CHAPTER 8: MOLECULAR AND CELLULAR DEVELOPMENT OF THE HEART

Cardiac development is a fascinating journey that starts early in embryonic development and brings cells from a homogeneous pluripotent state to a highly specialized array of differentiated tissues organized in a complex functional architecture. Heart development has been addressed extensively in monographic texts,^1,2 and here we will focus in summarizing the aspects more relevant to understand its origin, morphogenesis, and physiology. The heart is the first functional organ in the mammalian embryo; however, its full development spans the whole intrauterine period and is finished only in the postnatal period. Therefore, most of the complex morphogenetic events and the cell proliferation, migration, and differentiation processes involved in heart formation take place in the context of a beating heart that provides the circulatory function essential for embryonic, fetal, and postnatal development. Cardiac malformations are the most frequent congenital malformations in humans (around 1%), which correlates with the complexity of generating a four-chambered heart from an initially linear cardiac tube. Although genetic determinants of cardiac malformation are increasingly being discovered, the precise understanding of how specific developmental defects lead to a malformed newborn heart is still a difficult task and will likely demand the development of systems biology approaches capable of modeling cardiac development in a predictive manner. Importantly, during development, cardiomyocytes proliferate extensively. However, this ability is limited in the adult heart and so is cardiac regenerative capacity. The molecular and cellular pathways involved in cardiac development, therefore, have become a paradigm for the design of cardiac regenerative strategies in the adult heart.^3,4 In this chapter, we will address the main aspects of cardiac embryonic development, including the origin of cardiac precursors, the formation of the cardiac crescent, the morphogenesis of the primary heart tube, and the recruitment of additional lineages from the epicardium and the neural crest. We will also discuss the progressive incorporation of second heart field precursors and the differentiation of the different cardiomyocyte subtypes. In addition, we will present current knowledge on the morphogenesis of cardiac chambers, septa, and valves. Finally, we will review the knowledge on cardiomyocyte proliferation, differentiation, and hypertrophy and how the surrounding signaling and cellular environment influences these processes. Our knowledge of cardiac development mainly derives from studies in animal models in which experimental embryology and genetic engineering is accessible. We will describe human cardiac development,⁵ although the knowledge presented here on the mechanisms involved derives from experimental animal models, mainly mouse and chicken, which represent the genetic and embryological models closest to humans.

Allocation and Specification of Cardiac Progenitors during Gastrulation

Cardiac precursors are found shortly after gastrulation (the formation of the three embryonic layers: ectoderm, endoderm, mesoderm) within the mesodermal component of the splanchnopleural layer of the anteriormost lateral plate (Fig. 8–1).^2,6,7 This area is called the cardiogenic area and is formed by early gastrulating embryonic mesoderm. The cardiogenic area is single and crescent-shaped in the mouse and bilaterally paired in human and avian embryos (see Fig. 8–1).² Before gastrulation, the embryonic part of the conceptus consists of a pseudostratified epithelium of pluripotent cells known as the epiblast, which is flat in avian and human embryos and cup-shaped in the mouse. The epiblast epithelium is covered at its basal side by a layer of primitive endoderm (outer in the mouse and underneath in the human and chick). At gastrulation, cells at the central-posterior position in the epiblast undergo epithelial-to-mesenchymal transition (EMT), forming the so-called primitive streak (PS).⁸ Cells recruited to the EMT process at the PS migrate in between the epiblast and the endoderm to form the mesodermal layer and replace most of the primitive endoderm to form the definitive endoderm. PS formation starts posteriorly at the embryonic-extraembryonic interface and progresses in a posterior-to-anterior sequence. The pregastrulation epiblast already shows a large degree of regional specification and fate maps have been established identifying the prospective ectoderm and neuroectoderm, as well as various types of mesodermal compartments: cardiac, blood, vascular, axial paraxial, and lateral mesoderm.⁹ Despite this regional specification and despite the fact that certain signals are already regionalized before gastrulation, epiblast cells remain pluripotent until they are recruited to the PS. Transplantation studies have shown, for example, that epiblast regions fated to produce brain transplanted to the cardiac-forming region will form the heart and vice versa.⁹ These and other studies show that cells acquire their developmental fates depending on the specific position and timing of their transit through the PS.⁸ Therefore, the regional specification of the epiblast does not result from autonomous properties being established in epiblast cells but from the fact that gastrulation is a highly ordered process and thus the position of a cell in the epiblast predicts the timing and position of its ingression through the PS.⁸

As mentioned above, instructive signals are already regionalized in the embryo before and during gastrulation, and accumulating data indicate that the history of external signals cells receive during their journey through gastrulation determines the differentiation pathway they will adopt. The signal history includes not only the identity and intensity of the signals received but also the specific sequence in which these signals target the differentiating cells. The main factor determining an epiblast cell fate is, therefore, the position it occupies in the epiblast, because this will determine its gastrulation schedule and migration route and, thereby, the signal sequence to which it will be exposed.

As cells move away from the PS, they make a 90-degree turn towards an anterior direction, so that the anterior-posterior order of cells in the PS is transformed in mediolateral positions. Cells from the anteriormost PS contribute to axial mesoderm, those at immediately posterior positions contribute to paraxial mesoderm, those that are more posterior contribute to lateral mesoderm, and the most posterior ones colonize the extraembryonic region (see Fig. 8–1). Because gastrulating cells move anteriorly as they exit the PS, the earlier the cells exit the PS, the more they colonize the anterior region. For example, the earliest paraxial mesoderm will contribute to the head region and then, in a temporal sequence, to cervical, lumbar, sacral, and caudal somites. At the lateral plate level, the first mesodermal cells produced will form the septum transversum and heart, then forelimbs, other viscera, hind limbs, and the genitalia. Except for the extraembryonic mesoderm, the cardiac and hepatic mesoderm is thus the first mesoderm being produced in the embryo. In the mouse, the cardiac mesoderm first colonizes the rim between the head folds and the extraembryonic region initially lying at the most anteriolateral embryonic region, forming horseshoe-shaped primordium. In contrast, in the human and avian embryo, two cardiac primordia are formed bilaterally without continuity across the anterior midline (see Fig. 8–1).

