Development of the cardiac pacemaker

Cardiac Development at a Single-Cell Resolution

Mammalian cardiac development is a complex, multistage process. Though traditional lineage tracing studies have characterized the broad trajectories of cardiac progenitors, the advent and rapid optimization of single-cell RNA sequencing methods have yielded an ever-expanding toolkit for characterizing heterogeneous cell populations in the developing heart. Importantly, they have allowed for a robust profiling of the spatiotemporal transcriptomic landscape of the human and mouse heart, revealing the diversity of cardiac cells-myocyte and non-myocyte-over the course of development. These studies have yielded insights into novel cardiac progenitor populations, chamber-specific developmental signatures, the gene regulatory networks governing cardiac development, and, thus, the etiologies of congenital heart diseases. Furthermore, single-cell RNA sequencing has allowed for the exquisite characterization of distinct cardiac populations such as the hard-to-capture cardiac conduction system and the intracardiac immune population. Therefore, single-cell profiling has also resulted in new insights into the regulation of cardiac regeneration and injury repair. Single-cell multiomics approaches combining transcriptomics, genomics, and epigenomics may uncover an even more comprehensive atlas of human cardiac biology. Single-cell analyses of the developing and adult mammalian heart offer an unprecedented look into the fundamental mechanisms of cardiac development and the complex diseases that may arise from it.

Cadherin-5 facilitated the differentiation of human induced pluripotent stem cells into sinoatrial node-like pacemaker cells by regulating β-catenin

Our study was conducted to investigate whether cadherin-5 (CDH5), a vascular endothelial cell adhesion glycoprotein, could facilitate the differentiation of human induced pluripotent stem cells (hiPSCs) into sinoatrial node-like pacemaker cells (SANLPCs), following previous findings of silk-fibroin hydrogel-induced direct conversion of quiescent cardiomyocytes into pacemaker cells in rats through the activation of CDH5. In this study, the differentiating hiPSCs were treated with CDH5 (40 ng/mL) between Day 5 and 7 during cardiomyocytes differentiation. The findings in the present study demonstrated that CDH5 stimulated the expression of pacemaker-specific markers while suppressing markers associated with working cardiomyocytes, resulting in an increased proportion of SANLPCs among hiPSCs-derived cardiomyocytes (hiPSC-CMs) population. Moreover, CDH5 induced typical electrophysiological characteristics resembling cardiac pacemaker cells in hiPSC-CMs. Further mechanistic investigations revealed that the enriched differentiation of hiPSCs into SANLPCs induced by CDH5 was partially reversed by iCRT14, an inhibitor of β-catenin. Therefore, based on the aforementioned findings, it could be inferred that the regulation of β-catenin by CDH5 played a crucial role in promoting the enriched differentiation of hiPSCs into SANLPCs, which presents a novel avenue for the construction of biological pacemakers in forthcoming research.

Conversion of Unmodified Stem Cells to Pacemaker Cells by Overexpression of Key Developmental Genes

Arrhythmias of the heart are currently treated by implanting electronic pacemakers and defibrillators. Unmodified adipose tissue-derived stem cells (ASCs) have the potential to differentiate into all three germ layers but have not yet been tested for the generation of pacemaker and Purkinje cells. We investigated if-based on overexpression of dominant conduction cell-specific genes in ASCs-biological pacemaker cells could be induced. Here we show that by overexpression of certain genes that are active during the natural development of the conduction system, the differentiation of ASCs to pacemaker and Purkinje-like cells is feasible. Our study revealed that the most effective procedure consisted of short-term upregulation of gene combinations SHOX2-TBX5-HCN2, and to a lesser extent SHOX2-TBX3-HCN2. Single-gene expression protocols were ineffective. Future clinical implantation of such pacemaker and Purkinje cells, derived from unmodified ASCs of the same patient, could open up new horizons for the treatment of arrythmias.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Pacemaker activity and ion channels in the sinoatrial node cells: MicroRNAs and arrhythmia

The primary pacemaking activity of the heart is determined by a spontaneous action potential (AP) within sinoatrial node (SAN) cells. This unique AP generation relies on two mechanisms: membrane clocks and calcium clocks. Nonhomologous arrhythmias are caused by several functional and structural changes in the myocardium. MicroRNAs (miRNAs) are essential regulators of gene expression in cardiomyocytes. These miRNAs play a vital role in regulating the stability of cardiac conduction and in the remodeling process that leads to arrhythmias. Although it remains unclear how miRNAs regulate the expression and function of ion channels in the heart, these regulatory mechanisms may support the development of emerging therapies. This study discusses the spread and generation of AP in the SAN as well as the regulation of miRNAs and individual ion channels. Arrhythmogenicity studies on ion channels will provide a research basis for miRNA modulation as a new therapeutic target.

