The outflow tract of the heart in fishes: anatomy, genes and evolution

Laterality defect of the heart in non-teleost fish

Dextrocardia is a rare congenital malformation in humans in which most of the heart mass is positioned in the right hemithorax rather than on the left. The heart itself may be normal and dextrocardia is sometimes diagnosed during non-related explorations. A few reports have documented atypical positions of the cardiac chambers in farmed teleost fish. Here, we report the casual finding of a left-right mirrored heart in an 85 cm long wild-caught spiny dogfish (Squalus acanthias) with several organ malformations. Macroscopic observations showed an outflow tract originating from the left side of the ventricular mass, rather than from the right. Internal inspection revealed the expected structures and a looped cavity. The inner curvature of the loop comprised a large trabeculation, the bulboventricular fold, as expected. The junction between the sinus venosus and the atrium appeared normal, only mirrored. MRI data acquired at 0.7 mm isotropic resolution and subsequent 3D-modeling revealed the atrioventricular canal was to the right of the bulboventricular fold, rather than on the left. Spurred by the finding of dextrocardia in the shark, we revisit our previously published material on farmed Adriatic sturgeon (Acipenser naccarii), a non-teleost bony fish. We found several alevins with inverted (left-loop) hearts, amounting to an approximate incidence of 1%-2%. Additionally, an adult sturgeon measuring 90 cm in length showed abnormal topology of the cardiac chambers, but normal position of the abdominal organs. In conclusion, left-right mirrored hearts, a setting that resembles human dextrocardia, can occur in both farmed and wild non-teleost fish.

Sinus venosus adaptation models prolonged cardiovascular disease and reveals insights into evolutionary transitions of the vertebrate heart

How two-chambered hearts in basal vertebrates have evolved from single-chamber hearts found in ancestral chordates remains unclear. Here, we show that the teleost sinus venosus (SV) is a chamber-like vessel comprised of an outer layer of smooth muscle cells. We find that in adult zebrafish nr2f1a mutants, which lack atria, the SV comes to physically resemble the thicker bulbus arteriosus (BA) at the arterial pole of the heart through an adaptive, hypertensive response involving smooth muscle proliferation due to aberrant hemodynamic flow. Single cell transcriptomics show that smooth muscle and endothelial cell populations within the adapting SV also take on arterial signatures. Bulk transcriptomics of the blood sinuses flanking the tunicate heart reinforce a model of greater equivalency in ancestral chordate BA and SV precursors. Our data simultaneously reveal that secondary complications from congenital heart defects can develop in adult zebrafish similar to those in humans and that the foundation of equivalency between flanking auxiliary vessels may remain latent within basal vertebrate hearts.

Cephalochordate Hemocytes: First Demonstration for Asymmetron lucayanum (Bahamas Lancelet) Plus Augmented Description for Branchiostoma floridae (Florida Amphioxus)

AbstractWithin phylum Chordata, the subphylum Cephalochordata (amphioxus and lancelets) has figured large in considerations of the evolutionary origin of the vertebrates. To date, these discussions have been predominantly based on knowledge of a single cephalochordate genus (Branchiostoma), almost to the exclusion of the other two genera (Asymmetron and Epigonichthys). This uneven pattern is illustrated by cephalochordate hematology, until now known entirely from work done on Branchiostoma. The main part of the present study is to describe hemocytes in the dorsal aorta of a species of Asymmetron by serial block-face scanning electron microscopy. This technique, which demonstrates three-dimensional fine structure, showed that the hemocytes have a relatively uniform morphology characterized by an oval shape and scanty cytoplasm. Ancillary information is also included for Branchiostoma hemocytes, known from previous studies to have relatively abundant cytoplasm; our serial block-face scanning electron microscopy provides more comprehensive views of the highly variable shapes of these cells, which typically extend one or several pseudopodium-like protrusions. The marked difference in hemocyte morphology found between Asymmetron and Branchiostoma was unexpected and directs attention to investigating comparable cells in the genus Epigonichthys. A broader knowledge of the hemocytes in all three cephalochordate genera would provide more balanced insights into the evolution of vertebrate hematopoiesis.

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.

