Fate map of early avian cardiac progenitor cells

Cardiac competence of the paraxial head mesoderm fades concomitant with a shift towards the head skeletal muscle programme

The vertebrate head mesoderm provides the heart, the great vessels, some smooth and most head skeletal muscle, in addition to parts of the skull. It has been speculated that the ability to generate cardiac and smooth muscle is the evolutionary ground-state of the tissue. However, whether indeed the entire head mesoderm has generic cardiac competence, how long this may last, and what happens as cardiac competence fades, is not clear. Bone morphogenetic proteins (Bmps) are known to promote cardiogenesis. Using 41 different marker genes in the chicken embryo, we show that the paraxial head mesoderm that normally does not engage in cardiogenesis has the ability to respond to Bmp for a long time. However, Bmp signals are interpreted differently at different time points. Up to early head fold stages, the paraxial head mesoderm is able to read Bmps as signal to engage in the cardiac programme; the ability to upregulate smooth muscle markers is retained slightly longer. Notably, as cardiac competence fades, Bmp promotes the head skeletal muscle programme instead. The switch from cardiac to skeletal muscle competence is Wnt-independent as Wnt caudalises the head mesoderm and also suppresses Msc-inducing Bmp provided by the prechordal plate, thus suppressing both the cardiac and the head skeletal muscle programmes. Our study for the first time suggests a specific transition state in the embryo when cardiac competence is replaced by skeletal muscle competence. It sets the stage to unravel the cardiac-skeletal muscle antagonism that is known to partially collapse in heart failure.

Formation of the Heart: Defining Cardiomyocyte Progenitors at Single-Cell Resolution

PURPOSE OF REVIEW

Formation of the heart requires the coordinated addition of multiple progenitor sources which have undergone different pathways of specification and differentiation. In this review, I aim to put into context how recent studies defining cardiac progenitor heterogeneity build on our understanding of early heart development and also discuss the questions raised by this new insight.

RECENT FINDINGS

With the development of sequencing technologies and imaging approaches, it has been possible to define, at high temporal resolution, the molecular profile and anatomical location of cardiac progenitors at the single-cell level, during the formation of the mammalian heart. Given the recent progress in our understanding of early heart development and technical advances in high-resolution time-lapse imaging and lineage analysis, we are now in a position of great potential, allowing us to resolve heart formation at previously impossible levels of detail. Understanding how this essential organ forms not only addresses questions of fundamental biological significance but also provides a blueprint for strategies to both treat and model heart disease.

Quantifying endodermal strains during heart tube formation in the developing chicken embryo

In the early avian embryo, the developing heart forms when bilateral fields of cardiac progenitor cells, which reside in the lateral plate mesoderm, move toward the embryonic midline, and fuse above the anterior intestinal portal (AIP) to form a straight, muscle-wrapped tube. During this process, the precardiac mesoderm remains in close contact with the underlying endoderm. Previous work has shown that the endoderm around the AIP actively contracts to pull the cardiac progenitors toward the midline. The morphogenetic deformations associated with this endodermal convergence, however, remain unclear, as do the signaling pathways that might regulate this process. Here, we fluorescently labeled populations of endodermal cells in early chicken embryos and tracked their motion during heart tube formation to compute time-varying strains along the anterior endoderm. We then determined how the computed endodermal strain distributions are affected by the pharmacological inhibition of either myosin II or fibroblast growth factor (FGF) signaling. Our data indicate that a mediolateral gradient in endodermal shortening is present around the AIP, as well as substantial convergence and extension movements both anterior and lateral to the AIP. These active endodermal deformations are disrupted if either actomyosin contractility or FGF signaling are inhibited pharmacologically. Taken together, these results demonstrate how active deformations along the anterior endoderm contribute to heart tube formation within the developing embryo.

Inhibition of RhoA and Cdc42 by miR-133a Modulates Retinoic Acid Signalling during Early Development of Posterior Cardiac Tube Segment

It is well known that multiple microRNAs play crucial roles in cardiovascular development, including miR-133a. Additionally, retinoic acid regulates atrial marker expression. In order to analyse the role of miR-133a as a modulator of retinoic acid signalling during the posterior segment of heart tube formation, we performed functional experiments with miR-133a and retinoic acid by means of microinjections into the posterior cardiac precursors of both primitive endocardial tubes in chick embryos. Subsequently, we subjected embryos to whole mount in situ hybridisation, immunohistochemistry and qPCR analysis. Our results demonstrate that miR-133a represses RhoA and Cdc42, as well as Raldh2/Aldh1a2, and the specific atrial markers Tbx5 and AMHC1, which play a key role during differentiation. Furthermore, we observed that miR-133a upregulates p21 and downregulates cyclin A by repressing RhoA and Cdc42, respectively, thus functioning as a cell proliferation inhibitor. Additionally, retinoic acid represses miR-133a, while it increases Raldh2, Tbx5 and AMHC1. Given that RhoA and Cdc42 are involved in Raldh2 expression and that they are modulated by miR-133a, which is influenced by retinoic acid signalling, our results suggest the presence of a negative feedback mechanism between miR-133a and retinoic acid during early development of the posterior cardiac tube segment. Despite additional unexplored factors being possible contributors to this negative feedback mechanism, miR-133a might also be considered as a potential therapeutic tool for the diagnosis, therapy and prognosis of cardiac diseases.

