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Toward improved understanding of cardiac development and congenital heart disease: The advent of cardiac organoids

Published:February 23, 2022DOI:https://doi.org/10.1016/j.jtcvs.2022.02.028

      Key Words

      Human cardiac organoid systems hold significant promise for mechanistic studies of early heart morphogenesis and an improved understanding of congenital cardiac disease.
      This Invited Expert Opinion provides a perspective on the following papers: Cell. 2021 Jun 10;184(12):3299-3317.e22. https://doi.org/10.1016/j.cell.2021.04.034. and N Engl J Med. 2021 Aug 26;385(9):847-849. https://doi.org/10.1056/NEJMcibr2108627.
      Cardiac birth defects remain a significant health burden and cause of death in the United States and worldwide, with congenital heart malformations affecting 1% to 2% of live births.
      • van der Linde D.
      • Konings E.E.
      • Slager M.A.
      • Witsenburg M.
      • Helbing W.A.
      • Takkenberg J.J.M.
      • et al.
      Birth prevalence of congenital heart disease worldwide: a systematic review and meta-analysis.
      Surgical advances have promoted longer, healthier lives in those with severe genetic anomalies, but a more complete mechanistic understanding of early cardiac development and congenital structural disease could open doors to minimally invasive or nonsurgical interventions. To achieve such a lofty goal, the fields of developmental cardiology and congenital heart surgery require new models to provide critical insights into biological processes underlying early human cardiac development and disease. The in vitro self-organizing human cardiac organoid (CO) systems, such as those recently reported by Hofbauer and colleagues,
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      represent powerful platforms to study early cardiac morphogenic events and related abnormalities, offering the promise to eventually improve the current standard of care for patients with congenital heart disease via numerous applications (Figure 1).
      Figure thumbnail gr1
      Figure 1Progress and promise of cardiac organoids. Icons represent major cardiac organoid applications. Solid lines represent applications for which cardiac organoid technology is sufficiently progressed for immediate utility, such as variant screening. The dashed lines represent areas of promise, where the current complexity of cardiac organoids has not yet matured to meaningfully probe relevant research questions, such as cardiac structural disease. CRISPR, Clustered regularly interspaced palindromic repeats.