Experiments in zebrafish at the establishment of the cardiac fields¹⁰ have shown that retinoic acid signaling forms a gradient from posterior to anterior. This gradient establishes the border between the cardiac and forelimb fields through the regulation of Hox transcription factors and promotes the atrial versus the ventricular fate.

Soon after gastrulation starts, Hensen’s node is established at the anteriormost end of the PS. Oriented movements of Hensen’s node cell cilia establish left-right patterning by localizing signaling molecules essential to start specific left-right asymmetry molecular program.¹¹ This left-right asymmetry program promotes the specific location and left-right orientation of internal organs, including the heart.

Formation of the Primary Heart Tube and Establishment of the Second Heart Field

The outermost rim of the cardiac mesoderm closer to the extraembryonic region is the first region to show signs of cardiomyocyte differentiation and is known as first heart field (FHF) (see Fig. 8–1), which, at this stage, is arranged in the mouse in a crescent shape and thus named cardiac crescent.⁷ As part of the general embryonic folding process that brings the endoderm to the inside of the embryo, the heart precursors are brought to their definitive position posterior and ventral to the head (Fig. 8–2). During these movements, the heart forming regions are always in close contact with the pharyngeal endoderm, being definitely placed ventrally to the foregut pocket (see Fig. 8–2). The rest of mesodermal cardiac precursors positioned posteromedially and immediately adjacent to the cardiac crescent in the splanchnopleura are known as the second heart field (SHF) (see Fig. 8–2).^12,13,14,15 The FHF gives rise to posterior heart structures, including the left ventricle and most of the atria. The contribution of FHF to the heart tube takes place “all-at-once” by simultaneous folding, fusion, and remodeling of the splanchnopleural mesoderm, but the SHF is maintained as a pool of undifferentiated proliferating cardiac precursors for about 2 days, during which it progressively contributes cardiac tissue for (1) the sequential generation of the right ventricle and outflow tract (OFT) at the anterior pole and (2) part of the atria and inflow tract at the posterior pole (Fig. 8–3).^7,16,17 In the human and avian embryos, the initial cardiac fields occupy a paired bilateral position and do not span across the midline anterior to the head-forming region. Formation of the primary heart tube in these species thus involves the fusion of two primordial tubes initially formed bilaterally (see Fig. 8–2).^5,18 In the mouse, as mentioned above, the first cardiac cell differentiation takes place in a cardiac crescent already continuous across the midline and therefore does not require the same morphogenetic movements that take place in the human and avian embryos.² By the first signs of contractility, the cardiac tube is only a fold of the splanchnic mesodermal layer that is not closed dorsally (see Fig. 8–2). Endocardial cells beneath this fold, however, soon form a sealed tube enclosed between the endoderm and the primary cardiac tube and are therefore capable of circulating its contents as soon as contractions appear. With the closure of its dorsal aspect, the primary tube derived from the FHF is finished around 18 days in the human embryo (E8.0 in the mouse) (see Fig. 8–3).⁵ At this stage, it is composed by the primordium of the left ventricle, attached by short outflow and inflow tracts to the SHF progenitors located in the pharyngeal mesoderm (see Fig. 8–3).

Molecular Pathways Involved in Cardiac Progenitor Cell Specification

Signals

As mentioned above, the acquisition of cardiac fate takes place during gastrulation and is governed by the sequential exposure of initially naive epiblast precursors to a number of signals.^6,19,20 This sequence of signals in turn starts the hierarchical activation of epigenetic modifications and a set of transcription factors whose activity leads to the activation of cardiac differentiation genes.^6,20,21 Interestingly, human and mouse embryonic stem cells can be driven to cardiac differentiation on sequential exposure to the same molecular pathways involved in cardiac specification during embryonic development.^22,23,24 The first signals that the prospective cardiac cells are exposed to are the transforming growth factor (TGF) family molecule Nodal and canonical Wnt (cWnt). Both Nodal and cWnt signals show a graded distribution across the epiblast and high Nodal/cWnt signal at the posteroproximal region of the epiblast results in the promotion of gastrulation and cardiac mesodermal fate.^8,21 In particular, several cWnt ligands are specifically expressed in the PS and drive its formation. Prospective cardiac mesoderm thus experiences transient cWnt signaling as it moves through the PS.

As cardiac mesoderm is produced, it migrates from the primitive streak toward the anterior pole of the embryo in close proximity to the extraembryonic tissues (see Fig. 8–1), which produce high levels of bone morphogenetic protein 4 (BMP-4), and in contact with endoderm, which produces BMP-2.⁶ As the nascent mesoderm is subdivided in the somatic and splanchnic layers (see Fig. 8–1), BMP signaling stimulates the rapid differentiation of the splanchnopleural rim close to the extraembryonic region toward the cardiomyocyte lineage, forming the cardiac crescent in the mouse and the bilateral cardiac primordial in the human and chick. At the cardiac crescent stage, Sonic hedgehog (Shh) produced from the endoderm and notochord also contribute to promote the cardiac fate. cWnt signaling, initially required for early cardiac mesoderm specification, later needs to be repressed during cardiomyocyte differentiation.^8,21 Once the cardiac crescent is in place, cWnt ligands are produced by the neuroectoderm and the somatic mesoderm and restrict cardiac differentiation. After the primitive heart tube has formed (see Fig. 8–3), the SHF serves both as a reservoir of undifferentiated cardiac progenitors and as the source of new differentiating cardiomyocytes that will be added to both poles of the heart tube.¹⁹ At this stage, equilibrium between cardiac progenitor proliferation and differentiation toward cardiomyocyte is essential for proper heart formation (Fig. 8–4). Excessive differentiation may lead to premature exhaustion of cardiac SHF progenitors, whereas insufficient cardiomyocyte production could lead to accumulation of precursors, both resulting in failure to extend the primary heart tube. In the SHF, cWnt signaling, Shh, and fibroblast growth factor (FGF) promote cardiac progenitor proliferation and restrict differentiation (see Fig. 8–4). In contrast, noncanonical Wnt signaling (ncWnt) signaling, involved in cytoskeletal reorganization, is activated and required for cardiomyocyte differentiation along with BMP. As cells are added to the cardiac tube, high BMP signaling and ncWnt signaling promote cardiomyocyte differentiation and slower proliferation.^21,25 The graded regulation of BMP signaling is essential in this context, given that BMP activity is also required in the SHF and is promoted here by FGF. This fine regulation is achieved by cWnt signaling in the SHF, which represses BMP, so that only moderate levels of BMP unable to drive differentiation are allowed in the SHF. In addition, Notch signaling is essential to counteract cWnt function in the SHF, thereby limiting the expansion of progenitors in the SHF and promoting their differentiation²⁶ (see Fig. 8–4). Thus, the equilibrium between BMPs/ncWnt/Notch signals, which promote cardiomyocyte differentiation, and FGF/cWnt/Shh signals, which repress cardiomyocyte differentiation and promote cardiac progenitor proliferation, is essential for proper cardiac tube formation (see Fig. 8–4). As cardiogenesis proceeds, the specific tissues producing BMP and cWnt signaling and the specific ligands involved change accordingly with the new tissue disposition, but the effects on cardiomyocyte differentiation are maintained throughout cardiogenesis.

