The pharyngeal mesoderm (PM), located in the head region of the developing embryo, recently triggered renewed interest as the major source of cells contributing to broad regions of the heart as well as to the head musculature. What exactly is PM? In this review, we describe the anatomical and molecular characteristics of this mesodermal population and its relationship to the first and second heart fields in chick and mouse embryos. The regulatory network of transcription factors and signalling molecules that regulate PM development is also discussed. In addition, we summarize recent studies into the evolutionary origins of this tissue and its multipotential contributions to both cardiac and pharyngeal muscle progenitors.
Pharyngeal mesoderm: definition, location, and contribution
Pharyngeal mesoderm (PM) constitutes a subset of head mesoderm that surrounds the developing pharynx and is situated within the core of the pharyngeal arches (also known as branchial arches). Head mesoderm precursors undergo gastrulation in the primitive streak, prior to those of trunk mesoderm.1,2 The PM can be divided into two subdomains: the loosely connected, mesenchymal paraxial mesoderm, located on both sides of the neural tube and notochord (Figure 1A and B), and the medial splanchnic mesoderm, which is maintained as epithelial tissue, although there seems to be no clear border between these two domains (Figure 1A).
The lateral splanchnic mesoderm is located on the ventral side, beneath the floor of the pharynx (Figure 1B). Both paraxial and splanchnic mesoderm cells converge to form the mesodermal core within the pharyngeal arches3 (Figure 1C and D). Hence, the broad definition of the PM should include both paraxial and lateral mesoderm cells surrounding the pharynx (Figure 1, marked in green). Taken together, PM cells contribute to the core of the pharyngeal arches (Figure 1D). These cells give rise to significant parts of the heart and the pharyngeal muscles. PM cells are found in close proximity to the pharyngeal endoderm, ectoderm, and neural crest cells, which tightly influence PM development (Figure 1D; see below).
Studies in both chick and mouse embryos have shown that cardiac progenitor cells populating the cardiac outflow tract and right ventricle are progressively added during heart looping stages by PM cells, and are referred to as the anterior heart field.4–6 In the mouse, the anterior heart field is a subset of the second heart field, which contributes to the outflow tract and right ventricle, and will also contribute a majority of cells to the atria. Thus a subset of PM cells constitutes the second heart field, in contrast to the more lateral splanchnic mesoderm, known as the first heart field (Figure 1A and D, pink colour), which is contiguous with the PM, differentiates earlier, and eventually populates the left ventricle (reviewed in references 7–10).
The secondary heart field in the chick, which also gives rise to the arterial pole of the heart, is also part of the anterior heart field. The secondary heart field is situated slightly caudal to the anterior heart field, and gives rise to the myocardium and smooth muscle of the distal outflow tract.10 The cells added at the arterial and venous poles of the heart are derived from different regions of the mesoderm,11 and whether caudal PM contributes to the venous pole of the heart is currently not clear. Collectively, the anatomical terminology in the chick is such that the secondary heart field is a subdomain of the anterior heart field, which is a subdomain of the PM, which is part of the head mesoderm. In addition, recent studies indicate that PM progenitors contribute to both head muscles (pharyngeal muscles) and parts of the heart.3,4,12–16 Hence, PM cells are critical part of the cardio-craniofacial field during early embryogenesis.
The skeletal myogenic potential of PM cells and their contribution to pharyngeal muscles have long been documented.17 In contrast, the cardiogenic potential of these cells has only been revealed over the last decade (reviewed in references 7–10,18). In keeping with these findings, PM explants dissected from early chick embryos undergo cardiogenesis.19 This study further suggested that signals from the dorsal neural tube (e.g. Wnt1 and Wnt3a) attenuate PM-derived cardiogenesis.19 The in vivo cardiogenic potential of PM was further revealed in chick embryos.3,12,20 Furthermore, considerable overlap in the expression of head muscle markers [e.g. Myf5, Tcf21 (capsulin), Msc (MyoR), Tbx1, Pitx2] and cardiac markers such as Islet1 and Nkx2.5 has been documented in the PM, suggesting that these cells play a dual role in myogenesis and cardiogenesis.3,12,21 Likewise, lineage studies in the mouse demonstrated an overlap in progenitor populations contributing to pharyngeal muscles and second heart field derivatives.3,22–24 Thus, the genetic programme controlling pharyngeal muscles overlaps with that of the PM-derived heart.
