Developmental fate and cellular maturity encoded in human regulatory DNA landscapes.

Cellular-state information between generations of developing cells may be propagated via regulatory regions. We report consistent patterns of gain and loss of DNase I-hypersensitive sites (DHSs) as cells progress from embryonic stem cells (ESCs) to terminal fates. DHS patterns alone convey rich information about cell fate and lineage relationships distinct from information conveyed by gene expression. Developing cells share a proportion of their DHS landscapes with ESCs; that proportion decreases continuously in each cell type as differentiation progresses, providing a quantitative benchmark of developmental maturity. Developmentally stable DHSs densely encode binding sites for transcription factors involved in autoregulatory feedback circuits. In contrast to normal cells, cancer cells extensively reactivate silenced ESC DHSs and those from developmental programs external to the cell lineage from which the malignancy derives. Our results point to changes in regulatory DNA landscapes as quantitative indicators of cell-fate transitions, lineage relationships, and dysfunction.

A temporal chromatin signature in human embryonic stem cells identifies regulators of cardiac development.

Directed differentiation of human embryonic stem cells (ESCs) into cardiovascular cells provides a model for studying molecular mechanisms of human cardiovascular development. Although it is known that chromatin modification patterns in ESCs differ markedly from those in lineage-committed progenitors and differentiated cells, the temporal dynamics of chromatin alterations during differentiation along a defined lineage have not been studied. We show that differentiation of human ESCs into cardiovascular cells is accompanied by programmed temporal alterations in chromatin structure that distinguish key regulators of cardiovascular development from other genes. We used this temporal chromatin signature to identify regulators of cardiac development, including the homeobox gene MEIS2. Using the zebrafish model, we demonstrate that MEIS2 is critical for proper heart tube formation and subsequent cardiac looping. Temporal chromatin signatures should be broadly applicable to other models of stem cell differentiation to identify regulators and provide key insights into major developmental decisions.

Sequential Defects in Cardiac Lineage Commitment and Maturation Cause Hypoplastic Left Heart Syndrome.

BACKGROUND: Complex molecular programs in specific cell lineages govern human heart development. Hypoplastic left heart syndrome (HLHS) is the most common and severe manifestation within the spectrum of left ventricular outflow tract obstruction defects occurring in association with ventricular hypoplasia. The pathogenesis of HLHS is unknown, but hemodynamic disturbances are assumed to play a prominent role. METHODS: To identify perturbations in gene programs controlling ventricular muscle lineage development in HLHS, we performed whole-exome sequencing of 87 HLHS parent-offspring trios, nuclear transcriptomics of cardiomyocytes from ventricles of 4 patients with HLHS and 15 controls at different stages of heart development, single cell RNA sequencing, and 3D modeling in induced pluripotent stem cells from 3 patients with HLHS and 3 controls. RESULTS: Gene set enrichment and protein network analyses of damaging de novo mutations and dysregulated genes from ventricles of patients with HLHS suggested alterations in specific gene programs and cellular processes critical during fetal ventricular cardiogenesis, including cell cycle and cardiomyocyte maturation. Single-cell and 3D modeling with induced pluripotent stem cells demonstrated intrinsic defects in the cell cycle/unfolded protein response/autophagy hub resulting in disrupted differentiation of early cardiac progenitor lineages leading to defective cardiomyocyte subtype differentiation/maturation in HLHS. Premature cell cycle exit of ventricular cardiomyocytes from patients with HLHS prevented normal tissue responses to developmental signals for growth, leading to multinucleation/polyploidy, accumulation of DNA damage, and exacerbated apoptosis, all potential drivers of left ventricular hypoplasia in absence of hemodynamic cues. CONCLUSIONS: Our results highlight that despite genetic heterogeneity in HLHS, many mutations converge on sequential cellular processes primarily driving cardiac myogenesis, suggesting novel therapeutic approaches.

4HNE Impairs Myocardial Bioenergetics in Congenital Heart Disease-Induced Right Ventricular Failure.