Transcription Factors

The transcription factors (TFs) activated during the transit between the pluripotent epiblast cell and the differentiated fetal cardiomyocyte are in charge of regulating multiple genes across the genome to achieve the correct transcriptional profile typical of each stage.^20,27 The first sign of cardiac progenitor specification is the activation of the TF eomesodermin in the posterior PS.²⁸ Eomesodermin in turn activates Mesp TFs, which are not cardiac-specific and are not expressed in the cardiac crescent, but are essential for the activation of the cardiac specification program.²⁹ Shortly after gastrulation, as cells become allocated to the cardiac crescent, a set of TFs essential for cardiac specification appear (see Fig. 8–4). Some of these, such as Gata4, Nkx2.5, Mef2c, and Islet1, are expressed by most cardiac precursors in the FHF and SHF,^30,31,32 whereas others are restricted to regions contributing to specific structures of the heart. Tbx5 is restricted to the FHF or posterior tube, including the left ventricle and the atria³³; Hand2, to all anterior SHF derivatives, including the right ventricle and OFT³⁴; and Tbx1 to the part of the anterior SHF that contributes to the OFT. Tbx18 defines the posteriormost subpopulation of the SHF, contributing only to cava veins myocardium.^19,35,36 In addition, Pitx2c is expressed in the left side of the SHF, as part of its general role in left-right patterning of lateral plate derivatives in the embryo.³⁷

The specific combination of TFs expressed by each region of the cardiac fields is important for the proper generation and differentiation of the different parts of the heart tube, although the exact cellular functions of these factors is not completely understood. Interestingly, some of the factors, such as Tbx1, Meis1, Meis2, and Islet1, are expressed at the precursor stage but are downregulated as they incorporate to the heart tube and differentiate, being therefore specifically associated with cardiac precursor properties. Others, such as Nkx2.5, Mef2c, and Gata4, are expressed in both the precursors and all the differentiated cardiac cardiomyocyte lineages. Nkx2.5 in cooperation with Mef2c provides functions important both for the maintenance of the undifferentiated precursor population and for their differentiation to cardiomyocytes.³⁸ Nkx2.5 loss of function leads to cardiac tube truncation caused by the premature differentiation of the whole cardiac precursor pool toward the cardiomyocyte fate. However, proper cardiomyocyte differentiation is also blocked in the heart tube of Nxk2.5 mutant mice.³⁹

These results indicate that Nkx2.5 activity in the SHF needs to be selectively modulated to avoid the premature activation of its functions in cardiomyocyte differentiation. In part, this is achieved by the counteraction of Nkx2.5 by Islet1, which competes for binding to the same DNA regulatory sequences in key cardiac differentiation enhancers.⁴⁰ In cardiac precursors, in which both are expressed, the Nkx2.5 functions related to cardiomyocyte differentiation are repressed by Islet1, whereas Islet1 downregulation, as precursors incorporate to the heart tube, releases Nkx2.5 activity in promoting differentiation. In addition, Meis TFs have been identified as important hubs of the transcriptional control of cardiomyogenesis in embryonic stem cell differentiation models.^23,24 During cardiac tube formation, Meis TF is expressed in undifferentiated cardiac precursors at the SHF and in early differentiating cardiomyocytes of the OFT, where it competes with Nkx2.5 for DNA-binding sites in cardiac target genes.⁴¹

As mentioned above, the balance between activators and repressors of cardiomyocyte differentiation is controlled by the signals that control cardiac precursor maintenance versus those that promote cardiac differentiation (see Fig. 8–4). The TF Hopx is an essential regulator of the transition of cardiac precursors to differentiation as they are added to the heart tube.⁴² Hopx cooperates with BMP signaling to block cWnt signaling and promote cardiomyocyte differentiation. Expression of the TFs Smyd1⁴³ and Myocd⁴⁴ is then activated and required for the cardiomyocyte differentiation program (see Fig. 8–4).