Genetic studies in vertebrate embryos have begun to reveal the regulatory network of transcription factors expressed in the PM. The major players in this network include the transcription factors Tbx1, Pitx2, Tcf21 (capsulin), Msc (MyoR), and Isl1 (reviewed in references 8,13–15,25). Although these reviews describe muscle phenotypes resulting from knockout, singly or in combination, of these transcription factors in mouse embryos, exactly how they function in a hierarchical regulatory network to coordinate both myogenesis and cardiogenesis is far from clear.
Pharyngeal muscle heterogeneity
Myogenic programmes that lead to the formation of skeletal muscles vary considerably between the body and head regions. There are ∼60 distinct skeletal muscles in the vertebrate head which control food intake, facial expression, and eye movement; for example, the extraocular muscles (EOMs) move and rotate the eye in a highly coordinated manner; pharyngeal muscles control jaw movement and facial expression, as well as pharyngeal and laryngeal function. Muscles in the neck and tongue are derived from myoblasts originating in the most anterior set of somites located in the trunk (reviewed in reference 26). Importantly, while head mesoderm gives rise to all muscles of the head, the PM only contributes to subsets of these muscles. For example, EOMs derive from non-PM cells (e.g. prechordal mesoderm26) but it is not clear whether PM contributes in any way to EOMs.
Recent studies have begun to uncover an unexpected heterogeneity in head muscles with respect to their origins, genetic lineages, and transcriptional programmes, as well as their proliferative, differentiative, and regenerative properties.24,27,28 Heterogeneity in skeletal muscles can also be seen during adulthood, as reflected in distinct genetic signatures, and susceptibilities to muscle myopathies of both head and trunk skeletal muscles.29,30
Lineage mapping of head musculature in the mouse revealed genetic heterogeneity.24 This lineage map corroborates, at the cellular level, our understanding of craniofacial muscle phenotypes seen in several recent loss-of-function studies in mice. A complementary view of craniofacial muscle formation in mice, based on dissection of genetic programmes promoting myogenesis in distinct head muscles (e.g. EOMs vs. pharyngeal muscles) was demonstrated by the Tajbakhsh laboratory.27 Pharyngeal muscle progenitors are regulated by a combination of Tbx1 and Myf5 acting upstream of MyoD, as Tbx1:Myf5 doubled mutants lack these muscles completely, while EOMs in these mutants are spared. In contrast, EOMs are selectively lost in another genetic combination, the Myf5:Mrf4 double mutant, although other head and trunk muscles are present in the same double mutant.27 These observations, in addition to other mutant analyses (see below), suggest that the underlying mechanism that leads to selected myopathies may have an ontological aetiology.