The Medical versus Zoological Concept of Outflow Tract Valves of the Vertebrate Heart

The anatomical elements that in humans prevent blood backflow from the aorta and pulmonary artery to the left and right ventriclesare the aortic and pulmonary valves, respectively. Each valve regularly consists of three leaflets (cusps), each supported by its valvular sinus. From the medical viewpoint, each set of three leaflets and sinuses is regarded as a morpho-functional unit. This notion also applies to birds and non-human mammals. However, the structures that prevent the return of blood to the heart in other vertebrates are notably different. This has led to discrepancies between physicians and zoologists in defining what a cardiac outflow tract valve is. The aim here is to compare the gross anatomy of the outflow tract valvular system among several groups of vertebrates in order to understand the conceptual and nomenclature controversies in the field.

Zebrafish as a Model to Study Vascular Elastic Fibers and Associated Pathologies

Many extensible tissues such as skin, lungs, and blood vessels require elasticity to function properly. The recoil of elastic energy stored during a stretching phase is provided by elastic fibers, which are mostly composed of elastin and fibrillin-rich microfibrils. In arteries, the lack of elastic fibers leads to a weakening of the vessel wall with an increased risk to develop cardiovascular defects such as stenosis, aneurysms, and dissections. The development of new therapeutic molecules involves preliminary tests in animal models that recapitulate the disease and whose response to drugs should be as close as possible to that of humans. Due to its superior in vivo imaging possibilities and the broad tool kit for forward and reverse genetics, the zebrafish has become an important model organism to study human pathologies. Moreover, it is particularly adapted to large scale studies, making it an attractive model in particular for the first steps of investigations. In this review, we discuss the relevance of the zebrafish model for the study of elastic fiber-related vascular pathologies. We evidence zebrafish as a compelling alternative to conventional mouse models.

Branching patterns of the afferent branchial arteries and their phylogenetic significance in rays (Batoidea)

Rays of the superorder Batoidea comprise the most diverse group of chondrichthyans in terms of valid species and morphological disparity. Up to the present little agreement is observed in studies based on morphological and molecular data focused on uncovering the interrelationships within Batoidea. Morphology-based phylogenies of batoids have not included characters related to the afferent branchial arteries, and little is known about the variation in this anatomical complex in rays. Herein, representatives of 32 genera from 19 families currently recognized of rays were examined as well as some shark taxa. Seven new characters are proposed and tested in two different analyses, one on their own and in the other they were added to the morphological data matrix of the most recent analysis of interrelationships within Batoidea. The arrangement of afferent branchial arteries differs mainly among orders and families of batoids. The absence of a common trunk from which the three posteriormost afferent arteries branch is interpreted as a synapomorphy for Myliobatiformes and the presence of a coronary cranial artery as an autapomorphy for Mobula hypostoma. A close spatial relationship between the second and third afferent arteries within the common branch from the ventral aorta is proposed as a synapomorphy for Rajiformes with a secondary modification in Sympterygia. Data about patterns in afferent branchial arteries in additional taxa such as Squaliformes and Chimaeriformes are needed to better understand the evolution of this character complex among chondrichthyans.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Pathways Regulating Establishment and Maintenance of Cardiac Chamber Identity in Zebrafish

The vertebrate heart is comprised of two types of chambers-ventricles and atria-that have unique morphological and physiological properties. Effective cardiac function depends upon the distinct characteristics of ventricular and atrial cardiomyocytes, raising interest in the genetic pathways that regulate chamber-specific traits. Chamber identity seems to be specified in the early embryo by signals that establish ventricular and atrial progenitor populations and trigger distinct differentiation pathways. Intriguingly, chamber-specific features appear to require active reinforcement, even after myocardial differentiation is underway, suggesting plasticity of chamber identity within the developing heart. Here, we review the utility of the zebrafish as a model organism for studying the mechanisms that establish and maintain cardiac chamber identity. By combining genetic and embryological approaches, work in zebrafish has revealed multiple players with potent influences on chamber fate specification and commitment. Going forward, analysis of cardiomyocyte identity at the single-cell level is likely to yield a high-resolution understanding of the pathways that link the relevant players together, and these insights will have the potential to inform future strategies in cardiac tissue engineering.