Dynamic MicroRNA Expression Profiles During Embryonic Development Provide Novel Insights Into Cardiac Sinus Venosus/Inflow Tract Differentiation

MicroRNAs have been explored in different organisms and are involved as molecular switches modulating cellular specification and differentiation during the embryonic development, including the cardiovascular system. In this study, we analyze the expression profiles of different microRNAs during early cardiac development. By using whole mount in situ hybridization in developing chick embryos, with microRNA-specific LNA probes, we carried out a detailed study of miR-23b, miR-130a, miR-106a, and miR-100 expression during early stages of embryogenesis (HH3 to HH17). We also correlated those findings with putative microRNA target genes by means of mirWalk and TargetScan analyses. Our results demonstrate a dynamic expression pattern in cardiac precursor cells from the primitive streak to the cardiac looping stages for miR-23b, miR-130a, and miR-106a. Additionally, miR-100 is later detectable during cardiac looping stages (HH15-17). Interestingly, the sinus venosus/inflow tract was shown to be the most representative cardiac area for the convergent expression of the four microRNAs. Through in silico analysis we revealed that distinct Hox family members are predicted to be targeted by the above microRNAs. We also identified expression of several Hox genes in the sinus venosus at stages HH11 and HH15. In addition, by means of gain-of-function experiments both in cardiomyoblasts and sinus venosus explants, we demonstrated the modulation of the different Hox clusters, Hoxa, Hoxb, Hoxc, and Hoxd genes, by these microRNAs. Furthermore, we correlated the negative modulation of several Hox genes, such as Hoxa3, Hoxa4, Hoxa5, Hoxc6, or Hoxd4. Finally, we demonstrated through a dual luciferase assay that Hoxa1 is targeted by miR-130a and Hoxa4 is targeted by both miR-23b and miR-106a, supporting a possible role of these microRNAs in Hox gene modulation during differentiation and compartmentalization of the posterior structures of the developing venous pole of the heart.

Dissecting the Complexity of Early Heart Progenitor Cells

Early heart development depends on the coordinated participation of heterogeneous cell sources. As pioneer work from Adriana C. Gittenberger-de Groot demonstrated, characterizing these distinct cell sources helps us to understand congenital heart defects. Despite decades of research on the segregation of lineages that form the primitive heart tube, we are far from understanding its full complexity. Currently, single-cell approaches are providing an unprecedented level of detail on cellular heterogeneity, offering new opportunities to decipher its functional role. In this review, we will focus on three key aspects of early heart morphogenesis: First, the segregation of myocardial and endocardial lineages, which yields an early lineage diversification in cardiac development; second, the signaling cues driving differentiation in these progenitor cells; and third, the transcriptional heterogeneity of cardiomyocyte progenitors of the primitive heart tube. Finally, we discuss how single-cell transcriptomics and epigenomics, together with live imaging and functional analyses, will likely transform the way we delve into the complexity of cardiac development and its links with congenital defects.

A cellular platform for the development of synthetic living machines

Robot swarms have, to date, been constructed from artificial materials. Motile biological constructs have been created from muscle cells grown on precisely shaped scaffolds. However, the exploitation of emergent self-organization and functional plasticity into a self-directed living machine has remained a major challenge. We report here a method for generation of in vitro biological robots from frog (Xenopus laevis) cells. These xenobots exhibit coordinated locomotion via cilia present on their surface. These cilia arise through normal tissue patterning and do not require complicated construction methods or genomic editing, making production amenable to high-throughput projects. The biological robots arise by cellular self-organization and do not require scaffolds or microprinting; the amphibian cells are highly amenable to surgical, genetic, chemical, and optical stimulation during the self-assembly process. We show that the xenobots can navigate aqueous environments in diverse ways, heal after damage, and show emergent group behaviors. We constructed a computational model to predict useful collective behaviors that can be elicited from a xenobot swarm. In addition, we provide proof of principle for a writable molecular memory using a photoconvertible protein that can record exposure to a specific wavelength of light. Together, these results introduce a platform that can be used to study many aspects of self-assembly, swarm behavior, and synthetic bioengineering, as well as provide versatile, soft-body living machines for numerous practical applications in biomedicine and the environment.

The deployment of cell lineages that form the mammalian heart

The function of the mammalian heart depends on the interplay between different cardiac cell types. The deployment of these cells, with precise spatiotemporal regulation, is also important during development to establish the heart structure. In this Review, we discuss the diverse origins of cardiac cell types and the lineage relationships between cells of a given type that contribute to different parts of the heart. The emerging lineage tree shows the progression of cell fate diversification, with patterning cues preceding cell type segregation, as well as points of convergence, with overlapping lineages contributing to a given tissue. Several cell lineage markers have been identified. However, caution is required with genetic-tracing experiments in comparison with clonal analyses. Genetic studies on cell populations provided insights into the mechanisms for lineage decisions. In the past 3 years, results of single-cell transcriptomics are beginning to reveal cell heterogeneity and early developmental trajectories. Equating this information with the in vivo location of cells and their lineage history is a current challenge. Characterization of the progenitor cells that form the heart and of the gene regulatory networks that control their deployment is of major importance for understanding the origin of congenital heart malformations and for producing cardiac tissue for use in regenerative medicine.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Growth and Morphogenesis during Early Heart Development in Amniotes

In this review, we will focus on the growth and morphogenesis of the developing heart, an aspect of cardiovascular development to which Antoon Moorman and colleagues have extensively contributed. Over the last decades, genetic studies and characterization of regionally regulated gene programs have provided abundant novel insights into heart development essential to understand the basis of congenital heart disease. Heart morphogenesis, however, is inherently a complex and dynamic three-dimensional process and we are far from understanding its cellular basis. Here, we discuss recent advances in studying heart morphogenesis and regionalization under the light of the pioneering work of Moorman and colleagues, which allowed the reinterpretation of regional gene expression patterns under a new morphogenetic framework. Two aspects of early heart formation will be discussed in particular: (1) the initial formation of the heart tube and (2) the formation of the cardiac chambers by the ballooning process. Finally, we emphasize that in addition to analyses based on fixed samples, new approaches including clonal analysis, single-cell sequencing, live-imaging and quantitative analysis of the data generated will likely lead to novel insights in understanding early heart tube regionalization and morphogenesis in the near future.

