The anatomical components of the cardiac outflow tract of chondrichthyans and actinopterygians

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.

Coronary artery established through amniote evolution

Coronary arteries are a critical part of the vascular system and provide nourishment to the heart. In humans, even minor defects in coronary arteries can be lethal, emphasizing their importance for survival. However, some teleosts survive without coronary arteries, suggesting that there may have been some evolutionary changes in the morphology and function of coronary arteries in the tetrapod lineage. Here, we propose that the true ventricular coronary arteries were newly established during amniote evolution through remodeling of the ancestral coronary vasculature. In mouse (Mus musculus) and Japanese quail (Coturnix japonica) embryos, the coronary arteries unique to amniotes are established by the reconstitution of transient vascular plexuses: aortic subepicardial vessels (ASVs) in the outflow tract and the primitive coronary plexus on the ventricle. In contrast, amphibians (Hyla japonica, Lithobates catesbeianus, Xenopus laevis, and Cynops pyrrhogaster) retain the ASV-like vasculature as truncal coronary arteries throughout their lives and have no primitive coronary plexus. The anatomy and development of zebrafish (Danio rerio) and chondrichthyans suggest that their hypobranchial arteries are ASV-like structures serving as the root of the coronary vasculature throughout their lives. Thus, the ventricular coronary artery of adult amniotes is a novel structure that has acquired a new remodeling process, while the ASVs, which occur transiently during embryonic development, are remnants of the ancestral coronary vessels. This evolutionary change may be related to the modification of branchial arteries, indicating considerable morphological changes underlying the physiological transition during amniote evolution.

Ancestral developmental potentials in early bony fish contributed to vertebrate water-to-land transition

The water-to-land transition was a major step in vertebrate evolution and eventually gave rise to the tetrapods, including amphibians, reptiles, birds, and mammals. The first land invasion of our fish ancestors is considered to have occurred during the late Devonian period ~370 million years ago (Daeschler et al., 2006). Many fossils from important transitional species, such as Tiktaalik, Acanthostega, and Ichthyostega, have helped to identify key morphological and anatomical structures crucial to vertebrate terrestrial adaptation (Coates, 1996; Johanson & Ahlberg, 2001; Shubin et al., 2006). However, homologous analyses of these body forms and structures in more ancient species have suggested that some of the morphologies related to vertebrate land dispersal were already present in early bony fish species. For instance, the presence of shoulder girdles on the articular surface of the endoskeleton in Late Lochkovian Psarolepis indicates that stem sarcopterygians already possessed an endoskeletal fin pattern similar to that of tetrapod stylopods (Zhu & Yu, 2009). In addition, primitive lungs, which originated from the respiratory pharynx and were located on the ventral side of the alimentary tracts, can be observed in several extant basal actinopterygians (bichirs, reedfish) and all extant sarcopterygians, as well as some fossils of coelacanths and salamanders (Cupello et al., 2017; Tissier et al., 2017) (Figure 1). This evidence suggests that, instead of relying on genetic innovations evolving after the first fish left their water habitat, this transition may have been accomplished by adopting physical traits and genetic components that already existed far earlier than when the transition occurred. Whether such an ancestral developmental regulatory network was present or not and how far this ancestral network can be traced in history are challenging questions for paleontologists. Three recent papers published in Cell provide new insights into this hypothesis. Wang et al. (2021) sequenced the giant genome of lungfish, the closest fish species to tetrapods, and Bi et al. (2021) sequenced the genomes of multiple early divergent ray-finned fish. Comparative genomic analyses from these two studies confirmed the presence of ancestral genetic regulatory networks that likely played essential roles in the development and evolution of various biological functions related to vertebrate land invasion. Although certain ancestral features have been lost in teleosts, the most derived fish lineage to evolve after whole-genome duplication (Sato & Nishida, 2010), they have been recreated in zebrafish by modifying their genetic makeup to reactivate the ancestral genetic network (Hawkins et al., 2021).

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.

Tracing the genetic footprints of vertebrate landing in non-teleost ray-finned fishes

Rich fossil evidence suggests that many traits and functions related to terrestrial evolution were present long before the ancestor of lobe- and ray-finned fishes. Here, we present genome sequences of the bichir, paddlefish, bowfin, and alligator gar, covering all major early divergent lineages of ray-finned fishes. Our analyses show that these species exhibit many mosaic genomic features of lobe- and ray-finned fishes. In particular, many regulatory elements for limb development are present in these fishes, supporting the hypothesis that the relevant ancestral regulation networks emerged before the origin of tetrapods. Transcriptome analyses confirm the homology between the lung and swim bladder and reveal the presence of functional lung-related genes in early ray-finned fishes. Furthermore, we functionally validate the essential role of a jawed vertebrate highly conserved element for cardiovascular development. Our results imply the ancestors of jawed vertebrates already had the potential gene networks for cardio-respiratory systems supporting air breathing.

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.