Another important patterning system relevant in cardiac development is the Hox-TALE homeodomain transcription network. Hox TFs play essential and highly conserved roles in embryo patterning, being responsible for the specification of identities along the main embryonic axis. This role is conserved in metazoans and affects only the conserved bilaterian structures lying posterior to the midbrain-hindbrain junction. Cardiac mesoderm lies mostly anterior to this junction. However, areas of the SHF fated to the pulmonary branch of the OFT and some areas of the atria have been shown to derive from precursors of the most anteriorly expressed Hox genes.⁴⁵ In addition, Hox genes regulate cardiac neural crest specification, which is required for proper arterial trunk septation. Furthermore, mutations affecting the TALE homeodomain TFs of the Pbx and Meis families, essential cofactors of Hox proteins, produce OFT defects in mouse and humans.^46,47,48,49

Chromatin Regulation

Chromatin structure regulation is essential for filtering the accessibility of TFs to their regulated genes as well as to stably maintain the active or inactive states of the regulated genes. Chromatin structure is regulated at various levels by several large epigenetic regulatory complexes.⁵⁰ Key aspects of epigenetic regulation are histone modifications and nucleosome positioning, which determine the packaging degree of chromatin and the accessibility of regulatory elements in the DNA. Histone acetylation status represents a major level of chromatin regulation affecting transcriptional activation events. Whereas histone acetylation by the general transcriptional coactivators p300 and CBP promotes transcriptional activation, histone deacetylation by histone deacetylases (HDACs) leads to gene repression. During cardiac development, p300/CBP association with cardiac TFs is essential to direct histone acetylation to cardiac genes, while various associations of HDACs with individual factors, such as Hopx and Smyd1, or within large chromatin remodeling complexes, such as NurD, result in the essential repression of noncardiac genes.^51,52 In addition, the SWI/SNF (or BAF) remodeling complex also interacts with the cardiac TFs to reposition nucleosomes at gene regulatory elements and promote their activation.^53,54 Overexpression of the chromatin remodeling factor Baf60c together with the TFs Gata4 and Tbx5 has been shown sufficient to drive cardiomyocyte specification and differentiation in the mouse embryo.⁵³ A further level of chromatin packaging regulation is that exerted by the polycomb complexes PRC1 and PRC2, which promote histone methylation repressive marks and heritable silent chromatin states. Elimination of the catalytic subunit of PRC2, Ezh2, again results in defective cardiogenesis caused by the undesired expression of genes that should be repressed during cardiac differentiation.⁵⁵

As it is the case for TFs, the basis for the specificity of activity of large chromatin remodeling complexes during cardiogenesis remains largely unknown. An interesting observation in this context is the fact that the chromatin regulation complexes NurD, SWI/SNF, PRC1, and PRC2 incorporate tissue-specific variants for some of their components. Interestingly, the tissue-specific PRC1 factor Mel18 regulates a cardiac mesoderm differentiation program, showing that lineage-specific variants of chromatin complexes are essential in early specification of mesodermal precursors.⁵⁶ Additional specificity would be further gained through the interactions of the chromatin regulation complexes with the cardiac-specific TFs; however, the complexity of the interactions between TFs and chromatin regulation and how these are translated in the precise regulation of cardiac differentiation will require further investigation.

Cardiac Morphogenesis

Between 3.5 and 7 weeks of development (E8.5 to E11.5 in the mouse), the heart undergoes extensive growth and morphological modifications, leading to the formation of a partially septated four-chambered heart equipped with a set of primitive valves (Figs. 8–5 and 8–6). Subsequent OFT subdivision and complete interventricular and atrial septation lead, around the 12th week (~E16.5 in the mouse), to a heart bearing the gross morphological organization of a definitive adult heart (Fig. 8–7).⁵

Chamber Formation

The first stage of this process is chamber formation. The initial heart tube is composed of a primary myocardium that shows poor contractility and low conduction velocity. Interiorly, the primitive cardiac tube is lined by the endocardium, which is separated from the myocardium by a mass of cardiac jelly (see Fig. 8–2). The cardiac jelly is a gelatinous acellular material secreted by cardiomyocytes and formed mainly by a network of collagen fibrils. Chamber formation is first detected by the ballooning of the primary heart tube. The first discernable chamber is the left ventricle, initially located in a central posterior position of the linear tube, between the OFT and inflow tract (see Fig. 8–3). This stage is transitory as, immediately after, heart looping takes place. During heart looping, the cardiac tube undergoes a dextral bending that positions the right ventricle primordium in its definitive position with respect to the left ventricle (see Fig. 8–5). Heart looping is concomitant with continuous chamber growth and progressive addition of the right ventricle primordium at the anterior pole and the atria primordia at the posterior pole. Genetic lineage tracing, expression analyses, and four-dimensional proliferation maps have shown that chamber formation takes place by the hyperproliferation of discrete regions leading to local ballooning of the linear tube walls.^15,57 The forming chamber myocardium progressively acquires fast conduction and high contractility and increasingly differentiated sarcomeric structures. In contrast, the nonchamber myocardium retains immature features and, after extensive repositioning of the chambers, contributes to form the base of the ventricles and the atrioventricular (AV) valves as well as the OFT and inflow tract (see Fig. 8–5). In addition, cardiac jelly is excluded from the forming chambers but remains at the outflow and AV canal areas, where it exerts a resistance to blood flow that prevents blood reflux until proper valves are formed (see Fig. 8–3).⁵

The delimitation of the chamber-forming regions within the cardiac tube is governed by a transcriptional network with a predominant role for the T-box family factors Tbx2, 3, and 20.^6,35 The first signs of chamber initiation are an increase in local proliferation and activation of the atrial natriuretic peptide–coding gene Nppa.²⁰ Tbx20 is expressed throughout the heart tube and is essential for chamber formation. In contrast, Tbx2 and Tbx3 are both expressed specifically in the nonchamber myocardium and are needed to maintain the primitive myocardial character and repress chamber formation in these regions. Tbx2/3 are transcriptional repressors themselves and may also recruit HDACs to promote repressed chromatin states in chamber-promoting genes. Sustained high levels of BMP signaling are responsible for the maintenance of Tbx2/3 expression in the nonchamber myocardium, whereas Tbx20 repression of BMP signaling excludes Tbx2/3 from the chamber-forming regions, which allows the development of the working myocardium in these areas.⁶ A number of additional TFs such as those of the Iroquois family or Hand1 show chamber-specific expression and are likely involved in the establishment of chamber-specific differentiation and morphogenetic programs. The mechanisms by which the chamber-forming regions become specified in discrete regions of the initial cardiac tube, however, remain unknown.