      Methods for CO Generation

      Organoids are 3-dimensional (3D), self-assembled, cellular structures that can recapitulate important spatial and functional relationships in developing and adult mouse and human tissues.
      • Kim J.
      • Koo B.K.
      • Knoblich J.A.
      Human organoids: model systems for human biology and medicine.
      As such, organoids have been increasingly used to model early organ development, physiology, disease, and drug responses in vitro.
      • Kim J.
      • Koo B.K.
      • Knoblich J.A.
      Human organoids: model systems for human biology and medicine.
      Compared with traditional animal models, organoids have the primary advantage of using human cells and having improved ability to interrogate cellular processes, higher experimental throughput, and control over 3D cellular and matrix composition and structural organization. On the other hand, they have a small size due to the absence of perfusable vasculature, possess simplified architecture, and lack complex chemical and physical cues present in vivo. The typical protocols for organoid formation (Figure 2), involve culturing stem cells on a plate, reconstituting them into a single cell suspension, and embedding them in an extracellular matrix analog (eg, Matrigel [Corning Life Sciences], a solubilized basement membrane matrix secreted by mouse sarcoma cells). Over several days, the embedded cells self-aggregate into 3D spheroid structures, followed by differentiation, migration, and formation of solid or luminal structures, depending on the cell types used and modulation of specific signaling pathways.
      • Kim J.
      • Koo B.K.
      • Knoblich J.A.
      Human organoids: model systems for human biology and medicine.
      ,
      • Cakir B.
      • Xiang Y.
      • Tanaka Y.
      • Kural M.H.
      • Parent M.
      • Kang Y.-J.
      • et al.
      Engineering of human brain organoids with a functional vascular-like system.
      Organoids may be constructed out of terminally differentiated cells; however, the use of pluripotent stem cells (PSCs) or their multipotent derivatives uniquely allows studies of early lineage specification and tissue morphogenesis in a controlled, in vitro setting. Recent advances in organoid development have resulted in the successful generation of in vitro models of noncardiac tissues such as the intestines
      • Silva A.C.
      • Matthys O.B.
      • Joy D.A.
      • Kauss M.A.
      • Natarajan V.
      • Lai M.H.
      • et al.
      Co-emergence of cardiac and gut tissues promotes cardiomyocyte maturation within human iPSC-derived organoids.
      and brain.
      • Velasco S.
      • Kedaigle A.J.
      • Simmons S.K.
      • Nash A.
      • Rocha M.
      • Quadrato G.
      • et al.
      Individual brain organoids reproducibly form cell diversity of the human cerebral cortex.
      Self-organized COs mimicking early heart development
      • Silva A.C.
      • Matthys O.B.
      • Joy D.A.
      • Kauss M.A.
      • Natarajan V.
      • Lai M.H.
      • et al.
      Co-emergence of cardiac and gut tissues promotes cardiomyocyte maturation within human iPSC-derived organoids.
      ,
      • Moris N.
      • Anlas K.
      • van den Brink S.C.
      • Alemany A.
      • Schroder J.
      • Ghimire S.
      • et al.
      An in vitro model of early anteroposterior organization during human development.
      • Drakhlis L.
      • Biswanath S.
      • Farr C.M.
      • Lupanow V.
      • Teske J.
      • Ritzenhoff K.
      • et al.
      Human heart-forming organoids recapitulate early heart and foregut development.
      • Rossi G.
      • Broguiere N.
      • Miyamoto M.
      • Boni A.
      • Guiet R.
      • Girgin M.
      • et al.
      Capturing cardiogenesis in gastruloids.
      and chamberogenesis
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      ,
      • Lee J.
      • Sutani A.
      • Kaneko R.
      • Takeuchi J.
      • Sasano T.
      • Khoda T.
      • et al.
      In vitro generation of functional murine heart organoids via FGF4 and extracellular matrix.
      ,
      • Lewis-Israeli Y.R.
      • Wasserman A.H.
      • Gabalski M.A.
      • Volmert B.D.
      • Ming Y.
      • Ball K.A.
      • et al.
      Self-assembling human heart organoids for the modeling of cardiac development and congenital heart disease.
      are some of the latest additions to this rapidly evolving field.
      Figure thumbnail gr2
      Figure 2Schematics of generalized protocol for in vitro organoid formation. Pluripotent stem cells are cultured in standard tissue-culture dishes coated with laminin-based matrix. The cells are dissociated from a dish and reconstituted as a single cell suspension before being embedded at a desired density within an extracellular matrix analog. In the matrix, cells self-aggregate into a spherical organoid structure, whereas soluble factors are used to guide their differentiation and organization.
      In the original and increasingly used formulation of COs, multiple terminally differentiated cardiac cell types are coaxed together to form spheroid-shaped microtissues. For example, the COs made of human induced PSC-derived cardiomyocytes, cardiac fibroblasts, and endothelial cells can permit relatively high-throughput studies of drug responses,
      • Kim J.
      • Koo B.K.
      • Knoblich J.A.
      Human organoids: model systems for human biology and medicine.
      hypoxic insults,
      • Richards D.J.
      • Li Y.
      • Kerr C.M.
      • Yao J.
      • Beeson G.C.
      • Coyle R.C.
      • et al.
      Human cardiac organoids for the modelling of myocardial infarction and drug cardiotoxicity.
      or heterocellular interactions underlying heart diseases.
      • Giacomelli E.
      • Meraviglia V.
      • Campostrini G.
      • Cochrane A.
      • Cao X.
      • van Helden R.W.J.
      • et al.
      Human-iPSC-derived cardiac stromal cells enhance maturation in 3D cardiac microtissues and reveal non-cardiomyocyte contributions to heart disease.
      More complex structures made from multiple spheroid COs have been generated using 3D bioprinting to position and fuse organoids in predefined spatial patterns and study cardiac scarring.
      • Daly A.C.
      • Davidson M.D.
      • Burdick J.A.
      3D bioprinting of high cell-density heterogeneous tissue models through spheroid fusion within self-healing hydrogels.
      Although these methods do not replicate developmental organogenesis, they can recreate the realistic macroscopic architecture of an adult heart and its components, including ventricles, atria, valves, and coronary vasculature.
      • Kupfer M.E.
      • Lin W.H.
      • Ravikumar V.
      • Qiu K.
      • Wang L.
      • Gao L.
      • et al.
      In situ expansion, differentiation, and electromechanical coupling of human cardiac muscle in a 3D bioprinted, chambered organoid.
      Overall, whereas 2-dimensional micropatterning
      • Ma Z.
      • Wang J.
      • Loskill P.
      • Huebsch N.
      • Koo S.
      • Svedlund F.L.
      • et al.
      Self-organizing human cardiac microchambers mediated by geometric confinement.
      and 3D bioprinting
      • Lee A.
      • Hudson A.R.
      • Shiwarski D.J.
      • Tashman J.W.
      • Hinton T.J.
      • Yerneni S.
      • et al.
      3D bioprinting of collagen to rebuild components of the human heart.
      technologies can be used to preform complex organoid structures, they do not recreate dynamic spatiotemporal signals and self-organizing processes that drive early cardiac development and morphogenesis.

