The allocation of epiblast cells to the embryonic heart and other mesodermal lineages: the role of ingression and tissue movement during gastrulation

Early coordination of cell migration and cardiac fate determination during mammalian gastrulation

During gastrulation, mesodermal cells derived from distinct regions are destined to acquire specific cardiac fates after undergoing complex migratory movements. Here, we used light-sheet imaging of live mouse embryos between gastrulation and heart tube formation to track mesodermal cells and to reconstruct lineage trees and 3D migration paths for up to five cell divisions. We found independent progenitors emerging at specific times, contributing exclusively to left ventricle/atrioventricular canal (LV/AVC) or atrial myocytes. LV/AVC progenitors differentiated early to form the cardiac crescent, while atrial progenitors later generated the heart tube's Nr2f2+ inflow tract during morphogenesis. We also identified short-lived multipotent progenitors with broad potential, illustrating early developmental plasticity. Descendants of multipotent progenitors displayed greater dispersion and more diverse migratory trajectories within the anterior mesoderm than the progeny of uni-fated progenitors. Progenitors contributing to extraembryonic mesoderm (ExEm) exhibited the fastest and most dispersed migrations. In contrast, those giving rise to endocardial, LV/AVC, and pericardial cells showed a more gradual divergence, with late-stage behavioural shifts: endocardial cells increased in speed, while pericardial cells slowed down in comparison to LV/AVC cells. Together, these data reveal patterns of individual cell directionality and cardiac fate allocation within the seemingly unorganised migratory pattern of mesoderm cells.

Immune-mediated cardiac development and regeneration

The complex interplay between the immune and cardiovascular systems during development, homeostasis and regeneration represents a rapidly evolving field in cardiac biology. Single cell technologies, spatial mapping and computational analysis have revolutionised our understanding of the diversity and functional specialisation of immune cells within the heart. From the earliest stages of cardiogenesis, where primitive macrophages guide heart tube formation, to the complex choreography of inflammation and its resolution during regeneration, immune cells emerge as central orchestrators of cardiac fate. Translating these fundamental insights into clinical applications represents a major challenge and opportunity for the field. In this Review, we decode the immunological blueprint of heart development and regeneration to transform cardiovascular disease treatment and unlock the regenerative capacity of the human heart.

Relating normal human cardiac development to the anatomical findings in the congenitally malformed heart

A proper appreciation of cardiac development can now provide the necessary background to understand the anatomical findings in the congenitally malformed heart. We recently presented an account of human cardiac development based on reconstructions of histological datasets from human embryos aged between 3.5 and 8 weeks subsequent to conception. In this review, we summarize the changes observed relative to the findings when the heart is congenitally malformed. Beginning at the stage when it is first possible to recognize the primary heart tube, we describe the looping of its ventricular component, which occurs in the 5th week. We proceed with discussion of the formation of the atrial and ventricular chambers in the 6th week. The phases are successive, albeit partially overlapping. Separation of the circulations at the venous pole is completed at stage 17, equivalent to almost 6 weeks of development. During stages representing the 7th week of development, we concentrate on the remodeling of the outflow tract. This involves initially septation, but then separation of the developing circulations. The changes involve incorporation of the proximal outflow tract into the ventricles, with formation of the arterial roots in its middle part, and addition of a distal non-myocardial component to produce the intrapericardial arterial trunks. We pay particular attention to the changes occurring during remodeling of the interventricular foramen. We show that an understanding of this process provides the basis for understanding the functionally univentricular heart, as well as the arrangement found in double outlet right ventricle.

Organizer activity in the mouse embryo

The discovery of the embryonic organizer by Hilde Mangold and Hans Spemann in 1924 was one of the most ground-breaking achievements in the 1900 century for developmental biologists and beyond. Ever since the organizer was first described in newts, developmental biologists have been trying to uncover similar structures in other organisms. While the Spemann-Mangold organizer as an axis-inducing centre is evolutionary conserved in vertebrates, similar organizing centres have yet to be observed in mammals. In this review, we will provide a brief historical overview of the discovery of the mouse gastrula organizer and discuss its potential as an organizer throughout early post-implantation mouse development. We discuss cell migrations through the mouse organizer region and morphogenesis of organizer cells and tissues. Finally, we examine the evidence arguing for and against the existence of a head organizer in mice, and the role of the anterior visceral endoderm and the prechordal plate in organizing head structures.

Sonic Hedgehog signaling regulates the optimal differentiation pace from early-stage mesoderm to cardiogenic mesoderm in mice

Sonic Hedgehog (Shh), encoding an extracellular signaling molecule, is vital for heart development. Shh null mutants show congenital heart disease due to left-right asymmetry defects stemming from functional anomaly in the midline structure in mice. Shh signaling is also known to affect cardiomyocyte differentiation, endocardium development, and heart morphogenesis, particularly in second heart field (SHF) cardiac progenitor cells that contribute to the right ventricle, outflow tract, and parts of the atrium. Despite extensive studies, our understanding remains incomplete. Notably, Shh signaling is suggested to promote cardiac differentiation, while paradoxically preventing premature differentiation of SHF progenitors. In this study, we elucidate the role of Shh signaling in the earliest phase of cardiac differentiation. Our meta-analysis of single-cell RNA sequencing suggests that cardiogenic nascent mesoderm cells expressing the bHLH transcription factor Mesp1 interact with axial mesoderm via Hh signaling. Activation of Hh signaling using a Smoothened agonist delayed or suppressed the differentiation of primitive streak cells expressing T-box transcription factor T to Mesp1+ nascent mesoderm cells both in vitro and ex vivo. Conversely, inhibition of Hh signaling by cyclopamine facilitated cardiac differentiation. The reduction of Eomes, an inducer of Mesp1, by Hh signaling appears to be the underlying mechanism of this phenomenon. Our data suggest that SHH secreted from axial mesoderm inhibits premature differentiation of T+ cells to Mesp1+ nascent mesoderm cells, thereby regulating the pace of cardiac differentiation. These findings enhance our comprehension of Shh signaling in cardiac development, underscoring its crucial role in early cardiac differentiation.