Along theses same lines, Capsulin and MyoR act as upstream regulators of pharyngeal muscle development. In Capsulin/MyoR double mutants, the masseter, pterygoid, and temporalis muscles are missing, while lower jaw muscles (e.g. anterior digastric and mylohyoid), EOMs and tongue muscles are not affected.31 Similarly, pharyngeal muscles in Tbx1 mutants are frequently hypoplastic and asymmetric, whereas EOMs and tongue muscles are spared.32 Genetic lineage analyses of head musculature24 indicated that EOMs and tongue muscles derive from lineages distinct from those of pharyngeal muscles. In Pitx2 mutants, the EOMs and first arch muscles are affected22,33; first arch myoblasts marked by the Mef2c AHF-Cre lineage in the mouse16,23 were fewer in Pitx2 mutant embryos.22
Isl1 lineage-derived PM cells contribute to a broad range of cardiovascular and skeletal muscle progenitors
The LIM-homeodomain protein Islet1 (Isl1) is expressed in and required for a broad subset of cardiac progenitors in the mouse.34–37 Gene expression and lineage experiments in the chick have revealed that the core of the pharyngeal arch is divided along the proximal–distal axis such that paraxial mesoderm cells mainly contribute to the proximal region of the core, while the splanchnic mesoderm contributes to its distal region3 (Figure 2A–C). Isl1 is expressed in the distal part of the PM, and its expression is correlated with delayed differentiation of lower jaw muscles.3 Over-expression of Isl1 in the chick represses pharyngeal muscle differentiation.24
Lineage tracing experiments in the mouse using an Isl1-Cre line revealed a significant contribution of Isl1+ cells to the mesodermal core of the pharyngeal arches,3,24 as well as to the heart38 (Figure 2D). Isl1+ PM cells were shown to contribute to a subset of pharyngeal muscles, the mylohyoid, stylohyoid, and digastric, at the base of the mandible, facilitating its opening (Figure 2E and F). Isl1+ cells were also found in second arch-derived muscles controlling facial expression3,24 and, to a lesser extent, in the masseter, pterygoid and temporalis, the jaw closing muscles (Figure 2E, orange; Figure 2F, brown), indicating that this gene is not expressed in all cells of the PM. In both species, tongue muscles and EOM are not derived from the Isl1 lineage.3,24 A similar lineage map of Isl1+ PM cells was seen in adult pharyngeal muscles and their associated satellite cells.24 These findings highlight the link between myogenesis in the early embryo, and the generation of adult muscle progenitor pools required for muscle maintenance and regeneration. Taken together, Isl1 marks a subset of PM cells, and plays an important role in the development of distinct PM-derived cardiovascular and skeletal muscle progenitors. The direct role of Isl1 in pharyngeal muscle development has yet to be resolved, as Isl1 knockout embryos die at around E10.34
Pharyngeal mesoderm evolution: from pharyngeal muscles to the heart
The architecture, function, and physiology of muscle cells have been remarkably conserved throughout evolution. Hence, all muscle cells likely evolved from an ancestral developmental programme involving a single contractile myogenic cell type.39,40 The fact that the developmental programmes of the heart and pharyngeal muscles are tightly linked suggests that these tissues share common evolutionary origins13,14 (Figure 3).
Nematodes such as the worm, C. elegans, are invertebrates that do not possess a heart or defined circulatory system. Instead, their pharyngeal muscle contracts like a heart, and exhibits electrical activity similar to that of mammalian cardiomyocytes (Figure 3A). Moreover, it has been shown that development of the pharyngeal muscle in nematodes is regulated by the homeobox gene Nkx2.5 (ceh-22)41 and may be functionally replaced by the zebrafish nkx2.5 (42, reviewed in references 13,14,43).
Tunicates belong to the Chordata phylum, and are considered as the ‘sister group’ of vertebrates.44 The tunicate Ciona intestinalis is a sessile marine invertebrate. As in vertebrates, the Ciona heart is located ventrally and posterior to the pharynx, and anterior to the stomach; in the gastrulating embryo, its heart arises from a pair of blastomeres expressing the MesP gene. Several studies suggest significant similarities in the gene regulatory networks controlling cardiogenesis in vertebrates and tunicates.44–46 The heart and pharyngeal muscle cells in Ciona (Figure 3B) are seemingly distinct, based on the expression of different myosin heavy chain isoforms47; yet both are derived from MesP+ cells.