BACKGROUND: In patients with complex congenital heart disease, such as those with tetralogy of Fallot, the right ventricle (RV) is subject to pressure overload stress, leading to RV hypertrophy and eventually RV failure. The role of lipid peroxidation, a potent form of oxidative stress, in mediating RV hypertrophy and failure in congenital heart disease is unknown. METHODS: Lipid peroxidation and mitochondrial function and structure were assessed in right ventricle (RV) myocardium collected from patients with RV hypertrophy with normal RV systolic function (RV fractional area change, 47.3±3.8%) and in patients with RV failure showing decreased RV systolic function (RV fractional area change, 26.6±3.1%). The mechanism of the effect of lipid peroxidation, mediated by 4-hydroxynonenal ([4HNE] a byproduct of lipid peroxidation) on mitochondrial function and structure was assessed in HL1 murine cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes. RESULTS: RV failure was characterized by an increase in 4HNE adduction of metabolic and mitochondrial proteins (16 of 27 identified proteins), in particular electron transport chain proteins. Sarcomeric (myosin) and cytoskeletal proteins (desmin, tubulin) also underwent 4HNE adduction. RV failure showed lower oxidative phosphorylation (moderate RV hypertrophy, 287.6±19.75 versus RV failure, 137.8±11.57 pmol/[sec×mL]; P=0.0004), and mitochondrial structural damage. Using a cell model, we show that 4HNE decreases cell number and oxidative phosphorylation (control, 388.1±23.54 versus 4HNE, 143.7±11.64 pmol/[sec×mL]; P<0.0001). Carvedilol, a known antioxidant did not decrease 4HNE adduction of metabolic and mitochondrial proteins and did not improve oxidative phosphorylation. CONCLUSIONS: Metabolic, mitochondrial, sarcomeric, and cytoskeletal proteins are susceptible to 4HNE-adduction in patients with RV failure. 4HNE decreases mitochondrial oxygen consumption by inhibiting electron transport chain complexes. Carvedilol did not improve the 4HNE-mediated decrease in oxygen consumption. Strategies to decrease lipid peroxidation could improve mitochondrial energy generation and cardiomyocyte survival and improve RV failure in patients with congenital heart disease.

Cardiac involvement in classical or hypermobile Ehlers-Danlos syndrome is uncommon.

PURPOSE: Cardiac-valvular and vascular Ehlers-Danlos syndrome (EDS) have significant cardiovascular issues. The prevalence and significance of such abnormalities in classical (cEDS) or hypermobile EDS (hEDS) remain unclear. We report the prevalence of cardiac abnormalities in patients with cEDS and hEDS. METHODS: We identified 532 pediatric patients with potential EDS evaluated at our institution from January 2014 through April 2019 by retrospective chart review. Ninety-five patients (12 cEDS and 83 hEDS patients) met 2017 EDS diagnostic criteria and had an echocardiogram. One patient was excluded due to complex congenital heart disease, and two were excluded due to lack of images. We reviewed echocardiograms for all structural abnormalities. RESULTS: Of these 95 patients, 1 had mild aortic root dilation, and 1 had mild ascending aorta dilation in the setting of a bicuspid aortic valve. Eleven patients (11.6%) had a cardiac valve abnormality, all of which were trivial to mild. None of the patients required cardiac intervention. CONCLUSION: Our results demonstrate that aortic dilation and valvular anomalies are uncommon in cEDS or hEDS patients. Given the lack of evidence, we do not recommend echocardiographic evaluation and surveillance in patients with cEDS and hEDS in the absence of clinical findings or positive family history.

Cardiac Regeneration: Lessons From Development.

Palliative surgery for congenital heart disease has allowed patients with previously lethal heart malformations to survive and, in most cases, to thrive. However, these procedures often place pressure and volume loads on the heart, and over time, these chronic loads can cause heart failure. Current therapeutic options for initial surgery and chronic heart failure that results from failed palliation are limited, in part, by the mammalian heart's low inherent capacity to form new cardiomyocytes. Surmounting the heart regeneration barrier would transform the treatment of congenital, as well as acquired, heart disease and likewise would enable development of personalized, in vitro cardiac disease models. Although these remain distant goals, studies of heart development are illuminating the path forward and suggest unique opportunities for heart regeneration, particularly in fetal and neonatal periods. Here, we review major lessons from heart development that inform current and future studies directed at enhancing cardiac regeneration.