Synthetic embryology of the human heart

The evolution of stem cell-based heart models from cells and tissues to organoids and assembloids and recently synthetic embryology gastruloids, is poised to revolutionize our understanding of cardiac development, congenital to adult diseases, and patient customized therapies. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have already been integrated into transplantable patches and are in preclinical efforts to reverse fibrotic scarring from myocardial infarctions. To inform on the complexity of heart diseases, multi-tissue morphogenic heart models are needed that replicate fundamental components of heart function to heart organogenesis in vitro and which require a deep understanding of heart development. Organoid and assembloid models capture selected multicellular cardiac processes, such as chamber formation and priming events for vascularization. Gastruloid heart models offer deeper insights as synthetic embryology to mimic multi-staged developmental events of in vivo heart organogenesis including established heart fields, crescent formation and heart tube development along with vascular systemic foundation and even further steps. The human Elongating Multi-Lineage Organized Cardiac (EMLOC) gastruloid model captures these stages and additional events including chamber genesis, patterned vascularization, and extrinsic central and intrinsic cardiac nervous system (CNS-ICNS) integration guided by spatiotemporal and morphogenic processes with neural crest cells. Gastruloid synthetic embryology heart models offer new insights into previously hidden processes of development and provide powerful platforms for addressing heart disease that extends beyond cardiomyocytes, such as arrhythmogenic diseases, congenital defects, and systemic injury interactions, as in spinal cord injuries. The holistic view that is emerging will reveal heart development and disease in unprecedented detail to drive transformative state-of-the-art innovative applications for heart health.

In vitro modelling of anterior primitive streak patterning with human pluripotent stem cells identifies the path to notochord progenitors

Notochord progenitors (NotoPs) represent a scarce yet crucial embryonic cell population, playing important roles in embryo patterning and eventually giving rise to the cells that form and maintain intervertebral discs. The mechanisms regulating NotoPs emergence are unclear. This knowledge gap persists due to the inherent complexity of cell fate patterning during gastrulation, particularly within the anterior primitive streak (APS), where NotoPs first arise alongside neuro-mesoderm and endoderm. To gain insights into this process, we use micropatterning together with FGF and the WNT pathway activator CHIR9901 to guide the development of human embryonic stem cells into reproducible patterns of APS cell fates. We show that CHIR9901 dosage dictates the downstream dynamics of endogenous TGFβ signalling, which in turn controls cell fate decisions. While sustained NODAL signalling defines endoderm and NODAL inhibition is imperative for neuro-mesoderm emergence, timely inhibition of NODAL signalling with spatial confinement potentiates WNT activity and enables us to generate NotoPs efficiently. Our work elucidates the signalling regimes underpinning NotoP emergence and provides insights into the regulatory mechanisms controlling the balance of APS cell fates during gastrulation.

Three-dimensional stem cell models of mammalian gastrulation

Gastrulation is a key milestone in the development of an organism. It is a period of cell proliferation and coordinated cellular rearrangement, that creates an outline of the body plan. Our current understanding of mammalian gastrulation has been improved by embryo culture, but there are still many open questions that are difficult to address because of the intrauterine development of the embryos and the low number of specimens. In the case of humans, there are additional difficulties associated with technical and ethical challenges. Over the last few years, pluripotent stem cell models are being developed that have the potential to become useful tools to understand the mammalian gastrulation. Here we review these models with a special emphasis on gastruloids and provide a survey of the methods to produce them robustly, their uses, relationship to embryos, and their prospects as well as their limitations.

3D reconstruction of a gastrulating human embryo

Progress in understanding early human development has been impeded by the scarcity of reference datasets from natural embryos, particularly those with spatial information during crucial stages like gastrulation. We conducted high-resolution spatial transcriptomics profiling on 38,562 spots from 62 transverse sections of an intact Carnegie stage (CS) 8 human embryo. From this spatial transcriptomic dataset, we constructed a 3D model of the CS8 embryo, in which a range of cell subtypes are identified, based on gene expression patterns and positional register, along the anterior-posterior, medial-lateral, and dorsal-ventral axis in the embryo. We further characterized the lineage trajectories of embryonic and extra-embryonic tissues and associated regulons and the regionalization of signaling centers and signaling activities that underpin lineage progression and tissue patterning during gastrulation. Collectively, the findings of this study provide insights into gastrulation and post-gastrulation development of the human embryo.