FGF signaling enforces cardiac chamber identity in the developing ventricle

Atrial and ventricular cardiac chambers behave as distinct subunits with unique morphological, electrophysiological and contractile properties. Despite the importance of chamber-specific features, chamber fate assignments remain relatively plastic, even after differentiation is underway. In zebrafish, Nkx transcription factors are essential for the maintenance of ventricular characteristics, but the signaling pathways that operate upstream of Nkx factors in this context are not well understood. Here, we show that FGF signaling plays an essential part in enforcing ventricular identity. Loss of FGF signaling results in a gradual accumulation of atrial cells, a corresponding loss of ventricular cells, and the appearance of ectopic atrial gene expression within the ventricle. These phenotypes reflect important roles for FGF signaling in promoting ventricular traits, both in early-differentiating cells that form the initial ventricle and in late-differentiating cells that append to its arterial pole. Moreover, we find that FGF signaling functions upstream of Nkx genes to inhibit ectopic atrial gene expression. Together, our data suggest a model in which sustained FGF signaling acts to suppress cardiomyocyte plasticity and to preserve the integrity of the ventricular chamber.

A strategy to discover new organizers identifies a putative heart organizer

Organizers are regions of the embryo that can both induce new fates and impart pattern on other regions. So far, surprisingly few organizers have been discovered, considering the number of patterned tissue types generated during development. This may be because their discovery has relied on transplantation and ablation experiments. Here we describe a new approach, using chick embryos, to discover organizers based on a common gene expression signature, and use it to uncover the anterior intestinal portal (AIP) endoderm as a putative heart organizer. We show that the AIP can induce cardiac identity from non-cardiac mesoderm and that it can pattern this by specifying ventricular and suppressing atrial regional identity. We also uncover some of the signals responsible. The method holds promise as a tool to discover other novel organizers acting during development.

Negative Fgf8-Bmp2 feed-back is regulated by miR-130 during early cardiac specification

It is known that secreted proteins from the anterior lateral endoderm, FGF8 and BMP2, are involved in mesodermal cardiac differentiation, which determines the first cardiac field, defined by the expression of the earliest specific cardiac markers Nkx-2.5 and Gata4. However, the molecular mechanisms responsible for early cardiac development still remain unclear. At present, microRNAs represent a novel layer of complexity in the regulatory networks controlling gene expression during cardiovascular development. This paper aims to study the role of miR130 during early cardiac specification. Our model is focused on developing chick at gastrula stages. In order to identify those regulatory factors which are involved in cardiac specification, we conducted gain- and loss-of-function experiments in precardiac cells by administration of Fgf8, Bmp2 and miR130, through in vitro electroporation technique and soaked beads application. Embryos were subjected to in situ hybridization, immunohistochemistry and qPCR procedures. Our results reveal that Fgf8 suppresses, while Bmp2 induces, the expression of Nkx-2.5 and Gata4. They also show that Fgf8 suppresses Bmp2, and vice versa. Additionally, we observed that Bmp2 regulates miR-130 -a putative microRNA that targets Erk1/2 (Mapk1) 3'UTR, recognizing its expression in precardiac cells which overlap with Erk1/2 pattern. Finally, we evidence that miR-130 is capable to inhibit Erk1/2 and Fgf8, resulting in an increase of Bmp2, Nkx-2.5 and Gata4. Our data present miR-130 as a necessary linkage in the control of Fgf8 signaling, mediated by Bmp2, establishing a negative feed-back loop responsible to achieve early cardiac specification.

Smad1 transcription factor integrates BMP2 and Wnt3a signals in migrating cardiac progenitor cells

In vertebrate embryos, cardiac progenitor cells (CPCs) undergo long-range migration after emerging from the primitive streak during gastrulation. Together with other mesoderm progenitors, they migrate laterally and then toward the ventral midline, where they form the heart. Signals controlling the migration of different progenitor cell populations during gastrulation are poorly understood. Several pathways are involved in the epithelial-to-mesenchymal transition and ingression of mesoderm cells through the primitive streak, including fibroblast growth factors and wingless-type family members (Wnt). Here we focus on early CPC migration and use live video microscopy in chicken embryos to demonstrate a role for bone morphogenetic protein (BMP)/SMA and MAD related (Smad) signaling. We identify an interaction of BMP and Wnt/glycogen synthase kinase 3 beta (GSK3β) pathways via the differential phosphorylation of Smad1. Increased BMP2 activity altered migration trajectories of prospective cardiac cells and resulted in their lateral displacement and ectopic differentiation, as they failed to reach the ventral midline. Constitutively active BMP receptors or constitutively active Smad1 mimicked this phenotype, suggesting a cell autonomous response. Expression of GSK3β, which promotes the turnover of active Smad1, rescued the BMP-induced migration phenotype. Conversely, expression of GSK3β-resistant Smad1 resulted in aberrant CPC migration trajectories. De-repression of GSK3β by dominant negative Wnt3a restored normal migration patterns in the presence of high BMP activity. The data indicate the convergence of BMP and Wnt pathways on Smad1 during the early migration of prospective cardiac cells. Overall, we reveal molecular mechanisms that contribute to the emerging paradigm of signaling pathway integration in embryo development.