Trabeculation

An essential process during chamber formation is trabeculation.⁵⁸ Trabeculae are internal protrusions arising from the myocardial wall of the chambers and are specially abundant and complex in ventricles. From E9.5 until E14.5 in the mouse, a subset of the compact layer cardiomyocytes migrates to form ridges that evolve into myocardial projections toward the chamber lumen, giving rise to the trabeculated myocardium. Trabeculae increase cardiac output and permit myocardium oxygenation prior to coronary vascularization without increasing heart size. Several signaling pathways, such as BMP, Notch, and Neuregulin are involved in trabecule development through reciprocal signaling between endocardium and myocardium.^21,59 Trabeculae are transient structures that get incorporated to the compact myocardium as fetal development progresses. Alterations in developmental signals such as those in the Notch pathway lead to congenital ventricular noncompaction, a condition that may evolve to malignant arrhythmias and heart failure.^60,61

Septation

Establishment of the definitive cardiac anatomy requires division of the left and right aspects of the heart by septation.⁶² Septation defects are the most common congenital heart malformations in humans.^63,64 Atrial and ventricular septation involves proliferation and ingrowth of the interchamber myocardium for the separation of left and right heart sides (see Fig. 8–6). Ventricular septation is a relatively early and simple process, which is completed around the 7th week (E14.5 in the mouse). Ventricular septation is produced by the ingrowth of the myocardial wall at the interface between the right and left ventricles and is finished by the adjoining of the myocardial septum with the mesenchymal areas continuous with the valve-forming areas, forming the so-called membranous part of the septum (see Fig. 8–7).

In contrast, atrial septation involves the sequential ingrowth of two septa that fuse with a mesenchymal protrusion from the dorsal mesocardium and with the AV valve region to separate left and right atrial sides (see Fig. 8–6).⁶⁵ During atrial septation, communications between the left and right atria maintain left-right circulation. Formation of the septum primum involves the ingrowth of the myocardium to fuse with a dorsal mesocardial protrusion that invades the atria promoted by a Tbx5-Shh pathway.⁶⁶ As the septum primum forms, it leaves a basal region incompletely septated, constituting the foramen primum. Once the foramen primum is closed, a foramen secundum appears above in the septum primum around the 6th week. A septum secundum develops around the 7th week from dorsal to ventral on the right side of the septum primum and containing a third foramen, the foramen ovale. The noncoincident position of the two foramen and the higher flexibility of the septum primum results in a valvular function that allows right to left atrial circulation but not the reverse. This communication closes only after birth with the fusion of the two atrial septa in a single septum.⁶⁵

Venous Pole Development

Atrial septation is coordinated with the remodeling of the venous pole of the heart. Before cardiac septation, the paired embryonic veins (common cardinal, vitelline, and umbilical) drain to the sinus venosus, which opens independently into left and right atria. Before the septum primum develops (between the 4th and 5th week) the sinus venosus has partially incorporated to the atria and has undergone remodeling to drain exclusively into the right atrium, so that left and right common cardinal veins are connected to the right atrium (see Fig. 8–6). Later, the original left sinus is recruited to function as the coronary sinus and the former left cardinal vein constitutes the main coronary vein. In parallel, the left atrium develops the connections to the pulmonary veins. Initially, there is only one connection to a twice-bifurcated tree of pulmonary veins (see Fig. 8–6). The progressive incorporation of the roots of the pulmonary vein tree to the left atrium leads to the direct connection of four definitive independent veins (originally branches of the primary single vein) (see Fig. 8–7). In the definitive heart, all systemic return circulation drains through the superior and inferior vena cava in to the right atrium (see Fig. 8–7).

Valvulogenesis

Valves are essential to ensure unidirectional blood flow and form at the AV regions, connecting atria with ventricles, and at the base of the pulmonary and aortic trunks of the OFT (see Fig. 8–7). Valves derive from the original cardiac cushions, which are in turn derivatives of the original cardiac jelly, which gets shaped by morphogenetic processes and becomes populated by mesenchymal cells from various sources.⁶⁷ The OFT cushions are the primordium of the semilunar (aortic and pulmonary) valves, whereas the AV cushions are the primordium of the membranous part of the ventricular septum and the mitral and tricuspid valves with their associated connective tissues (see Fig. 8–6). Endocardial cushions are essential during early heart development, because they functionally divide aortic and pulmonary blood flows and chambers when valves and septa are not yet present.⁶ In both, the AV canal and the, cushions are populated by EMT of endocardial cells, in a process that is stimulated by BMP and TGFβ signaling from the myocardium and Notch signaling in the endocardium.⁶⁷ These signals are reinforced by a feedback loop with Tbx2, which also induces the expression of genes required for cardiac jelly deposition and endocardial cell migration.^68,69 The initial endothelial EMT (endocardial) phase requires nuclear factor of activated T cells (Nfat)-mediated repression of vascular endothelial growth factor (Vegf) in the myocardium. During a second phase, strong myocardial-produced Vegf stops endoEMT. Finally, calcineurin/Nfatc activity in the endocardium blocks Vegf and promotes valve differentiation.⁷⁰

The extent to which EMT contributes to AV and OFT cushions is different; while most of the AV cushion mesenchyme derives from endoEMT, most of the OFT cushion derives from pharyngeal mesoderm. Specific additional mesenchymal populations produced by EMT at later stages contribute to each of these two regions. In the region of the AV canal, a third mesenchymal population derives, again by EMT, from the epicardium (see below).