Human Cardiac Development

Many aspects of heart development are topographically complex and require three-dimensional (3D) reconstruction to understand the pertinent morphology. We have recently completed a comprehensive primer of human cardiac development that is based on firsthand segmentation of structures of interest in histological sections. We visualized the hearts of 12 human embryos between their first appearance at 3.5 weeks and the end of the embryonic period at 8 weeks. The models were presented as calibrated, interactive, 3D portable document format (PDF) files. We used them to describe the appearance and the subsequent remodeling of around 70 different structures incrementally for each of the reconstructed stages. In this chapter, we begin our account by describing the formation of the single heart tube, which occurs at the end of the fourth week subsequent to conception. We describe its looping in the fifth week, the formation of the cardiac compartments in the sixth week, and, finally, the septation of these compartments into the physically separated left- and right-sided circulations in the seventh and eighth weeks. The phases are successive, albeit partially overlapping. Thus, the basic cardiac layout is established between 26 and 32 days after fertilization and is described as Carnegie stages (CSs) 9 through 14, with development in the outlet component trailing that in the inlet parts. Septation at the venous pole is completed at CS17, equivalent to almost 6 weeks of development. During Carnegie stages 17 and 18, in the seventh week, the outflow tract and arterial pole undergo major remodeling, including incorporation of the proximal portion of the outflow tract into the ventricles and transfer of the spiraling course of the subaortic and subpulmonary channels to the intrapericardial arterial trunks. Remodeling of the interventricular foramen, with its eventual closure, is complete at CS20, which occurs at the end of the seventh week. We provide quantitative correlations between the age of human and mouse embryos as well as the Carnegie stages of development. We have also set our descriptions in the context of variations in the timing of developmental features.

Multi-chamber cardioids unravel human heart development and cardiac defects

The number one cause of human fetal death are defects in heart development. Because the human embryonic heart is inaccessible and the impacts of mutations, drugs, and environmental factors on the specialized functions of different heart compartments are not captured by in vitro models, determining the underlying causes is difficult. Here, we established a human cardioid platform that recapitulates the development of all major embryonic heart compartments, including right and left ventricles, atria, outflow tract, and atrioventricular canal. By leveraging 2D and 3D differentiation, we efficiently generated progenitor subsets with distinct first, anterior, and posterior second heart field identities. This advance enabled the reproducible generation of cardioids with compartment-specific in vivo-like gene expression profiles, morphologies, and functions. We used this platform to unravel the ontogeny of signal and contraction propagation between interacting heart chambers and dissect how mutations, teratogens, and drugs cause compartment-specific defects in the developing human heart.

Time space and single-cell resolved tissue lineage trajectories and laterality of body plan at gastrulation

Understanding of the molecular drivers of lineage diversification and tissue patterning during primary germ layer development requires in-depth knowledge of the dynamic molecular trajectories of cell lineages across a series of developmental stages of gastrulation. Through computational modeling, we constructed at single-cell resolution, a spatio-temporal transcriptome of cell populations in the germ-layers of gastrula-stage mouse embryos. This molecular atlas enables the inference of molecular network activity underpinning the specification and differentiation of the germ-layer tissue lineages. Heterogeneity analysis of cellular composition at defined positions in the epiblast revealed progressive diversification of cell types. The single-cell transcriptome revealed an enhanced BMP signaling activity in the right-side mesoderm of late-gastrulation embryo. Perturbation of asymmetric BMP signaling activity at late gastrulation led to randomization of left-right molecular asymmetry in the lateral mesoderm of early-somite-stage embryo. These findings indicate the asymmetric BMP activity during gastrulation may be critical for the symmetry breaking process.

Human pluripotent stem cell-based models of heart development and disease

The heart is a complex organ composed of distinct cell types, such as cardiomyocytes, cardiac fibroblasts, endothelial cells, smooth muscle cells, neuronal cells and immune cells. All these cell types contribute to the structural, electrical and mechanical properties of the heart. Genetic manipulation and lineage tracing studies in mice have been instrumental in gaining critical insights into the networks regulating cardiac cell lineage specification, cell fate and plasticity. Such knowledge has been of fundamental importance for the development of efficient protocols for the directed differentiation of pluripotent stem cells (PSCs) in highly specialized cardiac cell types. In this review, we summarize the evolution and current advances in protocols for cardiac subtype specification, maturation, and assembly in cardiac microtissues and organoids.

Gastrulation morphogenesis in synthetic systems

Recent advances in pluripotent stem cell culture allow researchers to generate not only most embryonic cell types, but also morphologies of many embryonic structures, entirely in vitro. This recreation of embryonic form from naïve cells, known as synthetic morphogenesis, has important implications for both developmental biology and regenerative medicine. However, the capacity of stem cell-based models to recapitulate the morphogenetic cell behaviors that shape natural embryos remains unclear. In this review, we explore several examples of synthetic morphogenesis, with a focus on models of gastrulation and surrounding stages. By varying cell types, source species, and culture conditions, researchers have recreated aspects of primitive streak formation, emergence and elongation of the primary embryonic axis, neural tube closure, and more. Here, we describe cell behaviors within in vitro/ex vivo systems that mimic in vivo morphogenesis and highlight opportunities for more complete models of early development.

Induced Pluripotent Stem Cell-Based Modeling of Single-Ventricle Congenital Heart Diseases

PURPOSE OF REVIEW

Congenital heart disease includes a wide variety of structural cardiac defects, the most severe of which are single ventricle defects (SVD). These patients suffer from significant morbidity and mortality; however, our understanding of the developmental etiology of these conditions is limited. Model organisms offer a window into normal and abnormal cardiogenesis yet often fail to recapitulate complex congenital heart defects seen in patients. The use of induced pluripotent stem cells (iPSCs) derived from patients with single-ventricle defects opens the door to studying SVD in patient-derived cardiomyocytes (iPSC-CMs) in a variety of different contexts, including organoids and chamber-specific cardiomyocytes. As the genetic and cellular causes of SVD are not well defined, patient-derived iPSC-CMs hold promise for uncovering mechanisms of disease development and serve as a platform for testing therapies. The purpose of this review is to highlight recent advances in iPSC-based models of SVD.