Endothelial TGF-β signaling instructs smooth muscle cell development in the cardiac outflow tract

The development of the cardiac outflow tract (OFT), which connects the heart to the great arteries, relies on a complex crosstalk between endothelial (ECs) and smooth muscle (SMCs) cells. Defects in OFT development can lead to severe malformations, including aortic aneurysms, which are frequently associated with impaired TGF-β signaling. To better understand the role of TGF-β signaling in OFT formation, we generated zebrafish lacking the TGF-β receptor Alk5 and found a strikingly specific dilation of the OFT: alk5-/- OFTs exhibit increased EC numbers as well as extracellular matrix (ECM) and SMC disorganization. Surprisingly, endothelial-specific alk5 overexpression in alk5-/- rescues the EC, ECM, and SMC defects. Transcriptomic analyses reveal downregulation of the ECM gene fibulin-5, which when overexpressed in ECs ameliorates OFT morphology and function. These findings reveal a new requirement for endothelial TGF-β signaling in OFT morphogenesis and suggest an important role for the endothelium in the etiology of aortic malformations.

Development of the ventricular myocardial trabeculae in Scyliorhinus canicula (Chondrichthyes): evolutionary implications

The development of the ventricular myocardial trabeculae occurs in three steps: emergence, trabeculation and remodeling. The whole process has been described in vertebrates with two different myocardial structural types, spongy (zebrafish) and compact (chicken and mouse). In this context, two alternative mechanisms of myocardial trabeculae emergence have been identified: (1) in chicken and mouse, the endocardial cells invade the two-layered myocardium; (2) in zebrafish, cardiomyocytes from the monolayered myocardium invaginate towards the endocardium. Currently, the process has not been studied in detail in vertebrates having a mixed type of ventricular myocardium, with an inner trabecular and an outer compact layer, which is presumptively the most primitive morphology in gnathostomes. We studied the formation of the mixed ventricular myocardium in the lesser spotted dogfish (Scyliorhinus canicula, Elasmobranchii), using light, scanning and transmission electron microscopy. Our results show that early formation of the mixed ventricular myocardium, specifically the emergence and the trabeculation steps, is driven by an endocardial invasion of the myocardium. The mechanism of trabeculation of the mixed ventricular myocardium in chondrichthyans is the one that best reproduces how this developmental process has been established from the beginning of the gnathostome radiation. The process has been apparently preserved throughout the entire group of sarcopterygians, including birds and mammals. In contrast, teleosts, at least those possessing a mostly spongy ventricular myocardium, seem to have introduced notable changes in their myocardial trabeculae development.

The lateral plate mesoderm

The lateral plate mesoderm (LPM) forms the progenitor cells that constitute the heart and cardiovascular system, blood, kidneys, smooth muscle lineage and limb skeleton in the developing vertebrate embryo. Despite this central role in development and evolution, the LPM remains challenging to study and to delineate, owing to its lineage complexity and lack of a concise genetic definition. Here, we outline the processes that govern LPM specification, organization, its cell fates and the inferred evolutionary trajectories of LPM-derived tissues. Finally, we discuss the development of seemingly disparate organ systems that share a common LPM origin.

Summary: The lateral plate mesoderm is the origin of several major cell types and organ systems in the vertebrate body plan. How this mesoderm territory emerges and partitions into its downstream fates provides clues about vertebrate development and evolution.

Cardiac function modulates endocardial cell dynamics to shape the cardiac outflow tract

Physical forces are important participants in the cellular dynamics that shape developing organs. During heart formation, for example, contractility and blood flow generate biomechanical cues that influence patterns of cell behavior. Here, we address the interplay between function and form during the assembly of the cardiac outflow tract (OFT), a crucial connection between the heart and vasculature that develops while circulation is under way. In zebrafish, we find that the OFT expands via accrual of both endocardial and myocardial cells. However, when cardiac function is disrupted, OFT endocardial growth ceases, accompanied by reduced proliferation and reduced addition of cells from adjacent vessels. The flow-responsive TGFβ receptor Acvrl1 is required for addition of endocardial cells, but not for their proliferation, indicating distinct modes of function-dependent regulation for each of these essential cell behaviors. Together, our results indicate that cardiac function modulates OFT morphogenesis by triggering endocardial cell accumulation that induces OFT lumen expansion and shapes OFT dimensions. Moreover, these morphogenetic mechanisms provide new perspectives regarding the potential causes of cardiac birth defects.