      Self-Organizing COs for Modeling Early Cardiac Development

      Recently, several studies have demonstrated generation of submilimeter to millimeter-sized COs where cells are initially mixed with Matrigel, followed by a staged addition of small molecules and growth factors to guide early cardiac differentiation, lineage specification, and formation of stratified ventricular walls and chamber-like lumens. Although the initial efforts to mimic early cardiac development in organoids have been limited to specification and rudimentary patterning of first and second heart fields with mouse PSCs,
      • Andersen P.
      • Tampakakis E.
      • Jimenez D.V.
      • Kannan S.
      • Miyamoto M.
      • Shin H.K.
      • et al.
      Precardiac organoids form two heart fields via Bmp/Wnt signaling.
      recent studies that utilize glycogen synthase kinase-3 inhibition to activate wingless and Int-1 (Wnt) signaling have demonstrated the formation of multilineage COs containing spatially distinct cardiac mesoderm and endodermal, gut-like regions.
      • Silva A.C.
      • Matthys O.B.
      • Joy D.A.
      • Kauss M.A.
      • Natarajan V.
      • Lai M.H.
      • et al.
      Co-emergence of cardiac and gut tissues promotes cardiomyocyte maturation within human iPSC-derived organoids.
      ,
      • Moris N.
      • Anlas K.
      • van den Brink S.C.
      • Alemany A.
      • Schroder J.
      • Ghimire S.
      • et al.
      An in vitro model of early anteroposterior organization during human development.
      ,
      • Rossi G.
      • Broguiere N.
      • Miyamoto M.
      • Boni A.
      • Guiet R.
      • Girgin M.
      • et al.
      Capturing cardiogenesis in gastruloids.
      Under exogenously supplied growth factors (ie, ascorbic acid, basic fibroblast growth factor, and vascular endothelial growth factor) and endogenous heterocellular cues, cardiac lineage cells within these COs undergo early differentiation and morphogenic events, eventually forming linear heart tube-like structures, but not proceeding to generate cardiac chamber.
      • Baar K.
      • Birla R.
      • Boluyt M.O.
      • Borschel G.H.
      • Arruda E.M.
      • Dennis R.G.
      Self-organization of rat cardiac cells into contractile 3-D cardiac tissue.
      A more accurate recapitulation of the interactions between human cardiac mesoderm and foregut endoderm
      • Drakhlis L.
      • Biswanath S.
      • Farr C.M.
      • Lupanow V.
      • Teske J.
      • Ritzenhoff K.
      • et al.
      Human heart-forming organoids recapitulate early heart and foregut development.
      resulted in multilineage organoids containing an outer layer of epicardial cells, cardiomyocytes, smooth muscle cells, and gut epithelial cells. Although this system could allow studies of early cardiac patterning events, including those involving aberrant NKX2 Homeobox 5 (NKX2.5), signaling, it did not result in cardiac chamberogenesis.