RECENT FINDINGS

Recent advances in patient-derived iPSC-CM differentiation, as well as the development of both chamber-specific and non-myocyte cardiac cell types, make it possible to model the complex genetic and molecular architecture involved in SVD development. Moreover, iPSC models have become increasingly complex with the generation of 3D organoids and engineered cardiac tissues which open the door to new mechanistic insight into SVD development. Finally, iPSC-CMs have been used in proof-of-concept studies that the molecular underpinnings of SVD may be targetable for future therapies. While each platform has its advantages and disadvantages, the use of patient-derived iPSC-CMs offers a window into patient-specific cardiogenesis and SVD development. Advancement in stem-cell based modeling of SVD promises to revolutionize our understanding of the developmental etiology of SVD and provides a tool for developing and testing new therapies.

Imaging Approaches and the Quantitative Analysis of Heart Development

Heart morphogenesis is a complex and dynamic process that has captivated researchers for almost a century. This process involves three main stages, during which the heart undergoes growth and folding on itself to form its common chambered shape. However, imaging heart development presents significant challenges due to the rapid and dynamic changes in heart morphology. Researchers have used different model organisms and developed various imaging techniques to obtain high-resolution images of heart development. Advanced imaging techniques have allowed the integration of multiscale live imaging approaches with genetic labeling, enabling the quantitative analysis of cardiac morphogenesis. Here, we discuss the various imaging techniques used to obtain high-resolution images of whole-heart development. We also review the mathematical approaches used to quantify cardiac morphogenesis from 3D and 3D+time images and to model its dynamics at the tissue and cellular levels.

Stiffness-Tunable Hydrogel-Sandwich Culture Modulates the YAP-Mediated Mechanoresponse in Induced-Pluripotent Stem Cell Embryoid Bodies and Augments Cardiomyocyte Differentiation

Microenvironmental factors, including substrate stiffness, regulate stem cell behavior and differentiation. However, the effects of substrate stiffness on the behavior of induced pluripotent stem cell (iPSC)- derived embryoid bodies (EB) remain unclear. To investigate the effects of mechanical cues on iPSC-EB differentiation, a 3D hydrogel-sandwich culture (HGSC) system is developed that controls the microenvironment surrounding iPSC-EBs using a stiffness-tunable polyacrylamide hydrogel assembly. Mouse iPSC-EBs are seeded between upper and lower polyacrylamide hydrogels of differing stiffness (Young's modulus [E'] = 54.3 ± 7.1 kPa [hard], 28.1 ± 2.3 kPa [moderate], and 5.1 ± 0.1 kPa [soft]) and cultured for 2 days. HGSC induces stiffness-dependent activation of the yes-associated protein (YAP) mechanotransducer and actin cytoskeleton rearrangement in the iPSC-EBs. Moreover, moderate-stiffness HGSC specifically upregulates the mRNA and protein expression of ectoderm and mesoderm lineage differentiation markers in iPSC-EBs via YAP-mediated mechanotransduction. Pretreatment of mouse iPSC-EBs with moderate-stiffness HGSC promotes cardiomyocyte (CM) differentiation and structural maturation of myofibrils. The proposed HGSC system provides a viable platform for investigating the role of mechanical cues on the pluripotency and differentiation of iPSCs that can be beneficial for research into tissue regeneration and engineering.

Postnatal oogenesis leads to an exceptionally large ovarian reserve in naked mole-rats

In the long-lived naked mole-rat (NMR), the entire process of oogenesis occurs postnatally. Germ cell numbers increase significantly in NMRs between postnatal days 5 (P5) and P8, and germs cells positive for proliferation markers (Ki-67, pHH3) are present at least until P90. Using pluripotency markers (SOX2 and OCT4) and the primordial germ cell (PGC) marker BLIMP1, we show that PGCs persist up to P90 alongside germ cells in all stages of female differentiation and undergo mitosis both in vivo and in vitro. We identified VASA+ SOX2+ cells at 6 months and at 3-years in subordinate and reproductively activated females. Reproductive activation was associated with proliferation of VASA+ SOX2+ cells. Collectively, our results suggest that highly desynchronized germ cell development and the maintenance of a small population of PGCs that can expand upon reproductive activation are unique strategies that could help to maintain the NMR's ovarian reserve for its 30-year reproductive lifespan.

Cre recombinase microinjection for single-cell tracing and localised gene targeting

Tracing and manipulating cells in embryos are essential to understand development. Lipophilic dye microinjections, viral transfection and iontophoresis have been key to map the origin of the progenitor cells that form the different organs in the post-implantation mouse embryo. These techniques require advanced manipulation skills and only iontophoresis, a demanding approach of limited efficiency, has been used for single-cell labelling. Here, we perform lineage tracing and local gene ablation using cell-permeant Cre recombinase (TAT-Cre) microinjection. First, we map the fate of undifferentiated progenitors to the different heart chambers. Then, we achieve single-cell recombination by titrating the dose of TAT-Cre, which allows clonal analysis of nascent mesoderm progenitors. Finally, injecting TAT-Cre to Mycnflox/flox embryos in the primitive heart tube revealed that Mycn plays a cell-autonomous role in maintaining cardiomyocyte proliferation. This tool will help researchers identify the cell progenitors and gene networks involved in organ development, helping to understand the origin of congenital defects.