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.

Leger J. Morphology of the Amazonian Teleost Genus Arapaima Using Advanced 3D Imaging

The arapaima is the largest of the extant air-breathing freshwater fishes. Their respiratory gas bladder is arguably the most striking of all the adaptations to living in the hypoxic waters of the Amazon basin, in which dissolved oxygen can reach 0 ppm (0 mg/l) at night. As obligatory air-breathers, arapaima have undergone extensive anatomical and physiological adaptations in almost every organ system. These changes were evaluated using magnetic resonance and computed tomography imaging, gross necropsy, and histology to create a comprehensive morphological assessment of this unique fish. Segmentation of advanced imaging data allowed for creation of anatomically accurate and quantitative 3D models of organs and their spatial relationships. The deflated gas bladder [1.96% body volume (BV)] runs the length of the coelomic cavity, and encompasses the kidneys (0.35% BV). It is compartmentalized by a highly vascularized webbing comprising of ediculae and inter-edicular septa lined with epithelium acting as a gas exchange surface analogous to a lung. Gills have reduced surface area, with severe blunting and broadening of the lamellae. The kidneys are not divided into separate regions, and have hematopoietic and excretory tissue interspersed throughout. The heart (0.21% BV) is encased in a thick layer of lipid rich tissue. Arapaima have an unusually large telencephalon (28.3% brain volume) for teleosts. The characteristics that allow arapaima to perfectly exploit their native environment also make them easy targets for overfishing. In addition, their habitat is at high risk from climate change and anthropogenic activities which are likely to result is fewer specimens living in the wild, or achieving their growth potential of up to 4.5 m in length.

Endocardiosis in Tetras (Family Characiformes)

Proliferative, myxomatous change was identified in the cardiac valves (i.e. valvular endocardiosis) with extension into some cardiac walls (i.e. mural endocardiosis) of five tetras (four neon tetras [Paracheirodon innesi] and one rummynose tetra [Hemigrammus rhodostomus]). Review of cardiac sections from tetras (family Characiformes) submitted to two diagnostic laboratories revealed a prevalence of endocardiosis in tetras of 4.3% and 5.7%, respectively. In four cases, concurrent disease that could be a primary cause of death was not identified, and endocardiosis was considered the primary cause of death; in the fifth case, egg binding was present concurrently. This retrospective case series suggests that endocardiosis is a significant concern in fish of the family Characiformes.

Development and evolution of the metazoan heart

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

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

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

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

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

Heart Anatomy of Rhincodon typus: Three-Dimensional X-Ray Computed Tomography of Plastinated Specimens

In this study, we examined the structure of the heart of the whale shark, Rhincodon typus, using a plastination technique and three-dimensional X-ray computer tomography (3DCT). Inspection of the atrium revealed a symmetric distribution of the pectinate muscles attached to the commissures of the sino-atrial valve, suggesting some functional advantages. The majority of the ventricular wall comprised spongiosa, and compacta accounted for only ~3% of the entire thickness. There were three major fiber orientations in the spongiosa: the fibers on the endocardial side formed trabeculae that were aligned with the blood flow tract, whereas those on the epicardial side formed a circular pattern around the flow tract. Transmural myofibers connected the inner and outer layers in the spongiosa, which may serve as an intraventricular conduction pathway. Plastination and 3DCT is a powerful combination that allowed for multifaceted visualization of the internal structure of rare heart specimens in a nondestructive manner. Anat Rec, 301:1801-1808, 2018. © 2018 Wiley Periodicals, Inc.