Myosin heavy chain isoforms in the myocardium of the atrioventricular junction of Scyliorhinus canicula (Chondrichthyes, Carcharhiniformes)

The atrioventricular junction of the fish heart, namely the segment interposed between the single atrium and the single ventricle, has been studied anatomically and histologically in several chondrichthyan and teleost species. Nonetheless, knowledge about myosin heavy chain (MyHC) in the atrioventricular myocardium remains scarce. The present report is the first one to provide data on the MyHC isoform distribution in the myocardium of the atrioventricular junction in chondrichthyans, specifically in the lesser spotted dogfish, Scyliorhinus canicula, a shark species whose heart reflects the primitive cardiac anatomical design in gnathostomes. Hearts from five dogfish were examined using histochemical and immunohistochemical techniques. The anti-MyHC A4.1025 antibody was used to detect differences in the occurrence of MyHC isoforms in the dogfish, as the fast-twitch isoforms MYH2 and MYH6 have a higher affinity for this antibody than the slow-twitch isoforms MYH7 and MYH7B. The histochemical findings show that myocardium of the atrioventricular junction connects the trabeculated myocardium of the atrium with the trabeculated layer of the ventricular myocardium. The immunohistochemical results indicate that the distribution of MyHC isoforms in the atrioventricular junction is not homogeneous. The atrial portion of the atrioventricular myocardium shows a positive reactivity against the A4.1025 antibody similar to that of the atrial myocardium. In contrast, the ventricular portion of the atrioventricular junction is not labelled, as is the case with the ventricular myocardium. This dual condition suggests that the myocardium of the atrioventricular junction has two contraction patterns: the myocardium of the atrial portion contracts in line with the atrial myocardium, whereas that of the ventricular portion follows the contraction pattern of the ventricular myocardium. Thus, the transition of the contraction wave from the atrium to the ventricle may be established in the atrioventricular segment because of its heterogeneous MyHC isoform distribution. The findings support the hypothesis that a distinct MyHC isoform distribution in the atrioventricular myocardium enables a synchronous contraction of inflow and outflow cardiac segments in vertebrates lacking a specialized cardiac conduction system.

Differential expression of myosin heavy chain isoforms in cardiac segments of gnathostome vertebrates and its evolutionary implications

Background

Immunohistochemical studies of hearts from the lesser spotted dogfish, Scyliorhinus canicula (Chondrichthyes) revealed that the pan-myosin heavy chain (pan-MyHC) antibody MF20 homogeneously labels all the myocardium, while the pan-MyHC antibody A4.1025 labels the myocardium of the inflow (sinus venosus and atrium) but not the outflow (ventricle and conus arteriosus) cardiac segments, as opposed to other vertebrates. We hypothesized that the conventional pattern of cardiac MyHC isoform distribution present in most vertebrates, i.e. MYH6 in the inflow and MYH7 in the outflow segments, has evolved from a primitive pattern that persists in Chondrichthyes. In order to test this hypothesis, we conducted protein detection techniques to identify the MyHC isoforms expressed in adult dogfish cardiac segments and to assess the pan-MyHC antibodies reactivity against the cardiac segments of representative species from different vertebrate groups.

Results

Western and slot blot results confirmed the specificity of MF20 and A4.1025 for MyHC in dogfish and their differential reactivity against distinct myocardial segments. HPLC-ESI-MS/MS and ESI-Quadrupole-Orbitrap revealed abundance of MYH6 and MYH2 in the inflow and of MYH7 and MYH7B in the outflow segments. Immunoprecipitation showed higher affinity of A4.1025 for MYH2 and MYH6 than for MYH7 and almost no affinity for MYH7B. Immunohistochemistry showed that A4.1025 signals are restricted to the inflow myocardial segments of elasmobranchs, homogeneous in all myocardial segments of teleosts and acipenseriforms, and low in the ventricle of polypteriforms.

Conclusions

The cardiac inflow and outflow segments of the dogfish show predominance of fast- and slow-twitch MyHC isoforms respectively, what can be considered a synapomorphy of gnathostomes. The myocardium of the dogfish contains two isomyosins (MYH2 and MYH7B) not expressed in the adult heart of other vertebrates. We propose that these isomyosins lost their function in cardiac contraction during the evolution of gnathostomes, the later acquiring a regulatory role in myogenesis through its intronic miRNA. Loss of MYH2 and MYH7B expression in the heart possibly occurred before the origin of Osteichthyes, being the latter reacquired in polypteriforms. We raise the hypothesis that the slow tonic MYH7B facilitates the peristaltic contraction of the conus arteriosus of fish with a primitive cardiac anatomical design and of the vertebrate embryo.

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.

The valves of the cardiac outflow tract of the starry ray, Raja asterias (Chondrichthyes; Rajiformes): Anatomical, histological and evolutionary aspects

The cardiac outflow tract of chondrichthyans is composed of the myocardial conus arteriosus, equipped with valves at its luminal side, and the bulbus arteriosus devoid of myocardium. Knowledge of the histomorphology of the conal valves is scarce despite their importance in preventing blood backflow to the heart. Current information on the subject refers to a single shark species. The present report is the first to describe the structure of the conal valves of a batoid species, namely, Raja asterias. Hearts from seven starry rays were examined using scanning electron microscopy and histochemical techniques for light microscopy. In all hearts, the conus showed four transverse rows of three pocket-like valves each. Each valve was composed of a leaflet and its supporting sinus. The leaflet had a stout central body, rich in glycosaminoglycans, which contained fibroblasts, collagen and elastin. The central body was surrounded by two thin fibrous layers, outer and inner, formed mainly by collagen. The valves of the anterior row, which were the largest of the valvular system, were attached proximally to the conus arteriosus and distally to the bulbus arteriosus, and not to the ventral aorta as previously reported for chondrichthyans. The arrangement of the anterior valves in the starry ray is an anatomical pattern that apparently has been preserved throughout the evolution of vertebrates.

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.