      Self-Organizing COs for Modeling Early Chamberogenesis

      Only recently have 2 studies with human
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      ,
      • Lewis-Israeli Y.R.
      • Wasserman A.H.
      • Gabalski M.A.
      • Volmert B.D.
      • Ming Y.
      • Ball K.A.
      • et al.
      Self-assembling human heart organoids for the modeling of cardiac development and congenital heart disease.
      and 1 with mouse
      • Lee J.
      • Sutani A.
      • Kaneko R.
      • Takeuchi J.
      • Sasano T.
      • Khoda T.
      • et al.
      In vitro generation of functional murine heart organoids via FGF4 and extracellular matrix.
      cells been able to create in vitro conditions for the spontaneous generation of chamber-like cavities within COs (Table 1). The cavity formation was induced by manipulating Wnt and/or retinoic acid signaling and did not require recreation of all developmental stages such as formation of the cardiac crescent, linear heart tube, or cardiac looping. Similar to other CO studies, the use of mouse PSCs yielded more advanced cardiac morphogenesis compared with use of human PSCs, with manipulation of extracellular matrix proteins and fibroblast growth factor 4 signaling resulting in the formation of primitive atrial and ventricular chambers.
      • Lee J.
      • Sutani A.
      • Kaneko R.
      • Takeuchi J.
      • Sasano T.
      • Khoda T.
      • et al.
      In vitro generation of functional murine heart organoids via FGF4 and extracellular matrix.
      A more rigorous structural and functional characterization of these COs and improved methods to decrease their variability in shape and cellular composition will be highly instructive for the future generation of developmentally mimetic COs made from human PSCs.
      Table 1Overview of cardiac organoid systems
      SpeciesArchitectureModulated pathwayFindingsAdvantages; DisadvantagesReference
      Organoids with single-cell-type composition
       HumanSpheroid with a microchamberWnt/β-cateninBiophysical cues dictate cell specification in cardiac organoidsStudying mechanical forces in cardiac development;

      Simple structure with 1 microchamber
      • Ma Z.
      • Wang J.
      • Loskill P.
      • Huebsch N.
      • Koo S.
      • Svedlund F.L.
      • et al.
      Self-organizing human cardiac microchambers mediated by geometric confinement.
       HumanSpheroid with a microchamberWntNoradrenaline and oxygen gradients can be used to study ischemiaMature cardiomyocytes relevant for adult disease;

      Simple structure with 1 microchamber
      • Richards D.J.
      • Li Y.
      • Kerr C.M.
      • Yao J.
      • Beeson G.C.
      • Coyle R.C.
      • et al.
      Human cardiac organoids for the modelling of myocardial infarction and drug cardiotoxicity.
      Multilineage organoid systems
       MousePrimitive, multilineage organoid without chambersBMP/WntBMP and Wnt signaling govern first and second heart field specificationCo-existence of both heart fields in single organoids; Use of murine instead of human cells
      • Andersen P.
      • Tampakakis E.
      • Jimenez D.V.
      • Kannan S.
      • Miyamoto M.
      • Shin H.K.
      • et al.
      Precardiac organoids form two heart fields via Bmp/Wnt signaling.
       MouseTube-like structure with cardiac and vascular progenitor cellsWntMouse ESCs can recapitulate key cardiac morphogenic events in vitroHigh fidelity recreation of cardiac crescent and heart tube; Use of murine instead of human cells
      • Rossi G.
      • Broguiere N.
      • Miyamoto M.
      • Boni A.
      • Guiet R.
      • Girgin M.
      • et al.
      Capturing cardiogenesis in gastruloids.
       HumanMulticell type spheroid (ECs, cardiomyocytes, fibroblasts)cAMPIntercellular crosstalk via cAMP signaling improves cardiomyocyte maturationIncreased cardiomyocyte maturation;

      Use of differentiated cells not akin to development
      • Giacomelli E.
      • Meraviglia V.
      • Campostrini G.
      • Cochrane A.
      • Cao X.
      • van Helden R.W.J.
      • et al.
      Human-iPSC-derived cardiac stromal cells enhance maturation in 3D cardiac microtissues and reveal non-cardiomyocyte contributions to heart disease.
       HumanMultilineage spheroid: cardiac, foregut endoderm, and vascularWntNKX2.5 knockout disrupts formation of compact myocardiumMimicking early cardiac development;

      No clear cardiac chamber formation
      • Drakhlis L.
      • Biswanath S.
      • Farr C.M.
      • Lupanow V.
      • Teske J.
      • Ritzenhoff K.
      • et al.
      Human heart-forming organoids recapitulate early heart and foregut development.
      Organoids with spontaneous formation of multiple chambers
       MouseMultilineage organoid with atrial and ventricular mimetic chambersBMP/Wnt, FGF4FGF4 supplementation gives rise to conduction cells, cardiomyocytes, fibroblasts, and ECsCardiac chamberogenesis with multiple resident cells;