Molecular regulation of cardiomyocyte differentiation.

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in ≈1 percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the causes of congenital heart disease coupled with the potential of using knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes, as well as regulation of myocardial trabeculation.

Transmembrane protein 88: a Wnt regulatory protein that specifies cardiomyocyte development.

Genetic regulation of the cell fate transition from lateral plate mesoderm to the specification of cardiomyocytes requires suppression of Wnt/ß-catenin signaling, but the mechanism for this is not well understood. By analyzing gene expression and chromatin dynamics during directed differentiation of human embryonic stem cells (hESCs), we identified a suppressor of Wnt/ß-catenin signaling, transmembrane protein 88 (TMEM88), as a potential regulator of cardiovascular progenitor cell (CVP) specification. During the transition from mesoderm to the CVP, TMEM88 has a chromatin signature of genes that mediate cell fate decisions, and its expression is highly upregulated in advance of key cardiac transcription factors in vitro and in vivo. In early zebrafish embryos, tmem88a is expressed broadly in the lateral plate mesoderm, including the bilateral heart fields. Short hairpin RNA targeting of TMEM88 during hESC cardiac differentiation increases Wnt/ß-catenin signaling, confirming its role as a suppressor of this pathway. TMEM88 knockdown has no effect on NKX2.5 or GATA4 expression, but 80% of genes most highly induced during CVP development have reduced expression, suggesting adoption of a new cell fate. In support of this, analysis of later stage cell differentiation showed that TMEM88 knockdown inhibits cardiomyocyte differentiation and promotes endothelial differentiation. Taken together, TMEM88 is crucial for heart development and acts downstream of GATA factors in the pre-cardiac mesoderm to specify lineage commitment of cardiomyocyte development through inhibition of Wnt/ß-catenin signaling.

Mechanical Stress Promotes Maturation of Human Myocardium From Pluripotent Stem Cell-Derived Progenitors.

Recent advances in pluripotent stem cell biology and directed differentiation have identified a population of human cardiovascular progenitors that give rise to cardiomyocytes, smooth muscle, and endothelial cells. Because the heart develops from progenitors in 3D under constant mechanical load, we sought to test the effects of a 3D microenvironment and mechanical stress on differentiation and maturation of human cardiovascular progenitors into myocardial tissue. Progenitors were derived from embryonic stem cells, cast into collagen hydrogels, and left unstressed or subjected to static or cyclic mechanical stress. Compared to 2D culture, the unstressed 3D environment increased cardiomyocyte numbers and decreased smooth muscle numbers. Additionally, 3D culture suppressed smooth muscle α-actin content, suggesting diminished cell maturation. Cyclic stress-conditioning increased expression of several cardiac markers, including ß-myosin heavy chain and cardiac troponin T, and the tissue showed enhanced calcium dynamics and force production. There was no effect of mechanical loading on cardiomyocyte or smooth muscle specification. Thus, 3D growth conditions favor cardiac differentiation from cardiovascular progenitors, whereas 2D conditions promote smooth muscle differentiation. Mechanical loading promotes cardiomyocyte structural and functional maturation. Culture in 3-D facilitates understanding how cues such as mechanical stress affect the differentiation and morphogenesis of distinct cardiovascular cell populations into organized, functional human cardiovascular tissue. Stem Cells 2015;33:2148-2157.

Combined lineage tracing and scRNA-seq reveals unexpected first heart field predominance of human iPSC differentiation.

During mammalian development, the left and right ventricles arise from early populations of cardiac progenitors known as the first and second heart fields, respectively. While these populations have been extensively studied in non-human model systems, their identification and study in vivo human tissues have been limited due to the ethical and technical limitations of accessing gastrulation-stage human embryos. Human-induced pluripotent stem cells (hiPSCs) present an exciting alternative for modeling early human embryogenesis due to their well-established ability to differentiate into all embryonic germ layers. Here, we describe the development of a TBX5/MYL2 lineage tracing reporter system that allows for the identification of FHF- progenitors and their descendants including left ventricular cardiomyocytes. Furthermore, using single-cell RNA sequencing (scRNA-seq) with oligonucleotide-based sample multiplexing, we extensively profiled differentiating hiPSCs across 12 timepoints in two independent iPSC lines. Surprisingly, our reporter system and scRNA-seq analysis revealed a predominance of FHF differentiation using the small molecule Wnt-based 2D differentiation protocol. We compared this data with existing murine and 3D cardiac organoid scRNA-seq data and confirmed the dominance of left ventricular cardiomyocytes (>90%) in our hiPSC-derived progeny. Together, our work provides the scientific community with a powerful new genetic lineage tracing approach as well as a single-cell transcriptomic atlas of hiPSCs undergoing cardiac differentiation.