Fate mapping identifies the origin of SHF/AHF progenitors in the chick primitive streak

Heart development depends on the spatio-temporally regulated contribution of progenitor cells from the primary, secondary and anterior heart fields. Primary heart field (PHF) cells are first recruited to form a linear heart tube; later, they contribute to the inflow myocardium of the four-chambered heart. Subsequently cells from the secondary (SHF) and anterior heart fields (AHF) are added to the heart tube and contribute to both the inflow and outflow myocardium. In amniotes, progenitors of the linear heart tube have been mapped to the anterior-middle region of the early primitive streak. After ingression, these cells are located within bilateral heart fields in the lateral plate mesoderm. On the other hand SHF/AHF field progenitors are situated anterior to the linear heart tube, however, the origin and location of these progenitors prior to the development of the heart tube remains elusive. Thus, an unresolved question in the process of cardiac development is where SHF/AHF progenitors originate from during gastrulation and whether they come from a region in the primitive streak distinct from that which generates the PHF. To determine the origin and location of SHF/AHF progenitors we used vital dye injection and tissue grafting experiments to map the location and ingression site of outflow myocardium progenitors in early primitive streak stage chicken embryos. Cells giving rise to the AHF ingressed from a rostral region of the primitive streak, termed region 'A'. During development these cells were located in the cranial paraxial mesoderm and in the pharyngeal mesoderm. Furthermore we identified region 'B', located posterior to 'A', which gave rise to progenitors that contributed to the primary heart tube and the outflow tract. Our studies identify two regions in the early primitive streak, one which generates cells of the AHF and a second from which cardiac progenitors of the PHF and SHF emerge.

Asymmetric fate of the posterior part of the second heart field results in unexpected left/right contributions to both poles of the heart

RATIONALE

The second heart field (SHF) contains progenitors of all heart chambers, excluding the left ventricle. The SHF is patterned, and the anterior region is known to be destined to form the outflow tract and right ventricle.

OBJECTIVE

The aim of this study was to map the fate of the posterior SHF (pSHF).

METHODS AND RESULTS

We examined the contribution of pSHF cells, labeled by lipophilic dye at the 4- to 6-somite stage, to regions of the heart at 20 to 25 somites, using mouse embryo culture. Cells more cranial in the pSHF contribute to the atrioventricular canal (AVC) and atria, whereas those more caudal generate the sinus venosus, but there is intermixing of fate throughout the pSHF. Caudal pSHF contributes symmetrically to the sinus venosus, but the fate of cranial pSHF is left/right asymmetrical. Left pSHF moves to dorsal left atrium and superior AVC, whereas right pSHF contributes to right atrium, ventral left atrium, and inferior AVC. Retrospective clonal analysis shows the relationships between AVC and atria to be clonal and that right and left progenitors diverge before first and second heart lineage separation. Cranial pSHF cells also contribute to the outflow tract: proximal and distal at 4 somites, and distal only at 6 somites. All outflow tract-destined cells are intermingled with those that will contribute to inflow and AVC.

CONCLUSIONS

These observations show asymmetric fate of the pSHF, resulting in unexpected left/right contributions to both poles of the heart and can be integrated into a model of the morphogenetic movement of cells during cardiac looping.

An unauthorized biography of the second heart field and a pioneer/scaffold model for cardiac development

The identification of subpharyngeal cardiac precursors has had a strong influence on the way we think about early cardiac development. From this discovery was born the concept of multiple heart fields. Early support for the concept came from gene expression, genetic retrospective fate mapping, and gene targeting studies, which collectively suggested the existence of a second heart field (SHF) on the basis of specific Islet-1 (Isl-1) expression, presence of two cardiac ancestral lineages, and compatible cardiac knockout phenotypes, respectively. A decade after the original studies, support for the SHF concept is dwindling. This is because in all bilaterian models studied, Isl expression in heart progenitors is not SHF-specific, because lineage data are best explained by alternative models including an older, truly ancestral, lineage of cardiac pioneers with unrestricted contribution to all cardiac segments and, finally, because the inflow-to-outflow segmental nature of the early vertebrate peristaltic heart has been reaffirmed with novel, less invasive, methodologies. Altogether, the paradigms derived from the discovery of subpharyngeal cardiac progenitors helped us shift from relatively simple models, which rely predominantly either on patterning, gene expression patterns or lineages, to a much more sophisticated body of knowledge in which all these parameters must be accounted. Thus, it is well possible that due consideration of the key elements contained in the inflow/outflow, pioneer/scaffold, ballooning, and SHF hypotheses may provide us with a unified framework of the early stages of cardiac development. Here, we advance into this direction by suggesting an intuitive model of early heart development based on the concept of an inflow/outflow scaffold erected by cardiac pioneers, one that is required to assemble all the subsequent cell contribution that emigrates from cardiac progenitor areas.