      Use of murine but not human cells
      • Lee J.
      • Sutani A.
      • Kaneko R.
      • Takeuchi J.
      • Sasano T.
      • Khoda T.
      • et al.
      In vitro generation of functional murine heart organoids via FGF4 and extracellular matrix.
       HumanMultiple primitive chambersWntModeling effects of gestational diabetes on heart developmentMultiple cardiac cell types and vascularization; rudimentary chambers
      • Lewis-Israeli Y.R.
      • Wasserman A.H.
      • Gabalski M.A.
      • Volmert B.D.
      • Ming Y.
      • Ball K.A.
      • et al.
      Self-assembling human heart organoids for the modeling of cardiac development and congenital heart disease.
       HumanChambered cardiac organoidWnt/BMP/HAND1Requisite need for HAND1 in the cardiac chamber formationFormation of ventricular-like chambers;

      No regeneration despite fetal cardiomyocytes
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      Wnt, Wingless and Int-1; BMP, bone morphogenic protein; ESCs, embryonic stem cells; cAMP, cyclic adenosine monophosphate; FGF4, fibroblast growth factor 4; HAND1, heart and neural crest derivatives expressed 1; ECs, endothelial cells.
      The most advanced method for forming a cavity resembling a primitive left ventricle in human COs has been reported by Hofbauer and colleagues.
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      The authors named these COs “cardioids,” using the term Baar and colleagues
      • Baar K.
      • Birla R.
      • Boluyt M.O.
      • Borschel G.H.
      • Arruda E.M.
      • Dennis R.G.
      Self-organization of rat cardiac cells into contractile 3-D cardiac tissue.
      originally coined to describe self-assembled aligned cylindrical cardiac tissues made from neonatal rat heart cells. Approximately 1 to 2 mm in size, each human cardioid was made starting from 5000 to 7500 PSCs, which, for improved reproducibility, were cultured within individual wells of a commercially available AggreWell (Stem Cell Technologies) dish. Upon induction of cardiac mesoderm, chamber-like cavities rapidly formed in the presence of Wnt, activin, and retinoic acid signals and were maintained in a stable fashion during subsequent cardiomyocyte specification and maturation for up to 3 months. Although exogenously induced and endogenous changes in vascular endothelial growth factor signaling led to partial lining of cavities with endothelial cells, neither endoderm nor endothelium was required for chamber formation. Instead, chambers were formed solely by self-organization of cardiac mesoderm, through a process regulated via the Wnt-bone morphogenic protein-heart and neural crest derivatives expressed 1 (HAND1) signaling axis. Specifically, and similar to cardiac development in vivo, chamber formation was critically dependent on the activity of HAND1, the genetic deletion of which resulted in small, poorly formed COs with rudimentary chambers. These experiments further revealed that cavity formation and cardiomyocyte specification in cardioids occur in parallel but are distinctly regulated. Interestingly, addition of PSC-derived epicardial cell aggregates to pre-formed cardioids generated stable structures in which epicardial cells both spread on the CO surface and, upon migration into the organoid, upregulated endothelial cells and fibroblast markers, thus yielding a more in vivo-like ventricle cell composition.

      Use of Cardioids to Study Injury Response

      The critical difference between fetal and adult mammalian hearts is the lack of proliferative capacity in adult cardiomyocytes, representing the primary barrier to robust regenerative response following myocardial injury.
      • Karra R.
      • Poss K.D.
      Redirecting cardiac growth mechanisms for therapeutic regeneration.
      In contrast, developing fetal hearts consist of cardiomyocytes with significant proliferative and regenerative ability.
      • Sturzu A.C.
      • Rajarajan K.
      • Passer D.
      • Plonowska K.
      • Riley A.
      • Tan T.C.
      • et al.
      Fetal mammalian heart generates a robust compensatory response to cell loss.
      The epicardium-supplemented cardioids by Hofbauer and colleagues
      • Hofbauer P.
      • Jahnel S.M.
      • Papai N.
      • Giesshammer M.
      • Deyett A.
      • Schmidt C.
      • et al.
      Cardioids reveal self-organizing principles of human cardiogenesis.
      did not regenerate after a localized cryoinjury and instead mounted a fibrotic response characterized by fibroblast, fibronectin, and collagen I accumulation at the injury site and no cardiomyocyte proliferation. As in mature hearts, this fibrogenic outcome in cardioids was aided by epicardial cells,
      • Quijada P.
      • Trembley M.A.
      • Small E.M.
      The role of the epicardium during heart development and repair.
      yet the lack of injury-induced proliferation of evidently immature cardiomyocytes is puzzling and suggests either a nonphysiological nature of cryoinjury or requisite roles for additional resident nonmyocytes or systemic influences (eg, immune system) in mounting a robust regenerative response in a fetal heart. The modular nature of cardioids can be used to gain further mechanistic insights into these clinically important phenomena.

