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 the 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
Early allocation of cardiac precursors. To the left, a schematic representation of a dorsal view of the 14-day human embryo (equivalent to approximately E7 mouse embryo). The arrows show the trajectories of the cardiac mesoderm during gastrulation from the primitive streak (PS) toward an anterolateral position. The dotted red arrow indicates the sectioning plane corresponding to the scheme on the right, showing the disposition of the embryonic and extraembryonic layers and the nascent celomic cavities, precursor of the pericardial cavity, are shown. The first heart field progenitors occupy the region closer to the extraembryonic membranes and those of the second heart field lie immediately medial to them. Interspersed endothelial precursors are present between the endoderm and the splanchnic mesoderm. EE, extraembryonic region. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
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 the human and avian embryo, in contrast, 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 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).
Formation of the paired primitive heart tubes. Schematic representation of the disposition of the primitive heart tubes in the 18-day human embryo (~E8 mouse embryo). The dotted red arrow indicates the sectioning plane corresponding to the scheme on the right. The first heart field precursors have differentiated into primary cardiomyocytes and folded toward the pericardial cavity to form two hemitubes. The endothelial precursors have formed a continuous endocardial tube along the primitive cardiac tubes. Although the myocardial wall has not closed dorsally, the continuous endocardial layer and the pharyngeal endoderm allow the enclosure of the cardiac lumen. Cardiomyocyte contraction has started in a peristaltic posterior-to-anterior fashion, which produces an incipient flow that starts to shape the peripheral vascular system (dotted gray arrows in left schematic). As a result of embryo folding, the cardiac fields have fused anteromedially and the first heart field–derived cardiac tubes now lie medial to the second heart field precursors, which remain attached to the pharyngeal endoderm. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
Formation of the primitive cardiac tube. The scheme on the left shows the shape and position of the cardiac tube in a 21-day human embryo (E8.5 in the mouse). Primordial ventricular and atrial chambers connecting with the arterial (outflow) and venous (inflow) poles of the linear heart tubes respectively are shown. The red arrows indicate the sectioning planes that are represented on the right. The two initial paired tubes have fused at the midline, forming a single tube that has become independent from the pharyngeal region, except at its inflow and outflow poles. The second heart field now forms a continuous sheet dorsal to the cardiac tube and in continuity with the outflow and inflow tracts. New cardiac precursors are continuously added to the forming heart tube from the second heart field though the arterial and venous poles (red arrowheads). As these precursors leave the pharyngeal region and incorporate to the heart tube, they progressively differentiate and start contracting. Chamber formation has started by increased local proliferation and working myocardium appears with higher contractile capacity and faster conductive ability than the primary myocardium. AVC, atrioventricular canal. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
Molecular Pathways Involved in Cardiac Progenitor Cell Specification
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.
Molecular regulation of cardiomyocyte differentiation from cardiac mesoderm. A pool of cardiac mesodermal precursors is established after gastrulation (left side of the diagram). These progenitors are primed for differentiation to cardiomyocytes but maintained on hold and proliferating by sonic hedgehog (Shh), fibroblast growth factor (FGF), and canonical Wnt (cWnt) signals. Low bone morphogenetic protein (BMP) is also required to maintain and promote the cardiac fate of these precursors. Precursors are recruited to differentiate (right side of the diagram) in the cardiac crescent by high BMP signaling and noncanonical Wnt (ncWnt) signaling. Once the primary tube is established, precursors in the second heart field and cardiomyocytes in the heart tube coexist. Precursors continue proliferating and are progressively added to the forming cardiac tube. As they incorporate to the tube, they transit from high FGF/Shh and cWnt expression to high BMP and ncWnt exposure, which promotes their differentiation and proliferation stop. cWnt and BMP signals counteract each other in part through the activity of Hpx, a transcription factor critical for the transition from the precursor state to differentiation. The transcription factors generally required for cardiac mesoderm establishment and differentiation to the cardiomyocyte lineage are represented. Nkx2.5, Gata, and Mef2c are required in both the precursor and differentiating stages, whereas transcription factors such as Islet or Meis are specific of the precursor stage and Smyd1 and Myocardin of the differentiation phase.
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 outflow tract34; and Tbx1 to the part of the anterior SHF that contributes to the outflow tract. 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 models23,24 During cardiac tube formation, Meis TFs is expressed in undifferentiated cardiac precursors at the SHF and in early differentiating cardiomyocytes of the outflow tract (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 tube42 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 bilaterians 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 outflow tract 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 outflow tract defects in mouse and humans.46,47,48,49
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 large chromatin remodeling complexes activity 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.