Strikingly, Isl1+ PM cells in both Ciona48 and vertebrates3,24 give rise to pharyngeal muscles (termed siphon muscles in Ciona). These findings suggest that the last common ancestor of tunicates and vertebrates had PM cells derived from MesP+ lineages that expressed Isl1, FoxF, and Nkx2.5, and had the potential to give rise to both heart tissue and pharyngeal muscles.48 With the increasing complexity of the vertebrate heart and, in particular, during the heart tube elongation that occurs in vertebrates, Isl1+ PM cells were recruited into the looping heart to give rise to cardiomyocytes (Figure 3C). Hence, this study suggests that reallocation of PM cells into the looping heart represents the emergence of the second heart field in vertebrates. In addition, these findings suggest distinct evolutionary origins for the two heart fields.
Signalling mechanisms affecting pharyngeal mesoderm development
PM cells are maintained in a proliferative and undifferentiated state by multiple signalling mechanisms that lately have begun to be defined. Abnormal transition of proliferating PM-derived cardiac progenitors into differentiating cardiomyocytes severely affects cardiac looping and outflow tract extension, processes which are generally associated with congenital heart disease.7,10 Many signalling pathways influence cardiac progenitor cell proliferation and differentiation (reviewed in references 8,10,49). For example, recent studies have shown that the Wnt/β-catenin pathway plays distinct roles at various stages of cardiac development by triggering the renewal and expansion of cardiac progenitors, and blocking their differentiation (reviewed in reference 50).
Bone morphogenic protein (BMP) and fibroblast growth factor (FGF) signals are initially required for cardiac specification and, later, for the differentiation of cardiac progenitors in chick and frog embryos. Loss-of-function studies in mice have underscored the essential roles of BMP and FGF signalling pathways in cardiogenesis (reviewed in references 8,49). BMP signalling affects PM cells in two phases: it is initially required to ‘lock’ these cells into the cardiogenic lineage (and block skeletal muscle markers);12 while at later stages, BMP signalling promotes PM cardiac differentiation by blocking FGF signalling, to facilitate the accurate deployment of these progenitors to the looping heart.51 Indeed, FGF signalling has been shown to play key roles in the survival and expansion of PM-derived cardiac progenitors,52–55 where down-regulation of this pathway is both required and sufficient for the differentiation of PM-derived cardiomyocytes.51 Hence, opposing activities of BMP and FGF signalling pathways constitute a major regulatory mechanism during cardiogenesis.51,56 In addition, BMP–FGF crosstalk regulates the epicardial vs. myocardial lineage switch at the inflow pole of the heart.57
What lies downstream of BMP signalling in the transition from cardiac progenitors to cardiomyocytes? In the chick, the transcription factor Msx1, expressed primarily by neural crest cells, has been shown to function between BMP and FGF pathways to promote cardiomyocyte differentiation.51 Likewise, double knockout of Msx1/2 in the mouse resulted in increased proliferation of second heart field cells.58 How Msx genes expressed in neural crest cells promote myocardial differentiation of PM cells is currently not clear. A recent study revealed another means by which BMP signalling can promote PM differentiation: the Martin laboratory found that the PM progenitor markers Isl1 and Tbx1 are repressed by BMP signalling, via the induction of microRNAs (miRNA 17–92 cluster).59
Regulation of pharyngeal mesoderm by other tissues
PM progenitors are exposed to signals from pharyngeal endoderm, ectoderm, and neural crest cells that, together, create a complex regulatory system (reviewed in references 8,49). Perturbation of the balance of signals within this system can lead to abnormal heart development. Neural crest ablation in the chick, for example, results in increased FGF signalling and elevated proliferation in PM.60–62 These studies suggest that both cardiac neural crest (affecting caudal PM progenitors) and cranial neural crest cells (affecting cranial PM) (shown in Figure 1D), buffer proliferative signals (presumably FGFs) secreted from the endoderm and ectoderm to promote PM migration and differentiation.