Phase 2 Study of Aficamten in Patients With Obstructive Hypertrophic Cardiomyopathy.

BACKGROUND: Left ventricular outflow tract (LVOT) obstruction is a major determinant of heart failure symptoms in obstructive hypertrophic cardiomyopathy (oHCM). Aficamten, a next-in-class cardiac myosin inhibitor, may lower gradients and improve symptoms in these patients. OBJECTIVES: This study aims to evaluate the safety and efficacy of aficamten in patients with oHCM. METHODS: Patients with oHCM and LVOT gradients ≥30 mm Hg at rest or ≥50 mm Hg with Valsalva were randomized 2:1 to receive aficamten (n = 28) or placebo (n = 13) in 2 dose-finding cohorts. Doses were titrated based on gradients and ejection fraction (EF). Safety and changes in gradient, EF, New York Heart Association functional class, and cardiac biomarkers were assessed over a 10-week treatment period and after a 2-week washout. RESULTS: From baseline to 10 weeks, aficamten reduced gradients at rest (mean difference: -40 ± 27 mm Hg, and -43 ± 37 mm Hg in Cohorts 1 and 2, P = 0.0003 and P = 0.0004 vs placebo, respectively) and with Valsalva (-36 ± 27 mm Hg and -53 ± 44 mm Hg, P = 0.001 and <0.0001 vs placebo, respectively). There were modest reductions in EF (-6% ± 7.5% and -12% ± 5.9%, P = 0.007 and P < 0.0001 vs placebo, respectively). Symptomatic improvement in ≥1 New York Heart Association functional class was observed in 31% on placebo, and 43% and 64% on aficamten in Cohorts 1 and 2, respectively (nonsignificant). With aficamten, N-terminal pro-B-type natriuretic peptide was reduced (62% relative to placebo, P = 0.0002). There were no treatment interruptions and adverse events were similar between treatment arms. CONCLUSIONS: Aficamten resulted in substantial reductions in LVOT gradients with most patients experiencing improvement in biomarkers and symptoms. These results highlight the potential of sarcomere-targeted therapy for treatment of oHCM.

Purification of Pluripotent Stem Cell-Derived Cardiomyocytes Using CRISPR/Cas9-Mediated Integration of Fluorescent Reporters.

Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have become critically important for the detailed study of cardiac development, disease modeling, and drug screening. However, directed differentiation of hiPSCs into cardiomyocytes often results in mixed populations of cardiomyocytes and other cell types, which may confound experiments that require pure populations of cardiomyocytes. Here, we detail the use of a CRISPR/Cas9 genome editing strategy to develop cardiomyocyte-specific reporters that allow for the isolation of hiPSC-derived cardiomyocytes and chamber-specific myocytes. Moreover, we describe a cardiac differentiation protocol to derive cardiomyocytes from hiPSCs, as well as a strategy to use fluorescence-activated cell sorting to isolate pure populations of fluorescently labeled cardiomyocytes for downstream applications.

CRISPR/Cas9-based targeting of fluorescent reporters to human iPSCs to isolate atrial and ventricular-specific cardiomyocytes.