High cardiomyocyte diversity in human early prenatal heart development

Cardiomyocytes play key roles during cardiogenesis, but have poorly understood features, especially in prenatal stages. Here, we characterized human prenatal cardiomyocytes, 6.5-7 weeks post-conception, by integrating single-cell RNA sequencing, spatial transcriptomics, and ligand-receptor interaction information. Using a computational workflow developed to dissect cell type heterogeneity, localize cell types, and explore their molecular interactions, we identified eight types of developing cardiomyocyte, more than double compared to the ones identified in the Human Developmental Cell Atlas. These have high variability in cell cycle activity, mitochondrial content, and connexin gene expression, and are differentially distributed in the ventricles, including outflow tract, and atria, including sinoatrial node. Moreover, cardiomyocyte ligand-receptor crosstalk is mainly with non-cardiomyocyte cell types, encompassing cardiogenesis-related pathways. Thus, early prenatal human cardiomyocytes are highly heterogeneous and develop unique location-dependent properties, with complex ligand-receptor crosstalk. Further elucidation of their developmental dynamics may give rise to new therapies.

Hippo-Yap Signaling Maintains Sinoatrial Node Homeostasis

BACKGROUND

The sinoatrial node (SAN) functions as the pacemaker of the heart, initiating rhythmic heartbeats. Despite its importance, the SAN is one of the most poorly understood cardiac entities because of its small size and complex composition and function. The Hippo signaling pathway is a molecular signaling pathway fundamental to heart development and regeneration. Although abnormalities of the Hippo pathway are associated with cardiac arrhythmias in human patients, the role of this pathway in the SAN is unknown.

METHODS

We investigated key regulators of the Hippo pathway in SAN pacemaker cells by conditionally inactivating the Hippo signaling kinases Lats1 and Lats2 using the tamoxifen-inducible, cardiac conduction system-specific Cre driver Hcn4CreERT2 with Lats1 and Lats2 conditional knockout alleles. In addition, the Hippo-signaling effectors Yap and Taz were conditionally inactivated in the SAN. To determine the function of Hippo signaling in the SAN and other cardiac conduction system components, we conducted a series of physiological and molecular experiments, including telemetry ECG recording, echocardiography, Masson Trichrome staining, calcium imaging, immunostaining, RNAscope, cleavage under targets and tagmentation sequencing using antibodies against Yap1 or H3K4me3, quantitative real-time polymerase chain reaction, and Western blotting. We also performed comprehensive bioinformatics analyses of various datasets.

RESULTS

We found that Lats1/2 inactivation caused severe sinus node dysfunction. Compared with the controls, Lats1/2 conditional knockout mutants exhibited dysregulated calcium handling and increased fibrosis in the SAN, indicating that Lats1/2 function through both cell-autonomous and non-cell-autonomous mechanisms. It is notable that the Lats1/2 conditional knockout phenotype was rescued by genetic deletion of Yap and Taz in the cardiac conduction system. These rescued mice had normal sinus rhythm and reduced fibrosis of the SAN, indicating that Lats1/2 function through Yap and Taz. Cleavage Under Targets and Tagmentation sequencing data showed that Yap potentially regulates genes critical for calcium homeostasis such as Ryr2 and genes encoding paracrine factors important in intercellular communication and fibrosis induction such as Tgfb1 and Tgfb3. Consistent with this, Lats1/2 conditional knockout mutants had decreased Ryr2 expression and increased Tgfb1 and Tgfb3 expression compared with control mice.

CONCLUSIONS

We reveal, for the first time to our knowledge, that the canonical Hippo-Yap pathway plays a pivotal role in maintaining SAN homeostasis.

Tbx18 promoted the conversion of human-induced pluripotent stem cell-derived cardiomyocytes into sinoatrial node-like pacemaker cells

Sinoatrial node (SAN) pacemaker cells originate from T-box transcription factor 18 (Tbx18)-expressing progenitor cells. The present study aimed to investigate whether overexpression of human transcription factor Tbx18 could reprogram human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) into SAN-like pacemaker cells (SANLPCs) in vitro. In the study, hiPSCs were first differentiated into hiPSC-CMs through regulating the Wnt/β-catenin pathway, then purified hiPSC-CMs were transfected by Tbx18 adenovirus (Tbx18-CMs group) or green fluorescent protein (GFP) adenovirus (GFP-CMs group). The beating frequency of the Tbx18-CMs group was significantly higher than that of the hiPSC-CMs group and GFP-CMs group. Compared with the other two groups, the expression levels of hyperpolarization-activated cyclic nucleotide-gated potassium channel isoform 4, connexin-45 in the Tbx18-CMs group were markedly upregulated, while the expressions of transcription factor NKX2.5, CX43 were significantly downregulated. Whole-cell patch-clamp results illustrated that action potential and "funny" current (I_f ) similar to SAN pacemaker cells could be recorded in the Tbx18-CMs group. In conclusion, this present study demonstrated that overexpression of Tbx18 promoted the conversion of hiPSC-CMs into SANLPCs.