Examples of Weak, If Not Absent, Form-Function Relations in the Vertebrate Heart

That form and function are related is a maxim of anatomy and physiology. Yet, form-function relations can be difficult to prove. Human subjects with excessive trabeculated myocardium in the left ventricle, for example, are diagnosed with non-compaction cardiomyopathy, but the extent of trabeculations may be without relation to ejection fraction. Rather than rejecting a relation between form and function, we may ask whether the salient function is assessed. Is there a relation to electrical propagation, mean arterial blood pressure, or propensity to form blood clots? In addition, how should the extent of trabeculated muscle be assessed? While reviewing literature on trabeculated muscle, we applied Tinbergen's four types of causation-how does it work, why does it work, how is it made, and why did it evolve-to better parse what is meant by form and function. The paper is structured around cases that highlight advantages and pitfalls of applying Tinbergen's questions. It further uses the evolution of lunglessness in amphibians to argue that lung reduction impacts on chamber septation and it considers the evolution of an arterial outflow in fishes to argue that reductions in energy consumption may drive structural changes with little consequences to function. Concerning trabeculations, we argue they relate to pumping function in the embryo in the few weeks before the onset of coronary circulation. In human fetal and postnatal stages, a spectrum of trabeculated-to-compact myocardium makes no difference to cardiac function and in this period, form and function may appear unrelated.

The developmental origin of heart size and shape differences in Astyanax mexicanus populations

Regulation of heart size and shape is one of the least understood processes in developmental biology. We have for the first time analysed the hearts of Astyanax mexicanus and identified several differences in heart morphology between the surface (epigean morph) and cave-dwelling (troglomorph) morphs. Examination of the adult revealed that the troglomorph possesses a smaller heart with a rounder ventricle in comparison to the epigean morph. The size differences identified appear to arise early in development, as early as 24 h post-fertilisation (hpf), while shape differences begin to appear at 2 days post-fertilisation. The heart of the first-generation cross between the cave-dwelling and river-dwelling morph shows uncoupling of different phenotypes observed in the parental populations and indicates that the cardiac differences have become embedded in the genome during evolution. The differences in heart morphology are accompanied by functional changes between the two morphs, with the cave-dwelling morph exhibiting a slower heart rate than the river-dwelling morph. The identification of morphological and functional differences in the A. mexicanus heart could allow us to gain more insight into how such parameters are regulated during cardiac development, with potential relevance to cardiac pathologies in humans.

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Differences in heart size, shape and tissue structure between Astyanax populations.

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Furthermore, differences in cardiac melanophore and adipocyte numbers.

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Heart size and shape differences are apparent early in development.

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Surface and Pachón show differences in heart rate during development and adulthood.

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F1 hybrids show uncoupling of features observed in surface and Pachón populations.

Continuous addition of progenitors forms the cardiac ventricle in zebrafish

The vertebrate heart develops from several progenitor lineages. After early-differentiating first heart field (FHF) progenitors form the linear heart tube, late-differentiating second heart field (SHF) progenitors extend the atrium and ventricle, and form inflow and outflow tracts (IFT/OFT). However, the position and migration of late-differentiating progenitors during heart formation remains unclear. Here, we track zebrafish heart development using transgenics based on the cardiopharyngeal gene tbx1. Live imaging uncovers a tbx1 reporter-expressing cell sheath that continuously disseminates from the lateral plate mesoderm towards the forming heart tube. High-speed imaging and optogenetic lineage tracing corroborates that the zebrafish ventricle forms through continuous addition from the undifferentiated progenitor sheath followed by late-phase accrual of the bulbus arteriosus (BA). FGF inhibition during sheath migration reduces ventricle size and abolishes BA formation, refining the window of FGF action during OFT formation. Our findings consolidate previous end-point analyses and establish zebrafish ventricle formation as a continuous process.

Late-differentiating second heart field progenitors contribute to atrium, ventricle, and outflow tract in the zebrafish heart but how remains unclear. Here, the authors image heart formation in transgenics based on the cardiopharyngeal gene tbx1 and show that progenitors are continuously added.

The bulbus arteriosus of the holocephalan heart: gross anatomy, histomorphology, pigmentation, and evolutionary significance