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 outflow tract 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
Heart looping, cushion formation, and trabeculation. The scheme on the left represents a frontal view of the cephalic region of a 24-day human embryo (~E9.5 in the mouse). The cardiac tube has increased in length mostly caused by the incorporation of new segments from the second heart field and has looped rightward, allowing the right and left ventricle aligning along the left–right axis. At the same time, the ventricles protrude, becoming positioned ventrally with respect to the atria. Internally (right scheme) the ventricular chambers have started to develop incipient trabeculae, and cushions have formed at the atrioventricular canal and the outflow tract, serving as functional valves at this stage. The aortic sac has established connections with the second branchial arch. Red dotted arrows indicate the trajectories of blood flow through the cardiac tube. AVC, atrioventricular canal; CNC, cardiac neural crest; IFT, inflow tract; LA, left atrium; LV, left ventricle; OFT, outflow tract; PEO, proepicardial organ; RA, right atrium; RV, right ventricle. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
The four-chambered heart. Schemes above show ventral, lateral and dorsal views of the 6-week human heart (~E11.5 in the mouse). The scheme below shows a frontal section along the plane indicated in the lateral view above. At this stage, the four chambers have become aligned in their definitive position. The four chambers have enlarged considerably and ventricles have continued their rotation until being placed caudally to the atria, so that the four chambers are aligned in their definitive positions. Compact and trabecular myocardium are now well established. The atrioventricular canal has split in two, as the atria become separated by the septum primum, so that each ventricle receives blood from its corresponding atrium. The systemic blood now returns only to the right atrium and the left atrium shows the pulmonary vein connections (two at this stage). Despite this defined compartmentalization of the cardiac elements, the left and right heart sides are still communicated. The inflowing blood (blue arrows) enters the left atrium through the ostium primum, a communication between the atria, and then passes into ventricles, which remain in communication, and it is pumped as outflowing blood (red arrows) through the common outflow tract. The outflow tract is still a common arterial trunk and internally contains helicoid cushions that prefigure the future septation. The cardiac neural crest progenitors colonize the cushions and will be essential for outflow tract septation. Epicardium derived from the covers ventricular and atrial chambers. AV, atrioventricular; ICV, inferior cava vein; IFT, inflow tract; LA, left atrium; LCCV, left common cardinal vein; LV, left ventricle; OFT, outflow tract; PEO, proepicardial organ; PVT, pulmonary veins tree; RA, right atrium; RCCV, right common cardinal vein; RV, right ventricle. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
The mature fetal heart. The scheme shows represents the fetal human heart from 12 weeks onward, when the fetal heart shows an anatomy very similar to the adult heart (from ~E16 in the mouse). The right atrium is now fed by the superior and inferior cava veins and the left atrium is fed by the four definitive pulmonary veins. The cushions have differentiated into the main heart valves, which are aligned in a single plane and encased in the annulus fibrosus, a structure also derived from the cushions and with a strong contribution from epicardial-derived cells (EPDCs). The inflow and outflow connections have evolved to their definitive disposition, with complete septation of the outflow common arterial trunk in the aortic and pulmonary trunks. Arterial end venous blood, however, still mixes through the atrial septa communication and the ductus arteriosus, which connects the pulmonary and arterial trunks. These communications will only disappear after birth on breathing of the newborn. In the ventricles, the compact layer has increased notably, trabeculae start compacting, and differentiation of the coronary plexus and colonization by EPDCs and their derivatives is advanced. Blue arrows show inflow blood, and red arrows show outflow blood. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. Modified with permission from Fuster V and Kalil Roberto: Cardiovascular Medicine, the Impact of Diseases. Atheneu; 2016.
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 outflow and inflow tracts (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 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 outflow and inflow tracts (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.
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
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
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 cava veins in to the right atrium (see Fig. 8–7).
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 outflow tract (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 outflow tract, 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 Vascular endothelial growth factor (Vegf) repression 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 outflow tract 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.
Aortic arch development. In the 20-day human embryo (E8 in the mouse), the first pair of pharyngeal arteries (I) have formed and connect the arterial pole of the heart with the paired dorsal aortas (DA). In the 30-day human embryo (~E10 in the mouse), the first arch artery has degenerated, and II, III, and IV are present and connected to the dorsal aortas and to the arterial pole of the heart (not shown) through the aortic sac (AS). The paired dorsal aortas have fused caudally to the heart. By 40 days (~E11 in the mouse), the arteries of the first two arches have regressed and those of arches III to VI are present. The pulmonary arteries (PAs) start sprouting out from the VI arch arteries, which are the precursors of the pulmonary trunk. In the 6-week embryo (E12.5 in the mouse), the right VI pharyngeal artery and the right dorsal aorta are in regression, and the left VI pharyngeal artery degenerate. The left VI pharyngeal arch artery splits from the aortic trunk to form the pulmonary trunk, which remains connected as the ductus arteriosus to the left dorsal aorta. By 7 weeks, the definitive aortic arch has formed with derivatives of the pharyngeal artery III forming the left and right common carotids (LCC and RCC) and those of the IV pharyngeal arteries making small contributions to the right subclavian artery and aortic arch. The left and right subclavian arteries (LSA and RSA) have remodeled their position to become connected, respectively, to the aortic arch and the brachycephalic artery (BCA; derived from the aortic sac). The right dorsal aorta has degenerated at this stage.
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 outflow tract 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. Outflow tract 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 outflow tract abnormalities and/or affection of the proper morphogenesis of semilunar valves. Most aortic arch congenital anomalies are as a result of abnormal retention or absence of the mosaic vascular segments that contribute to its formation.