Chronological and morphological study of heart development in the rat

Adult and embryonic laboratory rats have been used as a mammalian model organism in biomedical research, descriptive and experimental cardiac embryology, and experimental teratology. There have been, however, considerable variations and discrepancies concerning the developmental staging of the rat embryo in the reported literature, which have resulted in several controversies and inconsistencies. Therefore, we carried out a careful anatomical and histological study of rat cardiac morphogenesis from the premorphogenetic period to the mature heart in a newborn pup. A correlation between the chronology and morphological features of the heart and embryo or newborn was made. We provide a simple and comprehensive guide relating the developmental timing and fate of the embryonic components of the heart and their morphological changes in the rat based on in vivo labeling studies in the chick. We also compare the timing of heart development in rats, humans, and mice.

Not just inductive: a crucial mechanical role for the endoderm during heart tube assembly

The heart is the first functioning organ to form during development. During gastrulation, the cardiac progenitors reside in the lateral plate mesoderm but maintain close contact with the underlying endoderm. In amniotes, these bilateral heart fields are initially organized as a pair of flat epithelia that move towards the embryonic midline and fuse above the anterior intestinal portal (AIP) to form the heart tube. This medial motion is typically attributed to active mesodermal migration over the underlying endoderm. In this model, the role of the endoderm is twofold: to serve as a mechanically passive substrate for the crawling mesoderm and to secrete various growth factors necessary for cardiac specification and differentiation. Here, using computational modeling and experiments on chick embryos, we present evidence supporting an active mechanical role for the endoderm during heart tube assembly. Label-tracking experiments suggest that active endodermal shortening around the AIP accounts for most of the heart field motion towards the midline. Results indicate that this shortening is driven by cytoskeletal contraction, as exposure to the myosin-II inhibitor blebbistatin arrested any shortening and also decreased both tissue stiffness (measured by microindentation) and mechanical tension (measured by cutting experiments). In addition, blebbistatin treatment often resulted in cardia bifida and abnormal foregut morphogenesis. Moreover, finite element simulations of our cutting experiments suggest that the endoderm (not the mesoderm) is the primary contractile tissue layer during this process. Taken together, these results indicate that contraction of the endoderm actively pulls the heart fields towards the embryonic midline, where they fuse to form the heart tube.

Time-lapse imaging of chick cardiac precursor cells

The chick embryo is easily accessible and has therefore been widely used in developmental biology studies. In particular, the early embryo can be removed from the egg and cultured, which allows real-time observations and imaging. Here, we describe ex vivo electroporation followed by long-term time-lapse microscopy, image capture, and processing. We have applied this approach to characterise the migration route of cardiac progenitor cells (CPCs) in live embryos. The heart is the first organ to function during vertebrate development and it is essential for the continued growth and survival of the embryo. In the chick, cardiac progenitors have been mapped to the anterior and mid-primitive streak at Hamburger-Hamilton stage 3. However, until recently it was not possible to observe cell migration trajectories directly. Furthermore, we used grafting of beads or cell pellets or electroporation of expression plasmids to show that Wnt3a acts as a repulsive signal to guide the movement of cardiac progenitors.

Nodal signal is required for morphogenetic movements of epiblast layer in the pre-streak chick blastoderm

During axis formation in amniotes, posterior and lateral epiblast cells in the area pellucida undergo a counter-rotating movement along the midline to form primitive streak (Polonaise movements). Using chick blastoderms, we investigated the signaling involved in this cellular movement in epithelial-epiblast. In cultured posterior blastoderm explants from stage X to XI embryos, either Lefty1 or Cerberus-S inhibited initial migration of the explants on chamber slides. In vivo analysis showed that inhibition of Nodal signaling by Lefty1 affected the movement of DiI-marked epiblast cells prior to the formation of primitive streak. In Lefty1-treated embryos without a primitive streak, Brachyury expression showed a patchy distribution. However, SU5402 did not affect the movement of DiI-marked epiblast cells. Multi-cellular rosette, which is thought to be involved in epithelial morphogenesis, was found predominantly in the posterior half of the epiblast, and Lefty1 inhibited the formation of rosettes. Three-dimensional reconstruction showed two types of rosette, one with a protruding cell, the other with a ventral hollow. Our results suggest that Nodal signaling may have a pivotal role in the morphogenetic movements of epithelial epiblast including Polonaise movements and formation of multi-cellular rosette.

Identification of differentially expressed genes in the heart precursor cells of the chick embryo

Genetic evidence has implicated several genes as being critical for heart development. However, the inducers of these genes as well as their targets and pathways they are involved with, remain largely unknown. Previous studies in the avian embryo showed that at HH4 Cerberus (cCer) transcripts are detected in the anterior endomesoderm including the heart precursor cells and later in the left lateral plate mesoderm. We have identified a promoter element of chick cCer able to drive EGFP expression in a population of cells that consistently exit from the anterior primitive streak region, from as early as stage HH3+, and that later will populate the heart. Using this promoter element as a tool allowed us to identify novel genes previously not known to potentially play a role in heart development. In order to identify and study genes expressed and involved in the correct development and differentiation of the vertebrate heart precursor cell (HPC) lineages, a differential screening using Affymetrix GeneChip system technologies was performed. Remarkably, this screening led to the identification of more than 700 transcripts differentially expressed in the heart forming regions (HFR). Bioinformatic tools allowed us to filter the large amount of data generated from this approach and to select a few transcripts for in vivo validation. Whole-mount in situ hybridization and sectioning of selected genes showed heart and vascular expression patterns for these transcripts during early chick development. We have developed an effective strategy to specifically identify genes that are differentially expressed in the HPC lineages. Within this set we have identified several genes that are expressed in the heart, blood and vascular lineages, which are likely to play a role in their development. These genes are potential candidates for future functional studies on early embryonic patterning.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