E2A ablation enhances proportion of nodal-like cardiomyocytes in cardiac-specific differentiation of human embryonic stem cells

BACKGROUND

Human sinoatrial cardiomyocytes are essential building blocks for cell therapies of conduction system disorders. However, current differentiation protocols for deriving nodal cardiomyocytes from human pluripotent stem cells (hPSCs) are very inefficient.

METHODS

By employing the hPSCs to cardiomyocyte (CM) in vitro differentiation system and generating E2A-knockout hESCs using CRISPR/Cas9 gene editing technology, we analyze the functions of E2A in CM differentiation.

FINDINGS

We found that knockout of the transcription factor E2A substantially increased the proportion of nodal-like cells in hESC-derived CMs. The E2A ablated CMs displayed smaller cell size, increased beating rates, weaker contractile force, and other functional characteristics similar to sinoatrial node (SAN) cells. Transcriptomic analyses indicated that ion channel-encoding genes were up-regulated in E2A ablated CMs. E2A directly bounded to the promoters of genes key to SAN development via conserved E-box motif, and promoted their expression. Unexpect enhanced activity of NOTCH pathway after E2A ablation could also facilate to induct ventricle workingtype CMs reprogramming into SAN-like cells.

INTERPRETATION

Our study revealed a new role for E2A during directed cardiac differentiation of hESCs and may provide new clues for enhancing induction efficiency of SAN-like cardiomyocytes from hPSCs in the future.

FUNDING

This work was supported by the NSFC (No.82070391, N.S.; No.81870175 and 81922006, P.L.), the National Key R&D Program of China (2018YFC2000202, N.S.; 2017YFA0103700, P.L.), the Haiju program of National Children's Medical Center EK1125180102, and Innovative research team of high-level local universities in Shanghai and a key laboratory program of the Education Commission of Shanghai Municipality (ZDSYS14005).

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.

Genome-Wide Analysis Identifies an Essential Human TBX3 Pacemaker Enhancer

RATIONALE

The development and function of the pacemaker cardiomyocytes of the sinoatrial node (SAN), the leading pacemaker of the heart, are tightly controlled by a conserved network of transcription factors, including TBX3 (T-box transcription factor 3), ISL1 (ISL LIM homeobox 1), and SHOX2 (short stature homeobox 2). Yet, the regulatory DNA elements (REs) controlling target gene expression in the SAN pacemaker cells have remained undefined.

OBJECTIVE

Identification of the regulatory landscape of human SAN-like pacemaker cells and functional assessment of SAN-specific REs potentially involved in pacemaker cell gene regulation.

METHODS AND RESULTS

We performed Assay for Transposase-Accessible Chromatin using sequencing on human pluripotent stem cell-derived SAN-like pacemaker cells and ventricle-like cells and identified thousands of putative REs specific for either human cell type. We validated pacemaker cell-specific elements in the SHOX2 and TBX3 loci. CRISPR-mediated homozygous deletion of the mouse ortholog of a noncoding region with candidate pacemaker-specific REs in the SHOX2 locus resulted in selective loss of Shox2 expression from the developing SAN and embryonic lethality. Putative pacemaker-specific REs were identified up to 1 Mbp upstream of TBX3 in a region close to MED13L harboring variants associated with heart rate recovery after exercise. The orthologous region was deleted in mice, which resulted in selective loss of expression of Tbx3 from the SAN and (cardiac) ganglia and in neonatal lethality. Expression of Tbx3 was maintained in other tissues including the atrioventricular conduction system, lungs, and liver. Heterozygous adult mice showed increased SAN recovery times after pacing. The human REs harboring the associated variants robustly drove expression in the SAN of transgenic mouse embryos.

CONCLUSIONS

We provided a genome-wide collection of candidate human pacemaker-specific REs, including the loci of SHOX2, TBX3, and ISL1, and identified a link between human genetic variants influencing heart rate recovery after exercise and a variant RE with highly conserved function, driving SAN expression of TBX3.