Embryonic and extraembryonic tissues during mammalian development: shifting boundaries in time and space

The first few days of embryonic development in eutherian mammals are dedicated to the specification and elaboration of the extraembryonic tissues. However, where the fetus ends and its adnexa begins is not always as self-evident during the early stages of development, when the definitive body axes are still being laid down, the germ layers being specified and a discrete form or bodyplan is yet to emerge. Function, anatomy, histomorphology and molecular identities have been used through the history of embryology, to make this distinction. In this review, we explore them individually by using specific examples from the early embryo. While highlighting the challenges of drawing discrete boundaries between embryonic and extraembryonic tissues and the limitations of a binary categorization, we discuss how basing such identity on fate is the most universal and conceptually consistent. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The effects of β-catenin on cardiomyogenesis via Islet-1 and MLIP ubiquitination

Mesenchymal stem cells (MSCs) can treat myocardial injury-related diseases by differentiating into cardiomyocytes. Islet-1 plays an essential role in cardiac maturation. We have discovered that Islet-1 plays a crucial role in the histone acetylation regulation in this process. In addition, to increase GATA4/Nkx2.5 expression, Islet-1 may bind to Gcn5 and then guide Gcn5 to the GATA4/Nkx2.5 promoters, thereby facilitating the differentiation of MSCs into cardiomyocytes. Islet-1 is an important factor in the maturation of the heart. We have previously found that the pivotal factor in histone acetylation regulation in this process is Islet-1. Furthermore, Islet-1 and Gcn5 may boost GATA4/Nkx2.5 expression, which in turn promotes cardiomyocyte differentiation from MSCs. But the molecular mechanism of Islet-1 binding to GCN5 has not been elucidated. In this study, we found that the competitive binding relationship between Islet-1 and MLIP and GCN5 affected myocardial differentiation. The key enzymes of ubiquitination modification of MLIP and Islet-1 are UBE3C and WWP1, respectively. When short hairpin RNA (shRNA) was used to inhibit β-catenin expression, we found that the expression of UBE3C was upregulated, modifying MLIP ubiquitination and reducing its expression, and it upregulated Islet-1 by inhibiting the expression of WWP1. By using the chromatin immunoprecipitation (ChIP) and luciferase reporter system, we found that when MLIP binds to Islet-1, it significantly inhibits the transcriptional activity of Islet-1. In summary, our results show that decreasing β-catenin regulates the ubiquitination of Islet-1 and MLIP, affecting their expression, reducing the amount of Islet-1 binding to MLIP, and increasing the amount of binding to GCN5 in the nucleus. Therefore, the transcriptional activity of Islet-1 is significantly activated, inducing C3H10T1/2 cells to differentiate into myocytes. Further knowledge of biochemical pathways, including molecular signaling pathways, can provide more insights into the myocardial differentiation mechanism of MSCs.

Specification of the haematopoietic stem cell lineage: From blood-fated mesodermal angioblasts to haemogenic endothelium

Haematopoietic stem and progenitor cells emerge from specialized haemogenic endothelial cells in select vascular beds during embryonic development. Specification and commitment to the blood lineage, however, occur before endothelial cells are endowed with haemogenic competence, at the time of mesoderm patterning and production of endothelial cell progenitors (angioblasts). Whilst early blood cell fate specification has long been recognized, very little is known about the mechanisms that induce endothelial cell diversification and progressive acquisition of a blood identity by a subset of these cells. Here, we review the endothelial origin of the haematopoietic system and the complex developmental journey of blood-fated angioblasts. We discuss how recent technological advances will be instrumental to examine the diversity of the embryonic anatomical niches, signaling pathways and downstream epigenetic and transcriptional processes controlling endothelial cell heterogeneity and blood cell fate specification. Ultimately, this will give essential insights into the ontogeny of the cells giving rise to haematopoietic stem cells, that may aid in the development of novel strategies for their in vitro production for clinical purposes.

Sox2 levels regulate the chromatin occupancy of WNT mediators in epiblast progenitors responsible for vertebrate body formation

WNT signalling has multiple roles. It maintains pluripotency of embryonic stem cells, assigns posterior identity in the epiblast and induces mesodermal tissue. Here we provide evidence that these distinct functions are conducted by the transcription factor SOX2, which adopts different modes of chromatin interaction and regulatory element selection depending on its level of expression. At high levels, SOX2 displaces nucleosomes from regulatory elements with high-affinity SOX2 binding sites, recruiting the WNT effector TCF/β-catenin and maintaining pluripotent gene expression. Reducing SOX2 levels destabilizes pluripotency and reconfigures SOX2/TCF/β-catenin occupancy to caudal epiblast expressed genes. These contain low-affinity SOX2 sites and are co-occupied by T/Bra and CDX. The loss of SOX2 allows WNT-induced mesodermal differentiation. These findings define a role for Sox2 levels in dictating the chromatin occupancy of TCF/β-catenin and reveal how context-specific responses to a signal are configured by the level of a transcription factor.

Integrative analyses of biomarkers and pathways for heart failure

Background

Heart failure (HF) is the most common potential cause of death, causing a huge health and economic burden all over the world. So far, some impressive progress has been made in the study of pathogenesis. However, the underlying molecular mechanisms leading to this disease remain to be fully elucidated.

Methods

The microarray data sets of GSE76701, GSE21610 and GSE8331 were retrieved from the gene expression comprehensive database (GEO). After merging all microarray data and adjusting batch effects, differentially expressed genes (DEG) were determined. Functional enrichment analysis was performed based on Gene Ontology (GO) resources, Kyoto Encyclopedia of Genes and Genomes (KEGG) resources, gene set enrichment analysis (GSEA), response pathway database and Disease Ontology (DO). Protein protein interaction (PPI) network was constructed using string database. Combined with the above important bioinformatics information, the potential key genes were selected. The comparative toxicological genomics database (CTD) is used to explore the interaction between potential key genes and HF.