Mechanics of head fold formation: investigating tissue-level forces during early development

During its earliest stages, the avian embryo is approximately planar. Through a complex series of folds, this flat geometry is transformed into the intricate three-dimensional structure of the developing organism. Formation of the head fold (HF) is the first step in this cascading sequence of out-of-plane tissue folds. The HF establishes the anterior extent of the embryo and initiates heart, foregut and brain development. Here, we use a combination of computational modeling and experiments to determine the physical forces that drive HF formation. Using chick embryos cultured ex ovo, we measured: (1) changes in tissue morphology in living embryos using optical coherence tomography (OCT); (2) morphogenetic strains (deformations) through the tracking of tissue labels; and (3) regional tissue stresses using changes in the geometry of circular wounds punched through the blastoderm. To determine the physical mechanisms that generate the HF, we created a three-dimensional computational model of the early embryo, consisting of pseudoelastic plates representing the blastoderm and vitelline membrane. Based on previous experimental findings, we simulated the following morphogenetic mechanisms: (1) convergent extension in the neural plate (NP); (2) cell wedging along the anterior NP border; and (3) autonomous in-plane deformations outside the NP. Our numerical predictions agree relatively well with the observed morphology, as well as with our measured stress and strain distributions. The model also predicts the abnormal tissue geometries produced when development is mechanically perturbed. Taken together, the results suggest that the proposed morphogenetic mechanisms provide the main tissue-level forces that drive HF formation.

Second lineage of heart forming region provides new understanding of conotruncal heart defects

Abnormal heart development causes various congenital heart defects. Recent cardiovascular biology studies have elucidated the morphological mechanisms involved in normal and abnormal heart development. The primitive heart tube originates from the lateral-most part of the heart forming mesoderm and mainly gives rise to the left ventricle. Then, during the cardiac looping, the outflow tract is elongated by the addition of cardiogenic cells from the both pharyngeal and splanchnic mesoderm (corresponding to anterior and secondary heart field, respectively), which originate from the mediocaudal region of the heart forming mesoderm and are later located anteriorly (rostrally) to the dorsal region of the heart tube. Therefore, the heart progenitors that contribute to the outflow tract region are distinct from those that form the left ventricle. The knowledge that there are two different lineages of heart progenitors in the four-chambered heart provides new understanding of the morphological and molecular etiology of conotruncal heart defects.

Cardiogenesis: an embryological perspective

Cardiogenesis, considered as the formation of new heart tissue from embryonic, postnatal, or adult cardiac progenitors, is a pivotal concept to understand the rationale of advanced therapies to repair the damaged heart. In this review, we focus on the cellular and molecular regulation of cardiogenesis in the developing embryo, and we dissect the complex interactions that control the diversification and maturation of a variety of cardiac cell lineages. Our aim is to show how the sophisticated anatomical structure of the adult four-chambered heart strongly depends on the fine regulation of the differentiation of cardiac progenitor cells. These events are shown to be progressive and dynamic as well as plastic, so that the patterned differentiation of distinct heart domains is highly dependent on signals provided by nonmyocardial heart components and extracardiac tissues. Finally, we present the core of our knowledge on cardiac embryogenesis in a biomedical context to provide a critical analysis on the logic of cell therapies designed to treat the failing heart.

The role of secondary heart field in cardiac development

Although de la Cruz and colleagues showed as early as 1977 that the outflow tract was added after the heart tube formed, the source of these secondarily added cells was not identified for nearly 25 years. In 2001, three pivotal publications described a secondary or anterior heart field that contributed to the developing outflow tract. This review details the history of the heart field, the discovery and continuing elucidation of the secondarily adding myocardial cells, and how the different populations identified in 2001 are related to the more recent lineage tracing studies that defined the first and second myocardial heart fields/lineages. Much recent work has focused on secondary heart field progenitors that give rise to the myocardium and smooth muscle at the definitive arterial pole. These progenitors are the last to be added to the arterial pole and are particularly susceptible to abnormal development, leading to conotruncal malformations in children. The major signaling pathways (Wnt, BMP, FGF8, Notch, and Shh) that control various aspects of secondary heart field progenitor behavior are discussed.

Monitoring clonal growth in the developing ventricle

Understanding the etiology of congenital heart defects depends on a detailed knowledge of the morphogenetic events underlying cardiac development. Deciphering the developmental processes and cell behaviors resulting in the formation of a four-chambered heart requires techniques by which the destiny of individual cells can be traced during development. Ideally, such approaches provide information on progenitor cells and growth properties of clonally related myocytes. In the avian system, clonal analysis based on the use of replication-defective retroviral labeling led to a model for growth of the ventricular wall from polyclonal transmural cones of myocardial cells. In the mouse, the nlaacZ retrospective clonal analysis system has proved to be a powerful technique for studying different aspects of cardiac morphogenesis. Morphologic and histologic analyses of clonally related myocytes at early stages of development have provided genetic evidence for the formation of the heart tube from two cell lineages. Additional aspects of cardiac morphogenesis, including formation of the interventricular septum and myocardial outflow tract, and more recently, the origin of the ventricular conduction system, have been studied using this system. This brief review discusses how the nlaacZ system has provided new insights into the divergent properties of clonally related cells in these different regions of the developing heart.