Before septation, additional intercalated cushions appear in the OFT and lateral cushions in the AV canal.⁶² During septation, the cushions split between the left and right sides to contribute to the three leaflets of semilunar and tricuspid valves and two of the mitral valve. During maturation, valves elongate and are thinned by proliferation of mesenchymal cells at the growing edge and apoptosis at the base of the cushion (see Fig. 8–7).⁶²

Arterial Pole and Aortic Arch Development

When the heart tube forms, the endocardial layer is already connected to a preexisting plexus of endothelial cells generated by vasculogenesis in the yolk sac and aortic regions. As soon as the heart tube starts to contract, flow across the nascent plexus drives angiogenic remodeling and maturation of the embryonic circulatory system. Aortic arch development involves the sequential development and then involution of five aortic arch artery pairs, corresponding to branchial arches I to IV and VI; the V pair is vestigial (Fig. 8–8). The arterial pole is initially connected to the dorsal aortas by the first pair of pharyngeal arteries. As development progresses, the aortic sac is connected to progressively more posterior arches by new arterial pairs and gets disconnected from the anterior ones, which degenerate.¹⁹ Although the first two pharyngeal artery pairs and the right VI artery regress and do not produce any definitive contribution, after extensive remodeling, the left VI artery forms the pulmonary trunk and pulmonary arteries, the III pair forms the common carotid arteries, and the IV makes small contributions to the right subclavian artery and aortic arch (see Fig. 8–8). The pulmonary trunk remains attached to the aorta through the ductus arteriosus, which closes and involutes at birth, when the left and right side blood pressures change upon the onset of pulmonary function. Recent evidence supports a role for hemodynamic shear stress–responsive genes in the regulation of angiogenesis and aortic arch remodeling in the early embryo.

The arterial pole receives an essential third mesenchymal population deriving from the neural crest, which delaminates by EMT from the neural tube (see Fig. 8–4). This neural crest–derived population is essential for the septation of the initial OFT into the aortic and pulmonary trunks (see Fig. 8–5).⁷¹ However, it represents a transient population that undergoes cell death and does not contribute significantly to the definitive valve structures. OFT septation takes place in a distal-to-proximal progression and in a helicoid trajectory, resulting in the typical definitive arrangement of the pulmonary and aortic arteries (see Fig. 8–6).^72,73 Alterations in the SHF precursors of the arterial pole or in the neural crest migration will result in OFT abnormalities and/or affection of the proper morphogenesis of semilunar valves. Most aortic arch congenital anomalies are a result of abnormal retention or absence of the mosaic vascular segments that contribute to its formation.

The Cardiac Lineages, Origins, and Diversification

The adult heart is composed of cardiomyocytes and several other cell types, including vascular and perivascular cells, interstitial cells, and epicardial and endocardial cells (see Fig. 8–7). Most of these cellular components derive from the initial cardiac fields (see Fig. 8–1), which produce the cardiomyocyte and endocardial cells. However, later contributions from external sources such as the proepicardium and the cardiac neural crest (see Fig. 8–4) are produced after the generation of the primitive heart tube.⁷⁴ Although they are initially similar, cardiomyocytes soon specialize into different types of working myocardium (ventricular, atrial) or conduction system cardiomyocytes, each of which expresses a specific combination of TFs that drive their specific differentiation and functional program.²¹ The molecular aspects of the differentiation programs are orchestrated by networks of specific microRNAs and long noncoding RNAs (lncRNAs) that confer robustness to the molecular pathway choices.⁷⁵ The extent to which these specializations are determined by lineage or by external cues remains largely undescribed,⁷⁴ although hierarchical cardiac cell lineages have been proposed. Interestingly, the SHF progenitors have been shown to behave as pluripotent cardiac progenitors, which are able to generate cardiomyocytes, endocardial cells, smooth muscle cells, and other cardiac mesenchymal cells.⁷⁴

The Epicardium

The largest contribution to the non-cardiomyocyte populations of the heart derives from the epicardium.⁷⁶ The epicardium is the outermost layer of the heart. It derives from an extracardiac structure; the proepicardium, a mass of cells from the coelomic epithelium that forms soon after cardiac looping at the dorsal pericardial wall, caudal to the venous pole of the heart, and protruding to the pericardial cavity (see Fig. 8–5).⁷⁷ The first epicardial cells adhere at the AV canal level and later on extend to cover all the myocardial surface (see Fig. 8–6) with a characteristic “cobblestone” appearance. The epicardium then undergoes a process of EMT and gives rise to epicardial-derived cells (EPDCs) that invade the myocardial layer and differentiate into several cell types within the developing heart (see Fig. 8–7). EPDCs contribute to interstitial mesenchyme, and endothelial⁷⁸ and smooth muscle cells of the coronary vasculature.^79,80 Contribution of the epicardium to the cardiomyocyte population has also been reported,^81,82,83 although this remains a controversial issue.^84,85

Epicardial cells are heterogeneous and are composed of at least two proepicardial compartments containing progenitors with diverse differentiation potential. Although epicardial cells positive for the TFs Wt1 and Tbx18 give rise to mesenchymal and smooth muscle cells, those expressing the TF Scleraxis and the secreted factor Sema3a produce coronary endothelium and endocardium.^81,82,83,86 Whether these contributions derive from prespecified endothelial cells present in the proepicardium or from multipotent epicardial precursors is still debated. In addition to this heterogeneity established at the proepicardial organ, the epicardium receives a third population originated from hematopoietic precursors.⁸⁷ Starting at E12.5 in the mouse, cells expressing the hematopoietic marker CD45 and originated from the Vav1⁺ lineage (which includes embryonic hematopoietic precursors) are observed in the subepicardium and epicardium. In the first days after birth, these cells form epicardial clusters enclosed in extracellular matrix.⁸⁷

Following myocardial infarction in the adult mouse heart, both the Wt1 and the CD45+ epicardial cell populations undergo proliferative and invasive activation, giving rise to new vascular and perivascular cells in the myocardium.^87,88 Interestingly, stimulation with thymosin β4 activates epicardial cells to recapitulate an embryonic program and give rise to new vasculature and cardiomyocytes following a myocardial infarction.⁸⁹ These results support the notion that EPDCs in the adult heart can be a source of multipotent progenitor cells. However, treatment with thymosin β4 in a therapeutically relevant temporal window—after the myocardial infarction—does not reprogram epicardial cells into cardiomyocytes.⁹⁰

