Comparative developmental biology of the cardiac inflow tract

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.

Anatomy of the heart with the highest heart rate

Shrews occupy the lower extreme of the seven orders of magnitude mammals range in size. Their hearts are large relative to body weight and heart rate can exceed a thousand beats a minute. It is not known whether traits typical of mammal hearts scale to these extremes. We assessed the heart of three species of shrew (genus Sorex) following the sequential segmental analysis developed for human hearts. Using micro‐computed tomography, we describe the overall structure and find, in agreement with previous studies, a large and elongate ventricle. The atrial and ventricular septums and the atrioventricular (AV) and arterial valves are typically mammalian. The ventricular walls comprise mostly compact myocardium and especially the right ventricle has few trabeculations on the luminal side. A developmental process of compaction is thought to reduce trabeculations in mammals, but in embryonic shrews the volume of trabeculations increase for every gestational stage, only slower than the compact volume. By expression of Hcn4, we identify a sinus node and an AV conduction axis which is continuous with the ventricular septal crest. Outstanding traits include pulmonary venous sleeve myocardium that reaches farther into the lungs than in any other mammals. Typical proportions of coronary arteries‐to‐aorta do not scale and the shrew coronary arteries are proportionally enormous, presumably to avoid the high resistance to blood flow of narrow vessels. In conclusion, most cardiac traits do scale to the miniscule shrews. The shrew heart, nevertheless, stands out by its relative size, elongation, proportionally large coronary vessels, and extent of pulmonary venous myocardium.

Dynamic MicroRNA Expression Profiles During Embryonic Development Provide Novel Insights Into Cardiac Sinus Venosus/Inflow Tract Differentiation

MicroRNAs have been explored in different organisms and are involved as molecular switches modulating cellular specification and differentiation during the embryonic development, including the cardiovascular system. In this study, we analyze the expression profiles of different microRNAs during early cardiac development. By using whole mount in situ hybridization in developing chick embryos, with microRNA-specific LNA probes, we carried out a detailed study of miR-23b, miR-130a, miR-106a, and miR-100 expression during early stages of embryogenesis (HH3 to HH17). We also correlated those findings with putative microRNA target genes by means of mirWalk and TargetScan analyses. Our results demonstrate a dynamic expression pattern in cardiac precursor cells from the primitive streak to the cardiac looping stages for miR-23b, miR-130a, and miR-106a. Additionally, miR-100 is later detectable during cardiac looping stages (HH15-17). Interestingly, the sinus venosus/inflow tract was shown to be the most representative cardiac area for the convergent expression of the four microRNAs. Through in silico analysis we revealed that distinct Hox family members are predicted to be targeted by the above microRNAs. We also identified expression of several Hox genes in the sinus venosus at stages HH11 and HH15. In addition, by means of gain-of-function experiments both in cardiomyoblasts and sinus venosus explants, we demonstrated the modulation of the different Hox clusters, Hoxa, Hoxb, Hoxc, and Hoxd genes, by these microRNAs. Furthermore, we correlated the negative modulation of several Hox genes, such as Hoxa3, Hoxa4, Hoxa5, Hoxc6, or Hoxd4. Finally, we demonstrated through a dual luciferase assay that Hoxa1 is targeted by miR-130a and Hoxa4 is targeted by both miR-23b and miR-106a, supporting a possible role of these microRNAs in Hox gene modulation during differentiation and compartmentalization of the posterior structures of the developing venous pole of the heart.

Smooth Muscle in Cardiac Chambers is Common in Turtles and Extensive in the Emydid Turtle, Trachemys scripta

A prominent layer of smooth muscle lining the luminal side of the atria of freshwater turtles (Emydidae) was described more than a century ago. We recently demonstrated that this smooth muscle provides a previously unrecognized mechanism to change cardiac output in the emydid red-eared slider (Trachemys scripta) that possibly contributes to their tremendous diving capacity. The purpose of the present immunohistochemical study was firstly to screen major groups of vertebrates for the presence of cardiac smooth muscle. Secondly, we investigated the phylogenetic distribution of cardiac smooth muscle within the turtle order (Testudines), including terrestrial and aquatic species. Atrial smooth muscle was not detected in a range of vertebrates, including Xenopus laevis, Alligator mississippiensis, and Caiman crocodilus, all of which have pronounced diving capacities. However, we confirmed earlier reports that traces of smooth muscle are found in human atrial tissue. Only within the turtles (eight species) was there substantial amounts of nonvascular smooth muscle in the heart. This amount was greatest in the atria, while the amount in proportion to cardiac muscle was greater in the sinus venosus than in other chambers. T. scripta had more smooth muscle in the sinus venosus and atria than the other turtles. In some specimens, there was some smooth muscle in the ventricle and the pulmonary vein. Our study demonstrates that cardiac smooth muscle likely appeared early in turtle evolution and has become extensive within the Emydidae family, possibly in association with diving. Across other tetrapod clades, cardiac smooth muscle might not associate with diving. Anat Rec, 303:1327-1336, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association for Anatomy.

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.

Sinus venosus incorporation: contentious issues and operational criteria for developmental and evolutionary studies

The sinus venosus is a cardiac chamber upstream of the right atrium that harbours the dominant cardiac pacemaker. During human heart development, the sinus venosus becomes incorporated into the right atrium. However, from the literature it is not possible to deduce the characteristics and importance of this process of incorporation, due to inconsistent terminology and definitions in the description of multiple lines of evidence. We reviewed the literature regarding the incorporation of the sinus venosus and included novel electrophysiological data. Most mammals that have an incorporated sinus venosus show a loss of a functional valve guard of the superior caval vein together with a loss of the electrical sinuatrial delay between the sinus venosus and the right atrium. However, these processes are not necessarily intertwined and in a few species only the sinuatrial delay may be lost. Sinus venosus incorporation can be characterised as the loss of the sinuatrial delay of which the anatomical and molecular underpinnings are not yet understood.

Comparative analysis of avian hearts provides little evidence for variation among species with acquired endothermy

Mammals and birds acquired high performance hearts and endothermy during their independent evolution from amniotes with many sauropsid features. A literature review shows that the variation in atrial morphology is greater in mammals than in ectothermic sauropsids. We therefore hypothesized that the transition from ectothermy to endothermy was associated with greater variation in cardiac structure. We tested the hypothesis in 14 orders of birds by assessing the variation in 15 cardiac structures by macroscopic inspection and histology, with an emphasis on the atria as they have multiple features that lend themselves to quantification. We found bird hearts to have multiple features in common with ectothermic sauropsids (synapomorphies), such as the presence of three sinus horns. Convergent features were shared with crocodylians and mammals, such as the cranial offset of the left atrioventricular junction. Other convergent features, like the compact organization of the atrial walls, were shared with mammals only. Pacemaker myocardium, identified by Isl1 expression, was anatomically node‐like (Mallard), thickened (Chicken), or indistinct (Lesser redpoll, Jackdaw). Some features were distinctly avian, (autapomorphies) including the presence of a left atrial antechamber and the ventral merger of the left and right atrial auricles, which was found in some species of parrots and passerines. Most features, however, exhibited little variation. For instance, there were always three systemic veins and two pulmonary veins, whereas among mammals there are 2–3 and 1–7, respectively. Our findings suggest that the transition to high cardiac performance does not necessarily lead to a greater variation in cardiac structure.