Canonical Wnt Signaling Sets the Pace

In this issue of Developmental Cell, Ren et al. (2019) identify the embryonic origin of cardiac pacemaker cells in zebrafish and implicate Wnt5b in promoting their differentiation. Furthermore, canonical Wnt activation in human stem cell-derived cardiac progenitors produces functional pacemaker cells in vitro, advancing the therapeutic potential of biological pacemakers.

Subtype-specific cardiomyocytes for precision medicine: Where are we now?

Patient-derived pluripotent stem cells (PSCs) have greatly transformed the current understanding of human heart development and cardiovascular disease. Cardiomyocytes derived from personalized PSCs are powerful tools for modeling heart disease and performing patient-based cardiac toxicity testing. However, these PSC-derived cardiomyocytes (PSC-CMs) are a mixed population of atrial-, ventricular-, and pacemaker-like cells in the dish, hindering the future of precision cardiovascular medicine. Recent insights gleaned from the developing heart have paved new avenues to refine subtype-specific cardiomyocytes from patients with known pathogenic genetic variants and clinical phenotypes. Here, we discuss the recent progress on generating subtype-specific (atrial, ventricular, and nodal) cardiomyocytes from the perspective of embryonic heart development and how human pluripotent stem cells will expand our current knowledge on molecular mechanisms of cardiovascular disease and the future of precision medicine.

Differences in Expression of Genes Involved in Bone Development and Morphogenesis in the Walls of Internal Thoracic Artery and Saphenous Vein Conduits May Provide Markers Useful for Evaluation Graft Patency

Coronary artery bypass grafting (CABG) is one of the most efficient procedures for patients with advanced coronary artery disease. From all the blood vessels with the potential to be used in this procedure, the internal thoracic artery (ITA) and the saphenous vein (SV) are the most commonly applied as aortocoronary conduits. Nevertheless, in order to evaluate the graft patency and efficiency effectively, basic knowledge should be constantly expanding at the molecular level as well, as the understanding of predictive factors is still limited. In this study, we have employed the expressive microarray approach, validated with Real-Time Quantitative Polymerase Chain Reaction (RT-qPCR), to analyze the transcriptome of both venous and arterial grafts. Searching for potential molecular factors, we analyzed differentially expressed gene ontologies involved in bone development and morphogenesis, for the possibility of discovery of new markers for the evaluation of ITA and SV segment quality. Among three ontological groups of interest—“endochondral bone morphogenesis”, “ossification”, and “skeletal system development”—we found six genes common to all of them. BMP6, SHOX2, COL13A1, CSGALNACT1, RUNX2, and STC1 showed differential expression patterns in both analyzed vessels. STC1 and COL13A1 were upregulated in ITA samples, whereas others were upregulated in SV. With regard to the Runx2 protein function in osteogenic phenotype regulation, the RUNX2 gene seems to be of paramount importance in assessing the potential of ITA, SV, and other vessels used in the CABG procedure. Overall, the presented study provided valuable insight into the molecular background of conduit characterization, and thus indicated genes that may be the target of subsequent studies, also at the protein level. Moreover, it has been suggested that RUNX2 may be recognized as a molecular marker of osteogenic changes in human blood vessels.

Learn from Your Elders: Developmental Biology Lessons to Guide Maturation of Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cells (hPSCs) offer a multifaceted platform to study cardiac developmental biology, understand disease mechanisms, and develop novel therapies. Remarkable progress over the last two decades has led to methods to obtain highly pure hPSC-derived cardiomyocytes (hPSC-CMs) with reasonable ease and scalability. Nevertheless, a major bottleneck for the translational application of hPSC-CMs is their immature phenotype, resembling that of early fetal cardiomyocytes. Overall, bona fide maturation of hPSC-CMs represents one of the most significant goals facing the field today. Developmental biology studies have been pivotal in understanding the mechanisms to differentiate hPSC-CMs. Similarly, evaluation of developmental cues such as electrical and mechanical activities or neurohormonal and metabolic stimulations revealed the importance of these pathways in cardiomyocyte physiological maturation. Those signals cooperate and dictate the size and the performance of the developing heart. Likewise, this orchestra of stimuli is important in promoting hPSC-CM maturation, as demonstrated by current in vitro maturation approaches. Different shades of adult-like phenotype are achieved by prolonging the time in culture, electromechanical stimulation, patterned substrates, microRNA manipulation, neurohormonal or metabolic stimulation, and generation of human-engineered heart tissue (hEHT). However, mirroring this extremely dynamic environment is challenging, and reproducibility and scalability of these approaches represent the major obstacles for an efficient production of mature hPSC-CMs. For this reason, understanding the pattern behind the mechanisms elicited during the late gestational and early postnatal stages not only will provide new insights into postnatal development but also potentially offer new scalable and efficient approaches to mature hPSC-CMs.