Results

We identified 38 patients with heart failure and 16 normal controls. There were 315 DEGs among HF samples, including 278 up-regulated genes and 37 down-regulated genes. Pathway enrichment analysis showed that most DEGs were significantly enriched in BMP signal pathway, transmembrane receptor protein serine/threonine kinase signal pathway, extracellular matrix, basement membrane, glycosaminoglycan binding, sulfur compound binding and so on. Similarly, GSEA enrichment analysis showed that DEGs were mainly enriched in extracellular matrix and extracellular matrix related proteins. BBS9, CHRD, BMP4, MYH6, NPPA and CCL5 are central genes in PPI networks and modules.

Conclusions

The enrichment pathway of DEGs and GO may reveal the molecular mechanism of HF. Among them, target genes EIF1AY, RPS4Y1, USP9Y, KDM5D, DDX3Y, NPPA, HBB, TSIX, LOC28556 and XIST are expected to become new targets for heart failure. Our findings provide potential biomarkers or therapeutic targets for the further study of heart failure and contribute to the development of advanced prediction, diagnosis and treatment strategies.

A pictorial account of the human embryonic heart between 3.5 and 8 weeks of development

Heart development is topographically complex and requires visualization to understand its progression. No comprehensive 3-dimensional primer of human cardiac development is currently available. We prepared detailed reconstructions of 12 hearts between 3.5 and 8 weeks post fertilization, using Amira® 3D-reconstruction and Cinema4D®-remodeling software. The models were visualized as calibrated interactive 3D-PDFs. We describe the developmental appearance and subsequent remodeling of 70 different structures incrementally, using sequential segmental analysis. Pictorial timelines of structures highlight age-dependent events, while graphs visualize growth and spiraling of the wall of the heart tube. The basic cardiac layout is established between 3.5 and 4.5 weeks. Septation at the venous pole is completed at 6 weeks. Between 5.5 and 6.5 weeks, as the outflow tract becomes incorporated in the ventricles, the spiraling course of its subaortic and subpulmonary channels is transferred to the intrapericardial arterial trunks. The remodeling of the interventricular foramen is complete at 7 weeks.

Restoring Ravaged Heart: Molecular Mechanisms and Clinical Application of miRNA in Heart Regeneration

Human heart development is a complex and tightly regulated process, conserving proliferation, and multipotency of embryonic cardiovascular progenitors. At terminal stage, progenitor cell type gets suppressed for terminal differentiation and maturation. In the human heart, most cardiomyocytes are terminally differentiated and so have limited proliferation capacity. MicroRNAs (miRNAs) are non-coding single-stranded RNA that regulate gene expression and mRNA silencing at the post-transcriptional level. These miRNAs play a crucial role in numerous biological events, including cardiac development, and cardiomyocyte proliferation. Several cardiac cells specific miRNAs have been discovered. Inhibition or overexpression of these miRNAs could induce cardiac regeneration, cardiac stem cell proliferation and cardiomyocyte proliferation. Clinical application of miRNAs extends to heart failure, wherein the cell cycle arrest of terminally differentiated cardiac cells inhibits the heart regeneration. The regenerative capacity of the myocardium can be enhanced by cardiomyocyte specific miRNAs controlling the cell cycle. In this review, we focus on cardiac-specific miRNAs involved in cardiac regeneration and cardiomyocyte proliferation, and their potential as a new clinical therapy for heart regeneration.

An integrative multiscale view of early cardiac looping

The heart is the first organ to form and function during the development of an embryo. Heart development consists of a series of events believed to be highly conserved in vertebrates. Development of heart begins with the formation of the cardiac fields followed by a linear heart tube formation. The straight heart tube then undergoes a ventral bending prior to further bending and helical torsion to form a looped heart. The looping phase is then followed by ballooning, septation, and valve formation giving rise to a four-chambered heart in avians and mammals. The looping phase plays a central role in heart development. Successful looping is essential for proper alignment of the future cardiac chambers and tracts. As aberrant looping results in various congenital heart diseases, the mechanisms of cardiac looping have been studied for several decades by various disciplines. Many groups have studied anatomy, biology, genetics, and mechanical processes during heart looping, and have proposed multiple mechanisms. Computational modeling approaches have been utilized to examine the proposed mechanisms of the looping process. Still, the exact underlying mechanism(s) controlling the looping phase remain poorly understood. Although further experimental measurements are obviously still required, the need for more integrative computational modeling approaches is also apparent in order to make sense of the vast amount of experimental data and the complexity of multiscale developmental systems. Indeed, there needs to be an iterative interaction between experimentation and modeling in order to properly find the gap in the existing data and to validate proposed hypotheses. This article is categorized under: Cardiovascular Diseases > Genetics/Genomics/Epigenetics Cardiovascular Diseases > Computational Models Cardiovascular Diseases > Molecular and Cellular Physiology.

