediate inflow tract anomalies occurred in a higher percentage of embryos (approximately 30%), suggesting that TAPVR occurs at one end of a spectrum of defects. We show that the anomalous pulmonary venous connection seen in chick and mouse is highly similar to TAPVR discovered in an abnormal early stage embryo from the Kyoto human embryo collection. Whereas the embryology of the normal venous pole and PV is becoming understood, little is known about the embryogenesis or molecular pathogenesis of TAPVR. These models of TAPVR provide important insight into the pathogenesis of PV defects. Taken together, these data from human genetics and animal models support a role for PDGF-signaling in normal PV development, and in the pathogenesis of TAPVR.

Podoplanin deficient mice show a RhoA-related hypoplasia of the sinus venosus myocardium including the sinoatrial node

We investigated the role of podoplanin in development of the sinus venosus myocardium comprising the sinoatrial node, dorsal atrial wall, and primary atrial septum as well as the myocardium of the cardinal and pulmonary veins. We analyzed podoplanin wild-type and knockout mouse embryos between embryonic day 9.5-15.5 using immunohistochemical marker podoplanin; sinoatrial-node marker HCN4; myocardial markers MLC-2a, Nkx2.5, as well as Cx43; coelomic marker WT-1; and epithelial-to-mesenchymal transformation markers E-cadherin and RhoA. Three-dimensional reconstructions were made and myocardial morphometry was performed. Podoplanin mutants showed hypoplasia of the sinoatrial node, primary atrial septum, and dorsal atrial wall. Myocardium lining the wall of the cardinal and pulmonary veins was thin and perforated. Impaired myocardial formation is correlated with abnormal epithelial-to-mesenchymal transformation of the coelomic epithelium due to up-regulated E-cadherin and down-regulated RhoA, which are controlled by podoplanin. Our results demonstrate an important role for podoplanin in development of sinus venosus myocardium.