Cardiomyocyte Proliferation and Terminal Differentiation

The final adult myocardium structure and functionality is acquired through a series of maturational steps. Quantitative three-dimensional reconstruction of cell proliferation patterns has allowed defining two strategies for the generation of cardiomyocytes.^{5,57,91,92,93} Before primitive tube formation, the cardiogenic mesodermal area proliferates very actively. As the FHF cells are added to the tube, they undergo drastic reduction in cell proliferation, in coincidence with their differentiation to cardiomyocytes. For the next few days, the SHF cells, still residing in the pharyngeal area, continue proliferating at a high rate to provide new cardiac precursors that will be added sequentially to both the arterial and venous poles of the heart tube. As SHF cells are added to the heart tube and begin to differentiate into cardiomyocytes, they stop proliferating. The second strategy for cardiomyocyte proliferation starts around the early looping stage and consists of the reactivation of cardiomyocyte cell cycle in the chamber-forming regions of the heart tube. As a result of the sequential incorporation of precursors to the heart, the reactivation of cell proliferation in the prospective chambers overlaps in time with the proliferation of cardiac progenitors still in the SHF (see Fig. 8–3).¹⁵

Trabeculae begin to appear soon after chamber formation starts, and they present specific features that distinguish them from the chamber compact myocardium (see Fig. 8–5).²¹ In part, trabeculae maintain characteristics of the primary myocardium, showing poor sarcomeric maturation and low proliferation. However, they express high levels of connexins and are thought to function as the embryonic chamber conduction system and to generate the bundle branches and the peripheral ventricular conduction system of the adult myocardium. Although arising from a common precursor, trabeculated and compact myocardium show obvious structural and proliferative differences and give rise to the ventricular conduction system and the working myocardium, respectively. During development, trabecular cardiomyocytes drastically reduce proliferation and display poorly developed sarcomeric structures, compared to working myocardium of the compact layer.⁵ After septation of the chambers, the mammalian myocardium continues its maturational program; at birth, the sarcomeric structure is not fully organized and only within early postnatal life does the ventricular muscle acquire its characteristic rod-like shape and mature contractile apparatus.⁹⁴ During the first days after birth, cardiomyocytes still proliferate and are mononucleated, but soon they stop cycling and switch from hyperplasia (growth by increasing the number of cells) to hypertrophy (growth by increase in size of existing cells), coinciding with polyploidization by binucleation (mouse) or endoreplication (human).⁹⁵ This process is accompanied by definitive maturation of the sarcomeric structure and complete silencing of the fetal cardiogenic program.

Therefore, during development, new cardiomyocytes are first generated by migration and differentiation of cardiac precursors and afterward by proliferation of cardiomyocytes. Hence, there is a correlation between morphogenesis and heart proliferation that is further supported by the observation of differential gene expression regulating chamber formation.²⁰ Notably, progenitor recruitment and differentiation is used only during a limited embryonic period and generates a limited number of cardiomyocytes, while the major part of the cardiomyocyte pool generation is achieved by symmetric division.⁹⁶

An important component of the regulation of cardiomyocyte proliferation and differentiation is contributed by surrounding and supporting tissues. During heart growth, both epicardium and endocardium produce important signals for cardiomyocyte proliferation and maturation, including neuregulin from the endocardium and FGF from both endocardium and epicardium.⁹⁷ Moreover, signals from cardiac interstitial mesenchymal cells, mostly EPDCs, are essential to control the transit of cardiomyocytes from proliferation to cell cycle arrest and hypertrophy.⁹⁸

Recent evidence points to some ability of the mammalian heart to generate new cardiomyocytes, which has fueled interest in exploring the extent to which new cardiomyocytes may arise from symmetric division or from differentiation of a reservoir of cardiac precursor cells. A modest but clear proliferation ability has been described in adult human cardiomyocytes using carbon dating.⁹⁹ According to this study, at the end of a human life, close to 50% of the cardiomyocytes derive from cardiomyocytes generated postnatally, although most of these are generated in the first years of life. The methodology used in this study does not allow determin whether the new cardiomyocytes arise from preexisting cardiomyocytes or from a pool of stem/precursor cells. DNA labeling and genetic fate map analyses in the mouse similarly suggest a modest but constant rate of cardiomyocyte proliferation in the adult heart—but no significant cardiomyocyte turnover in homeostatic conditions.^100,101 Interestingly, cardiomyocyte proliferation seems to increase after myocardial infarction around the borders of the infarcted area.^100,101 Recently, a specific population of proliferating cardiomyocytes has been identified in the adult mouse heart.¹⁰² These proliferating cardiomyocytes are small and mononucleated and show activation of the hypoxia-inducible factor (Hif) pathway. Their frequency in the adult heart matches the previously described rates of adult mouse and human cardiomyocyte proliferation.

In contrast to the limited cardiomyocyte renewal ability in adult mammals,^99,101 heart homeostasis is effective throughout life in zebrafish via cardiomyocyte hyperplasia and epicardial signals.¹⁰³ Furthermore, both fish^104,105 and neonatal mammals¹⁰⁶ show a potent regenerative response against various types of cardiac injury. In both zebrafish^107,108 and neonatal mammals,¹⁰⁶ the regenerative response is mediated by symmetric cardiomyocyte proliferation, suggesting that stimulation of the limited endogenous cardiomyocyte proliferation ability might be an interesting strategy in cardiac regeneration.

The Conduction System

Efficient blood pumping depends on the rhythmic sequential contraction of the heart. Specialized myocytes of the cardiac conduction system are essential to coordinate sequential contraction of cardiac atria and ventricles. In the adult heart, contraction propagates from the atria, where it is initiated, to the ventricles, which contract from the apex to the base. Importantly, contraction of the ventricles is delayed in order to permit their complete filling with blood and the entry of the blood in the outflow vessels before a new atrial contraction occurs. A network of specialized cardiomyocytes, collectively referred to as the conduction system, initiates and propagates contraction throughout the myocardium.¹⁰⁹ The sinoatrial node (SAN), at the insertion of the superior vena cava in the right atrium works, as the apical pacemaker, initiating each contraction. A second essential component of the conduction system is the AV node (AVN), which lies at the base of the ventricles, and is in charge of the AV delay and further transmission of the electric impulse through the His bundle, the bundle branches, and the Purkinje fibers (derivatives of the trabecular myocardium) for coordinated and synchronous ventricular contraction.