Cardiac specification during gastrulation - The Yellow Brick Road leading to Tinman

The question of how the heart develops, and the genetic networks governing this process have become intense areas of research over the past several decades. This research is propelled by classical developmental studies and potential clinical applications to understand and treat congenital conditions in which cardiac development is disrupted. Discovery of the tinman gene in Drosophila, and examination of its vertebrate homolog Nkx2.5, along with other core cardiac transcription factors has revealed how cardiac progenitor differentiation and maturation drives heart development. Careful observation of cardiac morphogenesis along with lineage tracing approaches indicated that cardiac progenitors can be divided into two broad classes of cells, namely the first and second heart fields, that contribute to the heart in two distinct waves of differentiation. Ample evidence suggests that the fate of individual cardiac progenitors is restricted to distinct cardiac structures quite early in development, well before the expression of canonical cardiac progenitor markers like Nkx2.5. Here we review the initial specification of cardiac progenitors, discuss evidence for the early patterning of cardiac progenitors during gastrulation, and consider how early gene expression programs and epigenetic patterns can direct their development. A complete understanding of when and how the developmental potential of cardiac progenitors is determined, and their potential plasticity, is of great interest developmentally and also has important implications for both the study of congenital heart disease and therapeutic approaches based on cardiac stem cell programming.

Control of cardiomyocyte differentiation timing by intercellular signaling pathways

Congenital Heart Disease (CHD), malformations of the heart present at birth, is the most common class of life-threatening birth defect (Hoffman (1995) [1], Gelb (2004) [2], Gelb (2014) [3]). A major research challenge is to elucidate the genetic determinants of CHD and mechanistically link CHD ontogeny to a molecular understanding of heart development. Although the embryonic origins of CHD are unclear in most cases, dysregulation of cardiovascular lineage specification, patterning, proliferation, migration or differentiation have been described (Olson (2004) [4], Olson (2006) [5], Srivastava (2006) [6], Dunwoodie (2007) [7], Bruneau (2008) [8]). Cardiac differentiation is the process whereby cells become progressively more dedicated in a trajectory through the cardiac lineage towards mature cardiomyocytes. Defects in cardiac differentiation have been linked to CHD, although how the complex control of cardiac differentiation prevents CHD is just beginning to be understood. The stages of cardiac differentiation are highly stereotyped and have been well-characterized (Kattman et al. (2011) [9], Wamstad et al. (2012) [10], Luna-Zurita et al. (2016) [11], Loh et al. (2016) [12], DeLaughter et al. (2016) [13]); however, the developmental and molecular mechanisms that promote or delay the transition of a cell through these stages have not been as deeply investigated. Tight temporal control of progenitor differentiation is critically important for normal organ size, spatial organization, and cellular physiology and homeostasis of all organ systems (Raff et al. (1985) [14], Amthor et al. (1998) [15], Kopan et al. (2014) [16]). This review will focus on the action of signaling pathways in the control of cardiomyocyte differentiation timing. Numerous signaling pathways, including the Wnt, Fibroblast Growth Factor, Hedgehog, Bone Morphogenetic Protein, Insulin-like Growth Factor, Thyroid Hormone and Hippo pathways, have all been implicated in promoting or inhibiting transitions along the cardiac differentiation trajectory. Gaining a deeper understanding of the mechanisms controlling cardiac differentiation timing promises to yield insights into the etiology of CHD and to inform approaches to restore function to damaged hearts.

NOTCH Signaling in Aortic Valve Development and Calcific Aortic Valve Disease

NOTCH intercellular signaling mediates the communications between adjacent cells involved in multiple biological processes essential for tissue morphogenesis and homeostasis. The NOTCH1 mutations are the first identified human genetic variants that cause congenital bicuspid aortic valve (BAV) and calcific aortic valve disease (CAVD). Genetic variants affecting other genes in the NOTCH signaling pathway may also contribute to the development of BAV and the pathogenesis of CAVD. While CAVD occurs commonly in the elderly population with tri-leaflet aortic valve, patients with BAV have a high risk of developing CAVD at a young age. This observation indicates an important role of NOTCH signaling in the postnatal homeostasis of the aortic valve, in addition to its prenatal functions during aortic valve development. Over the last decade, animal studies, especially with the mouse models, have revealed detailed information in the developmental etiology of congenital aortic valve defects. In this review, we will discuss the molecular and cellular aspects of aortic valve development and examine the embryonic pathogenesis of BAV. We will focus our discussions on the NOTCH signaling during the endocardial-to-mesenchymal transformation (EMT) and the post-EMT remodeling of the aortic valve. We will further examine the involvement of the NOTCH mutations in the postnatal development of CAVD. We will emphasize the deleterious impact of the embryonic valve defects on the homeostatic mechanisms of the adult aortic valve for the purpose of identifying the potential therapeutic targets for disease intervention.

Segregation of brain and organizer precursors is differentially regulated by Nodal signaling at blastula stage

The blastula Chordin- and Noggin-expressing (BCNE) center comprises animal-dorsal and marginal-dorsal cells of the amphibian blastula and contains the precursors of the brain and the gastrula organizer. Previous findings suggested that the BCNE behaves as a homogeneous cell population that only depends on nuclear β-catenin activity but does not require Nodal and later segregates into its descendants during gastrulation. In contrast to previous findings, in this work, we show that the BCNE does not behave as a homogeneous cell population in response to Nodal antagonists. In fact, we found that chordin.1 expression in a marginal subpopulation of notochordal precursors indeed requires Nodal input. We also establish that an animal BCNE subpopulation of cells that express both, chordin.1 and sox2 (a marker of pluripotent neuroectodermal cells), and gives rise to most of the brain, persisted at blastula stage after blocking Nodal. Therefore, Nodal signaling is required to define a population of chordin.1+ cells and to restrict the recruitment of brain precursors within the BCNE as early as at blastula stage. We discuss our findings in Xenopus in comparison to other vertebrate models, uncovering similitudes in early brain induction and delimitation through Nodal signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: Nodal signaling is involved in the delimitation of the blastula cell populations that give rise to the brain and axial mesoderm in Xenopus.