Dynamic positional fate map of the primary heart-forming region

Here we show the temporal-spatial orchestration of early heart morphogenesis at cellular level resolution, in vivo, and reconcile conflicting positional fate mapping data regarding the primary heart-forming field(s). We determined the positional fates of precardiac cells using a precision electroporation approach in combination with wide-field time-lapse microscopy in the quail embryo, a warm-blooded vertebrate (HH Stages 4 through 10). Contrary to previous studies, the results demonstrate the existence of a "continuous" circle-shaped heart field that spans the midline, appearing at HH Stage 4, which then expands to form a wide arc of progenitors at HH Stages 5-7. Our time-resolved image data show that a subset of these cardiac progenitor cells do not overlap with the expression of common cardiogenic factors, Nkx-2.5 and Bmp-2, until HH Stage 10, when a tubular heart has formed, calling into question when cardiac fate is specified and by which key factors. Sub-groups and anatomical bands (cohorts) of heart precursor cells dramatically change their relative positions in a process largely driven by endodermal folding and other large-scale tissue deformations. Thus, our novel dynamic positional fate maps resolve the origin of cardiac progenitor cells in amniotes. The data also establish the concept that tissue motion contributes significantly to cellular position fate - i.e., much of the cellular displacement that occurs during assembly of a midline heart tube (HH Stage 9) is NOT due to "migration" (autonomous motility), a commonly held belief. Computational analysis of our time-resolved data lays the foundation for more precise analyses of how cardiac gene regulatory networks correlate with early heart tissue morphogenesis in birds and mammals.

Endothelial cell lineages of the heart

During early gastrulation, vertebrate embryos begin to produce endothelial cells (ECs) from the mesoderm. ECs first form primitive vascular plexus de novo and later differentiate into arterial, venous, capillary, and lymphatic ECs. In the heart, the five distinct EC types (endocardial, coronary arterial, venous, capillary, and lymphatic) have distinct phenotypes. For example, coronary ECs establish a typical vessel network throughout the myocardium, whereas endocardial ECs form a large epithelial sheet with no angiogenic sprouting into the myocardium. Neither coronary arteries, veins, and capillaries, nor lymphatic vessels fuse with the endocardium or open to the heart chamber. The developmental stage during which the specific phenotype of each cardiac EC type is determined remains unclear. The mechanisms involved in EC commitment and diversity can however be more precisely defined by tracking the migratory patterns and lineage decisions of the precursors of cardiac ECs.

Extrinsic regulation of cardiomyocyte differentiation of embryonic stem cells

Cardiovascular disease is one of leading causes of death throughout the U.S. and the world. The damage of cardiomyocytes resulting from ischemic injury is irreversible and leads to the development of progressive heart failure, which is characterized by the loss of functional cardiomyocytes. Because cardiomyocytes are unable to regenerate in the adult heart, cell-based therapy of transplantation provides a potential alternative approach to replace damaged myocardial tissue and restore cardiac function. A major roadblock toward this goal is the lack of donor cells; therefore, it is urgent to identify the cardiovascular cells that are necessary for achieving cardiac muscle regeneration. Pluripotent embryonic stem (ES) cells have enormous potential as a source of therapeutic tissues, including cardiovascular cells; however, the regulatory elements mediating ES cell differentiation to cardiomyocytes are largely unknown. In this review, we will focus on extrinsic factors that play a role in regulating different stages of cardiomyocyte differentiation of ES cells.

Patterning of the heart field in the chick

In human development, it is postulated based on histological sections, that the cardiogenic mesoderm rotates 180 degrees with the pericardial cavity. This is also thought to be the case in mouse development where gene expression data suggests that the progenitors of the right ventricle and outflow tract invert their position with respect to the progenitors of the atria and left ventricle. However, the inversion in both cases is inferred and has never been shown directly. We have used 3D reconstructions and cell tracing in chick embryos to show that the cardiogenic mesoderm is organized such that the lateralmost cells are incorporated into the cardiac inflow (atria and left ventricle) while medially placed cells are incorporated into the cardiac outflow (right ventricle and outflow tract). This happens because the cardiogenic mesoderm is inverted. The inversion is concomitant with movement of the anterior intestinal portal which rolls caudally to form the foregut pocket. The bilateral cranial cardiogenic fields fold medially and ventrally and fuse. After heart looping the seam made by ventral fusion will become the greater curvature of the heart loop. The caudal border of the cardiogenic mesoderm which ends up dorsally coincides with the inner curvature. Physical ablation of selected areas of the cardiogenic mesoderm based on this new fate map confirmed these results and, in addition, showed that the right and left atria arise from the right and left heart fields. The inversion and the new fate map account for several unexplained observations and provide a unified concept of heart fields and heart tube formation for avians and mammals.

The heart-forming fields: one or multiple?

The recent identification of a second mesodermal region as a source of cardiomyocytes has challenged the views on the formation of the heart. This second source of cardiomyocytes is localized centrally on the embryonic disc relative to the remainder of the classic cardiac crescent, a region also called the pharyngeal mesoderm. In this review, we discuss the concept of the primary and secondary cardiogenic fields in the context of folding of the embryo, and the subsequent temporal events involved in formation of the heart. We suggest that, during evolution, the heart developed initially only with the components required for a systemic circulation, namely a sinus venosus, a common atrium, a 'left' ventricle and an arterial cone, the latter being the myocardial outflow tract as seen in the heart of primitive fishes. These components developed in their entirety from the classic cardiac crescent. Only later in the course of evolution did the appearance of novel signalling pathways permit the central part of the cardiac crescent, and possibly the contiguous pharyngeal mesoderm, to develop into the cardiac components required for the pulmonary circulation. These latter components comprise the right ventricle, and that part of the left atrium that derives from the mediastinal myocardium, namely the dorsal atrial wall and the atrial septum. It is these elements which are now recognized as developing from the second field of pharyngeal mesoderm. We suggest that, rather than representing development from separate fields, the cardiac components required for both the systemic and pulmonary circulations are derived by patterning from a single cardiac field, albeit with temporal delay in the process of formation.