Transcriptomic Profiling of the Developing Cardiac Conduction System at Single-Cell Resolution

RATIONALE

The cardiac conduction system (CCS) consists of distinct components including the sinoatrial node, atrioventricular node, His bundle, bundle branches, and Purkinje fibers. Despite an essential role for the CCS in heart development and function, the CCS has remained challenging to interrogate because of inherent obstacles including small cell numbers, large cell-type heterogeneity, complex anatomy, and difficulty in isolation. Single-cell RNA-sequencing allows for genome-wide analysis of gene expression at single-cell resolution.

OBJECTIVE

Assess the transcriptional landscape of the entire CCS at single-cell resolution by single-cell RNA-sequencing within the developing mouse heart.

METHODS AND RESULTS

Wild-type, embryonic day 16.5 mouse hearts (n=6 per zone) were harvested and 3 zones of microdissection were isolated, including: Zone I-sinoatrial node region; Zone II-atrioventricular node/His region; and Zone III-bundle branch/Purkinje fiber region. Tissue was digested into single-cell suspensions, cells isolated, mRNA reverse transcribed, and barcoded before high-throughput sequencing and bioinformatics analyses. Single-cell RNA-sequencing was performed on over 22 000 cells, and all major cell types of the murine heart were successfully captured including bona fide clusters of cells consistent with each major component of the CCS. Unsupervised weighted gene coexpression network analysis led to the discovery of a host of novel CCS genes, a subset of which were validated using fluorescent in situ hybridization as well as whole-mount immunolabeling with volume imaging (iDISCO+) in 3 dimensions on intact mouse hearts. Further, subcluster analysis unveiled isolation of distinct CCS cell subtypes, including the clinically relevant but poorly characterized transitional cells that bridge the CCS and surrounding myocardium.

CONCLUSIONS

Our study represents the first comprehensive assessment of the transcriptional profiles from the entire CCS at single-cell resolution and provides a characterization in the context of development and disease.

Homeobox Genes and Homeodomain Proteins: New Insights into Cardiac Development, Degeneration and Regeneration

Cardiovascular diseases are the most common cause of human death in the developing world. Extensive evidence indicates that various toxic environmental factors and unhealthy lifestyle choices contribute to the risk, incidence and severity of cardiovascular diseases. Alterations in the genetic level of myocardium affects normal heart development and initiates pathological processes leading to various types of cardiac diseases. Homeobox genes are a large and highly specialized family of closely related genes that direct the formation of body structure, including cardiac development. Homeobox genes encode homeodomain proteins that function as transcription factors with characteristic structures that allow them to bind to DNA, regulate gene expression and subsequently control the proper physiological function of cells, tissues and organs. Mutations in homeobox genes are rare and usually lethal with evident alterations in cardiac function at or soon after the birth. Our understanding of homeobox gene family expression and function has expanded significantly during the recent years. However, the involvement of homeobox genes in the development of human and animal cardiac tissue requires further investigation. The phenotype of human congenital heart defects unveils only some aspects of human heart development. Therefore, mouse models are often used to gain a better understanding of human heart function, pathology and regeneration. In this review, we have focused on the role of homeobox genes in the development and pathology of human heart as potential tools for the future development of targeted regenerative strategies for various heart malfunctions.

Cardiac Stem Cells in the Postnatal Heart: Lessons from Development

Heart development in mammals is followed by a postnatal decline in cell proliferation and cell renewal from stem cell populations. A better understanding of the developmental changes in cardiac microenvironments occurring during heart maturation will be informative regarding the loss of adult regenerative potential. We reevaluate the adult heart's mitotic potential and the reported adult cardiac stem cell populations, as these are two topics of ongoing debate. The heart's early capacity for cell proliferation driven by progenitors and reciprocal signalling is demonstrated throughout development. The mature heart architecture and environment may be more restrictive on niches that can host progenitor cells. The engraftment issues observed in cardiac stem cell therapy trials using exogenous stem cells may indicate a lack of supporting stem cell niches, while tissue injury adds to a hostile microenvironment for transplanted cells. Engraftment may be improved by preconditioning the cultured stem cells and modulating the microenvironment to host these cells. These prospective areas of further research would benefit from a better understanding of cardiac progenitor interactions with their microenvironment throughout development and may lead to enhanced cardiac niche support for stem cell therapy engraftment.

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.