      Use of Cardioids for Modeling Developmental Defects

      Although essential roles of various transcription factors and their interactions have been extensively studied in cardiac specification, morphogenesis, function, and disease,
      • Luna-Zurita L.
      • Stirnimann C.U.
      • Glatt S.
      • Kaynak B.L.
      • Thomas S.
      • Baudin F.
      • et al.
      Complex interdependence regulates heterotypic transcription factor distribution and coordinates cardiogenesis.
      genetic manipulations in cardioid systems have revealed new, stage-specific changes in transcriptional hierarchy during early cardiogenesis. Specifically, by generating organoids from human PSCs and induced PSCs with HAND1 or NKX2.5 knockout, the authors determined that, in the cardiac mesoderm stage, HAND1 functions upstream of NKX2.5, whereas in the cardiomyocyte stage it functions downstream of NKX2.5. Because HAND1 and NKX2.5 mutations are known pathogenic drivers of ventricular anomalies, including hypoplastic left heart syndrome, it is conceivable that additional genetic manipulations of relevant transcription factors in cardioids (eg, GATA4, TBX5, MEF2, and SRF) could enhance our understanding of early cardiac morphogenetic defects.
      • Drakhlis L.
      • Biswanath S.
      • Farr C.M.
      • Lupanow V.
      • Teske J.
      • Ritzenhoff K.
      • et al.
      Human heart-forming organoids recapitulate early heart and foregut development.
      ,
      • Lewis-Israeli Y.R.
      • Wasserman A.H.
      • Gabalski M.A.
      • Volmert B.D.
      • Ming Y.
      • Ball K.A.
      • et al.
      Self-assembling human heart organoids for the modeling of cardiac development and congenital heart disease.
      On the other hand, cardioids are primarily made of first heart field derivatives and do not include second heart field or neural crest cells, which play important roles in outflow tract septation, valvulogenesis, and development of the cardiac conduction system.
      • Keyte A.
      • Hutson M.R.
      The neural crest in cardiac congenital anomalies.
      As such, cardioids do not model coemergence of a variety of intra- and extracardiac cell lineages that contribute the formation of linear heart tube, cardiac looping, and patterning of later, more complex cardiac structures (eg, right ventricle, atria, cardiac cushions, valves, Purkinje fibers, inflow/outflow tract, vasculature, and lymphatics
      • Meilhac S.M.
      • Buckingham M.E.
      The deployment of cell lineages that form the mammalian heart.
      ). Because most congenital malformations arise during these later stages of heart development, cardioids will require significant improvements to allow modeling of the common cardiac birth defects. Specifically, improving methods for human PSC differentiation and cardiomyocyte maturation
      • Shadrin I.Y.
      • Allen B.W.
      • Qian Y.
      • Jackman C.P.
      • Carlson A.L.
      • Juhas M.E.
      • et al.
      Cardiopatch platform enables maturation and scale-up of human pluripotent stem cell-derived engineered heart tissues.
      ,
      • Funakoshi S.
      • Fernandes I.
      • Mastikhina O.
      • Wilkinson D.
      • Tran T.
      • Dhahri W.
      • et al.
      Generation of mature compact ventricular cardiomyocytes from human pluripotent stem cells.
      and integration of advanced bioengineering methods
      • Pomeroy J.E.
      • Helfer A.
      • Bursac N.
      Biomaterializing the promise of cardiac tissue engineering.
      for 3D cell culture will be important first steps toward realizing this goal. Nevertheless, in their state-of-the-art form, COs can currently permit systematic studies of early cardiac self-patterning, including dissecting and simultaneously investigating cardiac morphogenetic and specification events.

      Conclusions

      Self-organizing COs derived from human PSCs represent important emerging platforms for studies of early heart development. Despite their structural simplicity, these in vitro systems may hold the key to improved understanding of early cardiac morphogenesis and mechanistic underpinnings of congenital heart disorders, eventually leading to improved patient care.

      Conflict of Interest Statement

      The authors reported no conflicts of interest.
      The Journal policy requires editors and reviewers to disclose conflicts of interest and to decline handling or reviewing manuscripts for which they may have a conflict of interest. The editors and reviewers of this article have no conflicts of interest.

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