Isolation of cardiac cells from E8.5 yolk sac by ALCAM (CD166) expression

It is known that the adhesion molecule ALCAM (CD166) mediates metastasis of malignant cells and organogenesis in embryos. We show here that embryonic day 8.5 (E8.5) murine yolk sac cells express ALCAM protein and that ALCAM expression can be used to define endothelial and cardiac precursors from hematopoietic precursors in E8.5 yolk sacs. ALCAM high+ cells exclusively give rise to endothelial and cardiac cells in matrigel assays but generate no hematopoietic colonies in methylcellulose assays. ALCAM low+ and ALCAM- populations predominantly give rise to hematopoietic cells in methylcellulose, but do not generate any cell clusters in matrigel. The ALCAM high+ population contains both Flk-1+ and Flk-1- cells. The former population exclusively contains endothelial cells whereas the latter give rise to cardiac cells when cultured on OP9 stromal cells. We also show that cardiac lineage marker genes such as Nkx-2.5, and the endothelial marker VE-cadherin are expressed in the ALCAM high+ fraction, whereas the hematopoietic marker GATA1 and Runx1 are expressed in the ALCAM low+/- fraction. However, we did not detect expression of the cardiac structural protein cTn-T in cells from yolk sac cells until these had had been differentiated on OP9 for 5 days. Altogether, these results indicate that cells retaining a potential to differentiate to the cardiac lineage are present in E8.5 yolk sacs and can be isolated as ALCAM high+, Flk-1- cells. Our report provides novel insights into the origin and differentiation process of cardiac cells in the formation of the circulatory system.

Vessel and blood specification override cardiac potential in anterior mesoderm

Organ progenitors arise within organ fields, embryonic territories that are larger than the regions required for organ formation. Little is known about the regulatory pathways that define organ field boundaries and thereby limit organ size. Here we identify a mechanism for restricting heart size through confinement of the developmental potential of the heart field. Via fate mapping in zebrafish, we locate cardiac progenitors within hand2-expressing mesoderm and demonstrate that hand2 potentiates cardiac differentiation within this region. Beyond the rostral boundary of hand2 expression, we find progenitors of vessel and blood lineages. In embryos deficient in vessel and blood specification, rostral mesoderm undergoes a fate transformation and generates ectopic cardiomyocytes. Therefore, induction of vessel and blood specification represses cardiac specification and delimits the capacity of the heart field. This regulatory relationship between cardiovascular pathways suggests strategies for directing progenitor cell differentiation to facilitate cardiac regeneration.

Trabeculated right ventricular free wall in the chicken heart forms by ventricularization of the myocardium initially forming the outflow tract

Recent molecular lineage analyses in mouse have demonstrated that the right ventricle is recruited from anterior mesoderm in later stages of cardiac development. This is in contrast to current views of development in the chicken heart, which suggest that the initial heart tube contains a subset of right ventricular precursors. We investigated the fate of the outflow tract myocardium using immunofluorescent staining of the myocardium, and lineage tracer, as well as cell death experiments. These analyses showed that the outflow tract is initially myocardial in its entirety, increasing in length up to HH24. The outflow tract myocardium, subsequently, shortens as a result of ventricularization, contributing to the trabeculated free wall, as well as the infundibulum, of the right ventricle. During this shortening, the overall length of the outflow tract is maintained because of the formation of a nonmyocardial portion between the distal myocardial border and the pericardial reflections. Cell death and transdifferentiation were found to play a more limited contribution to the initial shortening than is generally appreciated, if they play any part at all. Cell death, nonetheless, plays an important role in the disappearance of the myocardial collar that continues to invest the aorta and pulmonary trunk around HH30, and in the separation of the intrapericardial arterial vessels. Taken together, we show, as opposed to some current beliefs, the development of the arterial pole is similar in mammals and birds.

Signals from both sides: Control of cardiac development by the endocardium and epicardium

It is readily apparent that the process of heart development is an intricate one, in which cells derived from many embryonic sources coalesce and coordinate their behaviors and development, resulting in the mature heart. The behaviors and mechanisms of this process are complex, and still incompletely understood. However, it is readily apparent that communication between diverse cell types must be involved in this process. The signaling that emanates from epicardial and endocardial sources is the focus of this review.

Recruitment of New Cells into the Postnatal Heart: Potential Modification of Phenotype by Periostin

Establishment of the circulatory system occurs very early in development to support the rapid growth of the embryo. Therefore, the heart is the first functional organ to be formed during both avian and mammalian development. Historically, cardiac development has been considered to occur only during embryogenesis from cell sources located within the primordial structures that generate the myocardium and associated coronary vascular endothelium and smooth muscle and cardiac fibroblasts. Recently, however, contribution to the cardiac structures has been demonstrated to occur during embryonic development from extracardiac sources, like the anterior heart field, raising questions as to whether cardiogenesis may be an ongoing process that extends into adult life. In this brief article, we describe the contribution of circulating adult bone marrow hematopoietic stem cells to the cardiac cell populations and the potential regulation of their differentiation by the extracellular matrix protein, periostin.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.