Generating cardiomyocytes (CMs) from human induced pluripotent stem cells (hiPSCs) has represented a significant advance in our ability to model cardiac disease. Current differentiation protocols, however, have limited use due to their production of heterogenous cell populations, primarily consisting of ventricular-like CMs. Here we describe the creation of two chamber-specific reporter hiPSC lines by site-directed genomic integration using CRISPR-Cas9 technology. In the MYL2-tdTomato reporter, the red fluorescent tdTomato was inserted upstream of the 3' untranslated region of the Myosin Light Chain 2 (MYL2) gene in order faithfully label hiPSC-derived ventricular-like CMs while avoiding disruption of endogenous gene expression. Similarly, in the SLN-CFP reporter, Cyan Fluorescent Protein (CFP) was integrated downstream of the coding region of the atrial-specific gene, Sarcolipin (SLN). Purification of tdTomato+ and CFP+ CMs using flow cytometry coupled with transcriptional and functional characterization validated these genetic tools for their use in the isolation of bona fide ventricular-like and atrial-like CMs, respectively. Finally, we successfully generated a double reporter system allowing for the isolation of both ventricular and atrial CM subtypes within a single hiPSC line. These tools provide a platform for chamber-specific hiPSC-derived CM purification and analysis in the context of atrial- or ventricular-specific disease and therapeutic opportunities.

Cardiogenesis from human embryonic stem cells.

Over the past decade, the ability to culture and differentiate human embryonic stem cells (ESCs) has offered researchers a novel therapeutic that may, for the first time, repair regions of the damaged heart. Studies of cardiac development in lower organisms have led to identification of the transforming growth factor-ß superfamily (eg, activin A and bone morphogenic protein 4) and the Wnt/ß-catenin pathway as key inducers of mesoderm and cardiovascular differentiation. These factors act in a context-specific manner (eg, Wnt/ß-catenin is required initially to form mesoderm but must be antagonized thereafter to make cardiac muscle). Different lines of ESCs produce different levels of agonists and antagonists for these pathways, but with careful optimization, highly enriched populations of immature cardiomyocytes can be generated. These cardiomyocytes survive transplantation to infarcted hearts of experimental animals, where they create new human myocardial tissue and improve heart function. The grafts generated by cell transplantation have been small, however, leading to an exploration of tissue engineering as an alternate strategy. Engineered tissue generated from preparations of human cardiomyocytes survives poorly after transplantation, most likely because of ischemia. Creation of pre-organized vascular networks in the tissue markedly enhances survival, with human capillaries anastomosed to the host coronary circulation. Thus, pathways controlling formation of the human cardiovascular system are emerging, yielding the building blocks for tissue regeneration that may address the root causes of heart failure.

Intrinsic Endocardial Defects Contribute to Hypoplastic Left Heart Syndrome.

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart disease characterized by abnormalities in the left ventricle, associated valves, and ascending aorta. Studies have shown intrinsic myocardial defects but do not sufficiently explain developmental defects in the endocardial-derived cardiac valve, septum, and vasculature. Here, we identify a developmentally impaired endocardial population in HLHS through single-cell RNA profiling of hiPSC-derived endocardium and human fetal heart tissue with an underdeveloped left ventricle. Intrinsic endocardial defects contribute to abnormal endothelial-to-mesenchymal transition, NOTCH signaling, and extracellular matrix organization, key factors in valve formation. Endocardial abnormalities cause reduced cardiomyocyte proliferation and maturation by disrupting fibronectin-integrin signaling, consistent with recently described de novo HLHS mutations associated with abnormal endocardial gene and fibronectin regulation. Together, these results reveal a critical role for endocardium in HLHS etiology and provide a rationale for considering endocardial function in regenerative strategies.

Wnt Activation and Reduced Cell-Cell Contact Synergistically Induce Massive Expansion of Functional Human iPSC-Derived Cardiomyocytes.

Modulating signaling pathways including Wnt and Hippo can induce cardiomyocyte proliferation in vivo. Applying these signaling modulators to human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in vitro can expand CMs modestly (<5-fold). Here, we demonstrate massive expansion of hiPSC-CMs in vitro (i.e., 100- to 250-fold) by glycogen synthase kinase-3ß (GSK-3ß) inhibition using CHIR99021 and concurrent removal of cell-cell contact. We show that GSK-3ß inhibition suppresses CM maturation, while contact removal prevents CMs from cell cycle exit. Remarkably, contact removal enabled 10 to 25 times greater expansion beyond GSK-3ß inhibition alone. Mechanistically, persistent CM proliferation required both LEF/TCF activity and AKT phosphorylation but was independent from yes-associated protein (YAP) signaling. Engineered heart tissues from expanded hiPSC-CMs showed comparable contractility to those from unexpanded hiPSC-CMs, demonstrating uncompromised cellular functionality after expansion. In summary, we uncovered a molecular interplay that enables massive hiPSC-CM expansion for large-scale drug screening and tissue engineering applications.