This study was designed to determine whether the outflow tract of the holocephalan heart is composed of a myocardial conus arteriosus and a non-myocardial bulbus arteriosus, as is the case in elasmobranchs. This is a key issue to verify the hypothesis that these two anatomical components existed from the onset of the jawed vertebrate radiation. The Holocephali are the sister group of the elasmobranchs, sharing with them a common, still unknown Palaeozoic ancestor. The sample examined herein consisted of hearts from individuals of four species, two of them belonging to the Chimaeridae and the other two to the Rhinochimaeridae. In all specimens, the cardiac outflow tract consisted of a conus arteriosus, with myocardium in its walls and two rows of valves at its luminal side, and an intrapericardial bulbus arteriosus shorter than the conus and devoid of valves. The bulbus, mainly composed of elastin and smooth musculature, was covered by the epicardium and crossed longitudinally by coronary artery trunks. These findings give added support to the viewpoint that the outflow tract of the primitive heart of the gnathostomes was not composed of a single component, but two, the conus and the bulbus. All rabbitfish (Chimaera monstrosa) examined had pigment cells over the surface of the heart. The degree of pigmentation, which varied widely between individuals, was particularly intense in the cardiac outflow tract. Pigment cells also occurred in the bulbus arteriosus of one of the two hearts of the straightnose rabbitfish (Rhinochimaera atlantica) included in the study. The cells containing pigment, presumably derived from the neural crest, were located in the subepicardium.

Heart fossilization is possible and informs the evolution of cardiac outflow tract in vertebrates

Elucidating cardiac evolution has been frustrated by lack of fossils. One celebrated enigma in cardiac evolution involves the transition from a cardiac outflow tract dominated by a multi-valved conus arteriosus in basal actinopterygians, to an outflow tract commanded by the non-valved, elastic, bulbus arteriosus in higher actinopterygians. We demonstrate that cardiac preservation is possible in the extinct fish Rhacolepis buccalis from the Brazilian Cretaceous. Using X-ray synchrotron microtomography, we show that Rhacolepis fossils display hearts with a conus arteriosus containing at least five valve rows. This represents a transitional morphology between the primitive, multivalvar, conal condition and the derived, monovalvar, bulbar state of the outflow tract in modern actinopterygians. Our data rescue a long-lost cardiac phenotype (119-113 Ma) and suggest that outflow tract simplification in actinopterygians is compatible with a gradual, rather than a drastic saltation event. Overall, our results demonstrate the feasibility of studying cardiac evolution in fossils.

Advances in the Study of Heart Development and Disease Using Zebrafish

Animal models of cardiovascular disease are key players in the translational medicine pipeline used to define the conserved genetic and molecular basis of disease. Congenital heart diseases (CHDs) are the most common type of human birth defect and feature structural abnormalities that arise during cardiac development and maturation. The zebrafish, Danio rerio, is a valuable vertebrate model organism, offering advantages over traditional mammalian models. These advantages include the rapid, stereotyped and external development of transparent embryos produced in large numbers from inexpensively housed adults, vast capacity for genetic manipulation, and amenability to high-throughput screening. With the help of modern genetics and a sequenced genome, zebrafish have led to insights in cardiovascular diseases ranging from CHDs to arrhythmia and cardiomyopathy. Here, we discuss the utility of zebrafish as a model system and summarize zebrafish cardiac morphogenesis with emphasis on parallels to human heart diseases. Additionally, we discuss the specific tools and experimental platforms utilized in the zebrafish model including forward screens, functional characterization of candidate genes, and high throughput applications.

Immunolocalization of immune cells and cell cycle proteins in the bulbus arteriosus of Atlantic salmon (Salmo salar L.)

The bulbus arteriosus is the most anterior chamber of the teleost heart. The present study aimed to establish the presence, and to provide semi-quantitative information on the abundance, of several immune and cell-cycle proteins in the bulbus arteriosus of healthy Atlantic salmon (Salmo salar L.). Using immunohistochemistry, lymphocyte-like cells were identified in the bulbus arteriosus using antibodies to CD3ε and MHC class IIβ. Few PCNA positive cells were identified in post-smolt fish as compared to moderate levels of staining in fresh water fry. Interestingly no staining was evident in adult fish (1-3 kg), thus there was a loss of cells expressing cell-cycle regulatory proteins with ontogeny/progressive life-history stages. Eosinophilic granulocytes (EGCs) were identified in the bulbus arteriosus using TNFα and HIF1α antibodies. Anti-caspase 3 immune-reaction identified a strong endothelial cytoplasmic staining in the bulbus arteriosus. Taken together, the immunolocalization of immune-related molecules (CD3, MHC class II and TNFα), cell-cycle regulatory proteins (PCNA and HIF1α) and apoptosis markers (TUNEL, caspase 3) suggest that the bulbus arteriosus may have an immune component within its functional repertoire.

Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel-shaped and contains small amounts of myocardium. The AV valve is formed by two pocket-like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well-defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen-muscle boundary marks the SV-atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853-865, 2016. © 2016 Wiley Periodicals, Inc.

Structure and vascularization of the ventricular myocardium in Holocephali: their evolutionary significance

It was generally assumed that the ventricle of the primitive vertebrate heart was composed of trabeculated, or spongy, myocardium, supplied by oxygen-poor luminal blood. In addition, it was presumed that the mixed ventricular myocardium, consisting of a compacta and a spongiosa, and its supply through coronary arteries appeared several times throughout fish evolution. Recent work has suggested, however, that a fully vascularized, mixed myocardium may be the primitive condition in gnathostomes. The present study of the heart ventricles of four holocephalan species aimed to clarify this controversy. Our observations showed that the ventricular myocardium of Chimaera monstrosa and Harriotta raleighana consists of a very thin compacta overlying a widespread spongiosa. The ventricle of Hydrolagus affinis is composed exclusively of trabeculated myocardium. In these three species there is a well-developed coronary artery system. The main coronary artery trunks run along the outflow tract, giving off subepicardial ventricular arteries. The trabeculae of the spongiosa are irrigated by branches of the subepicardial arteries and by penetrating arterial vessels arising directly from the main coronary trunks at the level of the conoventricular junction. The ventricle of Rhinochimaera atlantica has only spongy myocardium supplied by luminal blood. Small coronary arterial vessels are present in the subepicardium, but they do not enter the myocardial trabeculae. The present findings show for the first time that in a wild living vertebrate species, specifically H. affinis, an extensive coronary artery system supplying the whole cardiac ventricle exists in the absence of a well-developed compact ventricular myocardium. This is consistent with the notion derived from experimental work that myocardial cell proliferation and coronary vascular growth rely on distinct developmental programs. Our observations, together with data in the literature on elasmobranchs, support the view that the mixed ventricular myocardium is primitive for chondrichthyans. The reduction or even lack of compacta in holocephali has to be regarded as a derived anatomical trait. Our findings also fit in with the view that the mixed myocardium was the primitive condition in gnathostomes, and that the absence of compact ventricular myocardium in different actinopterygian groups is the result of a repeated loss of such type of cardiac muscle during fish evolution.

Evolution of the fish heart by sub/neofunctionalization of an elastin gene

The evolution of phenotypic traits is a key process in diversification of life. However, the mechanisms underlying the emergence of such evolutionary novelties are largely unknown. Here we address the origin of bulbus arteriosus (BA), an organ of evolutionary novelty seen in the teleost heart outflow tract (OFT), which sophisticates their circulatory system. The BA is a unique organ that is composed of smooth muscle while the OFTs in other vertebrates are composed of cardiac muscle. Here we reveal that the teleost-specific extracellular matrix (ECM) gene, elastin b, was generated by the teleost-specific whole-genome duplication and neofunctionalized to contribute to acquisition of the BA by regulating cell fate determination of cardiac precursor cells into smooth muscle. Furthermore, we show that the mechanotransducer yap is involved in this cell fate determination. Our findings reveal a mechanism of generating evolutionary novelty through alteration of cell fate determination by the ECM.

The bulbus arteriosus is an organ unique to the heart of teleosts, composed of specialized smooth muscle. Here, the authors show that the gene elastin b, which regulates cell fate of cardiac precursor cells into smooth muscle, evolved after whole-genome duplication and neofunctionalization in teleosts.

KCTD10 is critical for heart and blood vessel development of zebrafish

KCTD10 is a member of the PDIP1 family, which is highly conserved during evolution, sharing a lot of similarities among human, mouse, and zebrafish. Recently, zebrafish KCTD13 has been identified to play an important role in the early development of brain and autism. However, the specific function of KCTD10 remains to be elucidated. In this study, experiments were carried out to determine the expression pattern of zebrafish KCTD10 mRNA during embryonic development. It was found that KCTD10 is a maternal gene and KCTD10 is of great importance in the shaping of heart and blood vessels. Our data provide direct clues that knockdown of KCTD10 resulted in severe pericardial edema and loss of heart formation indicated by morphological observation and crucial heart markers like amhc, vmhc, and cmlc2. The heart defect caused by KCTD10 is linked to RhoA and PCNA. Flk-1 staining revealed that intersomitic vessels were lost in the trunk, although angioblasts could migrate to the midline. These findings could be helpful to better understand the determinants responsible for the heart and blood vessel defects.