Other examples for the roles of cardiac/cranial neural crest in the regulation of PM cells may be found in mice lacking either Smad4, a member of the Smad family of signal transduction proteins involved in BMP signalling63 or the BMP receptor ALK264 in neural crest cells. Both of these mouse models resulted in abnormal differentiation of PM progenitors that led to severe cardiac outflow defects. Furthermore, these studies are consistent with a BMP-dependent signalling mechanism involving neural crest cells that regulate these progenitors. Similarly, interference with Notch signalling in neural crest, PM or endothelial cells, all result in arterial pole and arch artery defects (reviewed in references 8,65). Taken together, these studies demonstrate that multiple signals from various tissues are intertwined in an as-yet unclear manner, to control the development of PM cells.
It has long been suggested that neural crest—PM crosstalk is involved in patterning of the head musculature (reviewed in reference 66). Several recent studies suggest that pharyngeal muscle patterning and differentiation are tightly regulated by the interaction of PM-derived muscle progenitors with adjacent cranial neural crest cells that give rise to most of the vertebrate skeletal system, including bones, cartilage, and connective tissues in the face.62,67–70 It is therefore believed that CNC-derived connective tissue progressively imposes the characteristic anatomical musculoskeletal architecture upon PM muscle progenitors.
Pharyngeal mesoderm cells are multipotent cardiac and skeletal muscle progenitors
While it is now well-established that PM progenitors contribute to both pharyngeal muscles and heart progenitors within the arterial pole of the heart,3,4,12 the question remained whether these head and heart cells originated from single PM cells or from a pre-determined, mixed population of PM cells. Using a retrospective clonal analysis (that they developed) in the mouse, the Buckingham laboratory demonstrated that head muscles and second heart field derivatives originate from multipotent PM progenitors.16 Two myogenic lineages linking groups of head muscles to different parts of the heart were identified (Figure 4A–C): The first muscle lineage (blue) gives rise to the temporalis and masseter, two first pharyngeal arch muscles, as well as to the EOMs. Strikingly, this single-cell lineage also contributes to myocardial cells in the right ventricle (Figure 4C). The second lineage gives rise to a broad range of muscles controlling facial expression, which derive from mesoderm of the second pharyngeal arch, and also contributes myocardial cells to the arterial pole of the heart (Figure 4B and C). These findings highlight the dynamic posterior shift in the alignment of the cardiac outflow tract with the pharyngeal arches, and the spatiotemporal deployment of PM into heart and head muscles. Interestingly, Mef2c-AHF-Cre, activated in PM progenitors,22,23 can distinguish between first- and second-pharyngeal arch myogenic lineages. Further sublineages distinguish myocardium at the base of the aorta or pulmonary trunk, with a clonal relationship to right or left head muscles, respectively.16
In conclusion, we have attempted to summarize novel insights into how pharyngeal muscles and certain parts of the heart arise from PM. In doing so, we emphasized key findings concerning the anatomical, cellular, and molecular characteristics of PM progenitors. We also discussed signalling mechanisms that regulate PM proliferation and differentiation, and the evolution of PM-derived cardiac and skeletal muscle progenitors. The developmental paths that lead to the formation of skeletal muscles in the head appear to be distinct from those operating in the trunk. Considerable cellular and genetic variations among the different craniofacial muscle groups are also seen. A deeper understanding of PM development is instrumental in deciphering the aetiology of heart and craniofacial birth defects.
The Tzahor laboratory is supported by research grants from the Association Française Contre les Myopathies; the Israel Science Foundation; the United States-Israel Binational Science Foundation (BSF, with S.M.E); the Helen and Martin Kimmel Institute for Stem Cell Research; the German Israeli Foundation (GIF); and the Minerva Foundation. S.M.E. would like to acknowledge funding from NIH and BSF.
We deeply thank Miriam Brodt-Ivenshitz and Itamar Harel from the Tzahor laboratory for the artwork. We thank Brad Davidson and Mike Levine for insightful discussions. E.T. is the incumbent of the Gertrude and Philip Nollman Career Development Chair.
Conflict of interest: none declared.