Risk factors associated with the development of double-inlet ventricle congenital heart disease.

BACKGROUND: Congenital heart disease (CHD) is the most common birth defect group and a significant contributor to neonatal and infant death. CHD with single ventricle anatomy, including hypoplastic left heart syndrome (HLHS), tricuspid atresia (TA), and various double-inlet ventricle (DIV) malformations, is the most complex with the highest mortality. Prenatal risk factors associated with HLHS have been studied, but such data for DIV are lacking. METHODS: We analyzed DIV cases and nonmalformed controls in the National Birth Defects Prevention Study, a case-control, multicenter population-based study of birth defects. Random forest analysis identified potential predictor variables for DIV, which were included in multivariable models to estimate effect magnitude and directionality. RESULTS: Random forest analysis identified pre-pregnancy diabetes, history of maternal insulin use, maternal total lipid intake, paternal race, and intake of several foods and nutrients as potential predictors of DIV. Logistic regression confirmed pre-pregnancy diabetes, maternal insulin use, and paternal race as risk factors for having a child with DIV. Additionally, higher maternal total fat intake was associated with a reduced risk. CONCLUSIONS: Maternal pre-pregnancy diabetes and history of insulin use were associated with an increased risk of having an infant with DIV, while maternal lipid intake had an inverse association. These novel data provide multiple metabolic pathways for investigation to identify better the developmental etiologies of DIV and suggest that public health interventions targeting diabetes prevention and management in women of childbearing age could reduce CHD risk.

Multi-disciplinary evaluation of a 5-month-old with hypertrophic cardiomyopathy related to a functional adrenocortical tumor.

Background Hypertrophic cardiomyopathy (HCM) in childhood is a rare diagnosis, and associations with adrenocortical tumors (ACTs) have been rarely reported in the pediatric literature. Case presentation We present a case of a 5-month-old who presented with HCM and during the evaluation for hypertension was found to have elevated glucocorticoids, mineralocorticoids, androgens and urine metanephrines. During preoperative evaluation, he developed shock followed by cardiogenic collapse requiring extracorporeal membrane oxygenation (ECMO); however, he did not survive. Pathology revealed an ACT with hormone production that contributed to his demise. Conclusions Adrenocortical tumors associated with hypertrophic cardiomyopathy can be life-threatening. We discuss the complex interplay of unrestricted cortical hormone production in the setting of hypertrophic cardiomyopathy that may lead to rapid decline and poor clinical outcomes.

Nkx2.5+ Cardiomyoblasts Contribute to Cardiomyogenesis in the Neonatal Heart.

During normal lifespan, the mammalian heart undergoes limited renewal of cardiomyocytes. While the exact mechanism for this renewal remains unclear, two possibilities have been proposed: differentiated myocyte replication and progenitor/immature cell differentiation. This study aimed to characterize a population of cardiomyocyte precursors in the neonatal heart and to determine their requirement for cardiac development. By tracking the expression of an embryonic Nkx2.5 cardiac enhancer, we identified cardiomyoblasts capable of differentiation into striated cardiomyocytes in vitro. Genome-wide expression profile of neonatal Nkx2.5+ cardiomyoblasts showed the absence of sarcomeric gene and the presence of cardiac transcription factors. To determine the lineage contribution of the Nkx2.5+ cardiomyoblasts, we generated a doxycycline suppressible Cre transgenic mouse under the regulation of the Nkx2.5 enhancer and showed that neonatal Nkx2.5+ cardiomyoblasts mature into cardiomyocytes in vivo. Ablation of neonatal cardiomyoblasts resulted in ventricular hypertrophy and dilation, supporting a functional requirement of the Nkx2.5+ cardiomyoblasts. This study provides direct lineage tracing evidence that a cardiomyoblast population contributes to cardiogenesis in the neonatal heart. The cell population identified here may serve as a promising therapeutic for pediatric cardiac regeneration.