From the earliest stages of heart tube formation, pacemaker activity is localized to the inflow pole of the heart. Strong expression of the hyperpolarization-activated cation-selective nucleotide-gated channel 4 is associated with the pacemaker activity during all development, and somewhat lower expression is typical of the rest of the conduction system.¹¹⁰ The SAN cardiomyocytes originate from Nkx2.5-negative and Tbx18-positive progenitor cells localized posterior to the primitive heart tube.^111,112 The TF Tbx3 is an essential component of SAN formation, because it activates the pacemaker gene program at the expense of the working myocardium gene program,¹¹³ while both Tbx2 and Tbx 3 cooperate in AVN specification.¹¹⁴ The crucial role of Tbx3 is further confirmed by its ability to reprogram working cardiomyocytes into a pacemaker fate.^113,115

Lineage tracing experiments have shown that the conduction system receives contribution from both the first and second heart fields, and, for example, the SAN is a hybrid derived from both fields,¹¹⁶ indicating that the origin is less important than the adoption of a cardiac conduction differentiation pathway. In fact, “in situ”, relatively late recruitment from ventricular myocytes has been found as a mechanism for the specification of the ventricular cardiac conduction system with the exception of the AVN and His bundle.^116,117 Contrary to working cardiomyocytes, conduction system cardiomyocytes display poorly organized sarcomeres and few mitochondria. During development, cardiomyocytes of the conduction system show very early signs of divergence from working myocardium, displaying a specific gene expression program and lower contractility and proliferation than the working myocardium. Purkinje fibers derive from trabeculae that derive from cardiomyocytes of the compact myocardium, which, as they are recruited, stop proliferation and differentiate as conductive cardiomyocytes. Although the mechanisms by which cardiomyocytes are recruited to the conduction differentiation program are unknown, a network of TFs has been shown essential to confer specific properties to the ventricular conductive cardiomyocytes, including Irx3, Id2, Nkx2.5, and Tbx5. ^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,119} Trabecule formation depends on differential Notch signaling in the inner ventricular wall, as discussed above.

The Coronary Vasculature

The early developing myocardium is a thin layer of a few cells in thickness and is relatively close to the endocardium, where oxygenated blood flows. During development, however, the ventricular walls develop a thick compact layer that sustains the heart contractile work load. Cardiomyocytes use fundamentally aerobic energy production, and therefore a strong demand of oxygen that cannot be supplied from the endocardium is needed as soon as compact myocardium increases its thickness. The coronary vasculature is in charge of supplying the required oxygen flow to the working myocardium and, accordingly, it starts to be functional around midgestation, when ventricular wall thickness increases.

The mature coronary plexus is fed from two main coronaries connected to the base of the aorta and feeds a network of fine and dense capillaries intimately intermingled with cardiomyocytes. The coronary venous system collects the blood from these capillaries and drains it to the right ventricle through the coronary sinus.¹²⁰ The earliest coronary endothelial precursors appear sometime after the heart has been covered by the epicardial epithelium. Initially, they are evident around the AV junctions and in 2 to 3 days they extend to cover both ventricles and atria.^77,120 The coronary precursors initially form an immature plexus that is not connected to the circulation and has not invaded the myocardium, being placed at the subepicardial space, where the future coronary veins will form. In a second step, an endothelial plexus appears at deeper positions within the compact myocardium, where future arteries will form. Formation of this primary plexus requires an FGF-Shh signaling relay in cardiomyocytes.¹²¹ Connection of the coronary plexus to the aorta takes place through the ingrowth of capillaries that develop independently at early stages in the OFT, before the chamber coronary precursors appear. These capillaries initially form multiple small connections that later coalesce into the two ostia that connect the base of the aorta with the stem of the coronary arteries. This process is regulated by the Hif-Vegf axis.¹²² Once the blood flow is established, the coronary plexus remodels into a stereotyped arrangement of hierarchical vessels. Maturation of the vascular plexus involves also the recruitment of perivascular smooth muscle and mesenchymal cells, largely derived from the epicardium.⁷⁷ The origin of the coronary endothelium is very likely from multiple sources. In the avian model, proepicardium transplantation has shown epicardial contribution to coronary vascular endothelium.^78,123 In mammals, genetic lineage tracing revealed no endothelial potential of the epicardial Tbx18 lineage⁸² and only a minor contribution from the Wt1 epicardial lineage.⁸¹ However, the proepicardial Scleraxis and Sema3a lineages do contribute to coronary endothelium and other epicardial derivatives.⁸³ One important source of primary coronary endothelium is the sinus venosus endothelium. Angiogenesis from the sinus venosus promoted by angiopoietin-1 promotes the generation of the primary subepicardial plexus.¹²⁴ Clonal and lineage tracing analyses have shown that at least part of the coronary artery endothelium derives from the sinus venosus and that this contribution might take place by recruitment of the initial subepicardial endothelial plexus to deeper layers of the myocardium.^124,125 Additionally, the ventricular endocardium has been shown to contribute endothelium to coronary arteries by budding of the sac-like digitations of the endocardium between trabeculae.¹²⁶ More recently it has been described that the endocardium functions as a source for de novo created coronary vessels in the inner half of the ventricular chamber during neonatal trabecular compaction.¹²⁷ Therefore, most likely, the coronary endothelium arises from various origins, although the extent to which each of these sources contributes and the exact sequence of the contributions remains incompletely understood.

The coronary vasculature also contains a lymphatic system that develops relative late from two different sources of lymphatic endothelial cells; those derived from venous sprouting and those derived from hemogenic endothelium.^128,129

Finally, coronary development is also important for the innervation pattern of the heart. Sympathetic innervation of the heart originates in the stellate ganglia, which have a neural crest origin. Arterial vascular smooth muscle cells mediate proximal sympathetic axon extension by secretion of artemin,^130,131 neurotrophin 3,^132,133 and endothelins.¹³⁴ Once they arrive at the heart, the sympathetic fibers are directed by the coronary veins and arteries through the production of nerve growth factor by the vascular smooth muscle cells associated to them.¹³⁵

We thank Bradley B. Keller, James B. Hoying, and Roger R. Markwald for their inspiring contribution to previous editions of this chapter.