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.

Zebrafish in the study of early cardiac development

Heart development is a complex process that involves cell specification and differentiation, as well as elaborate tissue morphogenesis and remodeling, to generate a functional organ. The zebrafish has emerged as a powerful model system to unravel the basic genetic, molecular, and cellular mechanisms of cardiac development and function. We summarize and discuss recent discoveries on early cardiac specification and the identification of the second heart field in zebrafish. In addition to the inductive signals regulating cardiac specification, these studies have shown that heart development also requires a repressive mechanism imposed by retinoic acid signaling to select cardiac progenitors from a multipotent population. Another recent advance in the study of early zebrafish cardiac development is the identification of the second heart field. These studies suggest that the molecular mechanisms that regulate the second heart field development are conserved between zebrafish and other vertebrates including mammals and provide insight into the evolution of the second heart field and its derivatives.

Complex innervation patterns of the conus arteriosus in the heart of the longnose gar, Lepisosteus osseus

Anatomical and functional studies of the autonomic innervation in the conus arteriosus of the garfishes are lacking. This study reveals that the conus arteriosus of the longnose gar is primarily myocardial in nature, but additionally, large numbers of smooth muscle cells are present in the subendocardium. A well-developed system of adrenergic, cholinergic, substance P (SP) and neuronal nitric oxide synthase (nNOS) positive nerve terminals are found in the wall of the conus arteriosus. Coronary blood vessels running in the adventitia receive a rich supply of nNOS positive nerve fibers, thus suggesting their importance in the nitrergic control of blood flow in the conus arteriosus. The present data show that the patterns of autonomic innervation of the garfish conus arteriosus are more complex than previously appreciated.

Zebrafish cardiac development requires a conserved secondary heart field

The secondary heart field is a conserved developmental domain in avian and mammalian embryos that contributes myocardium and smooth muscle to the definitive cardiac arterial pole. This field is part of the overall heart field and its myocardial component has been fate mapped from the epiblast to the heart in both mammals and birds. In this study we show that the population that gives rise to the arterial pole of the zebrafish can be traced from the epiblast, is a discrete part of the mesodermal heart field, and contributes myocardium after initial heart tube formation, giving rise to both smooth muscle and myocardium. We also show that Isl1, a transcription factor associated with undifferentiated cells in the secondary heart field in other species, is active in this field. Furthermore, Bmp signaling promotes myocardial differentiation from the arterial pole progenitor population, whereas inhibiting Smad1/5/8 phosphorylation leads to reduced myocardial differentiation with subsequent increased smooth muscle differentiation. Molecular pathways required for secondary heart field development are conserved in teleosts, as we demonstrate that the transcription factor Tbx1 and the Sonic hedgehog pathway are necessary for normal development of the zebrafish arterial pole.

Phylogeny informs ontogeny: a proposed common theme in the arterial pole of the vertebrate heart

In chick and mouse embryogenesis, a population of cells described as the secondary heart field (SHF) adds both myocardium and smooth muscle to the developing cardiac outflow tract (OFT). Following this addition, at approximately HH stage 22 in chick embryos, for example, the SHF can be identified architecturally by an overlapping seam at the arterial pole, where beating myocardium forms a junction with the smooth muscle of the arterial system. Previously, using either immunohistochemistry or nitric oxide indicators such as diaminofluorescein 2-diacetate, we have shown that a similar overlapping architecture also exists in the arterial pole of zebrafish and some shark species. However, although recent work suggests that development of the zebrafish OFT may also proceed by addition of a SHF-like population of cells, the presence of a true SHF in zebrafish and in many other developmental biological models remains an open question. We performed a comprehensive morphological study of the OFT of a wide range of vertebrates. Our data suggest that all vertebrates possess three fundamental OFT components: a proximal myocardial component, a distal smooth muscle component, and a middle component that contains overlapping myocardium and smooth muscle surrounding and supporting the outflow valves. Because the middle OFT component of avians and mammals is derived from the SHF, our observations suggest that a SHF may be an evolutionarily conserved theme in vertebrate embryogenesis.