Spatiotemporal sequence of mesoderm and endoderm lineage segregation during mouse gastrulation

Anterior mesoderm (AM) and definitive endoderm (DE) progenitors represent the earliest embryonic cell types that are specified during germ layer formation at the primitive streak (PS) of the mouse embryo. Genetic experiments indicate that both lineages segregate from Eomes-expressing progenitors in response to different Nodal signaling levels. However, the precise spatiotemporal pattern of the emergence of these cell types and molecular details of lineage segregation remain unexplored. We combined genetic fate labeling and imaging approaches with single-cell RNA sequencing (scRNA-seq) to follow the transcriptional identities and define lineage trajectories of Eomes-dependent cell types. Accordingly, all cells moving through the PS during the first day of gastrulation express Eomes AM and DE specification occurs before cells leave the PS from Eomes-positive progenitors in a distinct spatiotemporal pattern. ScRNA-seq analysis further suggested the immediate and complete separation of AM and DE lineages from Eomes-expressing cells as last common bipotential progenitor.

Mouse gastrulation: Coordination of tissue patterning, specification and diversification of cell fate

During mouse embryonic development a mass of pluripotent epiblast tissue is transformed during gastrulation to generate the three definitive germ layers: endoderm, mesoderm, and ectoderm. During gastrulation, a spatiotemporally controlled sequence of events results in the generation of organ progenitors and positions them in a stereotypical fashion throughout the embryo. Key to the correct specification and differentiation of these cell fates is the establishment of an axial coordinate system along with the integration of multiple signals by individual epiblast cells to produce distinct outcomes. These signaling domains evolve as the anterior-posterior axis is established and the embryo grows in size. Gastrulation is initiated at the posteriorly positioned primitive streak, from which nascent mesoderm and endoderm progenitors ingress and begin to diversify. Advances in technology have facilitated the elaboration of landmark findings that originally described the epiblast fate map and signaling pathways required to execute those fates. Here we will discuss the current state of the field and reflect on how our understanding has shifted in recent years.

Hedgehog-FGF signaling axis patterns anterior mesoderm during gastrulation

The mechanisms used by embryos to pattern tissues across their axes has fascinated developmental biologists since the founding of embryology. Here, using single-cell technology, we interrogate complex patterning defects and define a Hedgehog (Hh)-fibroblast growth factor (FGF) signaling axis required for anterior mesoderm lineage development during gastrulation. Single-cell transcriptome analysis of Hh-deficient mesoderm revealed selective deficits in anterior mesoderm populations, culminating in defects to anterior embryonic structures, including the pharyngeal arches, heart, and anterior somites. Transcriptional profiling of Hh-deficient mesoderm during gastrulation revealed disruptions to both transcriptional patterning of the mesoderm and FGF signaling for mesoderm migration. Mesoderm-specific Fgf4/Fgf8 double-mutants recapitulated anterior mesoderm defects and Hh-dependent GLI transcription factors modulated enhancers at FGF gene loci. Cellular migration defects during gastrulation induced by Hh pathway antagonism were mitigated by the addition of FGF4 protein. These findings implicate a multicomponent signaling hierarchy activated by Hh ligands from the embryonic node and executed by FGF signals in nascent mesoderm to control anterior mesoderm patterning.

Cell cycle regulators control mesoderm specification in human pluripotent stem cells

The mesoderm is one of the three germ layers produced during gastrulation from which muscle, bones, kidneys, and the cardiovascular system originate. Understanding the mechanisms that control mesoderm specification could inform many applications, including the development of regenerative medicine therapies to manage diseases affecting these tissues. Here, we used human pluripotent stem cells to investigate the role of cell cycle in mesoderm formation. To this end, using small molecules or conditional gene knockdown, we inhibited proteins controlling G1 and G2/M cell cycle phases during the differentiation of human pluripotent stem cells into lateral plate, cardiac, and presomitic mesoderm. These loss-of-function experiments revealed that regulators of the G1 phase, such as cyclin-dependent kinases and pRb (retinoblastoma protein), are necessary for efficient mesoderm formation in a context-dependent manner. Further investigations disclosed that inhibition of the G2/M regulator cyclin-dependent kinase 1 decreases BMP (bone morphogenetic protein) signaling activity specifically during lateral plate mesoderm formation while reducing fibroblast growth factor/extracellular signaling-regulated kinase 1/2 activity in all mesoderm subtypes. Taken together, our findings reveal that cell cycle regulators direct mesoderm formation by controlling the activity of key developmental pathways.

The mouse fetal-placental arterial connection: A paradigm involving the primitive streak and visceral endoderm with implications for human development

In Placentalia, the fetus depends upon an organized vascular connection with its mother for survival and development. Yet, this connection was, until recently, obscure. Here, we summarize how two unrelated tissues, the primitive streak, or body axis, and extraembryonic visceral endoderm collaborate to create and organize the fetal-placental arterial connection in the mouse gastrula. The primitive streak reaches into the extraembryonic space, where it marks the site of arterial union and creates a progenitor cell pool. Through contact with the streak, associated visceral endoderm undergoes an epithelial-to-mesenchymal transition, contributing extraembryonic mesoderm to the placental arterial vasculature, and to the allantois, or pre-umbilical tissue. In addition, visceral endoderm bifurcates into the allantois where, with the primitive streak, it organizes the nascent umbilical artery and promotes allantoic elongation to the chorion, the site of fetal-maternal exchange. Brachyury mediates streak extension and vascular patterning, while Hedgehog is involved in visceral endoderm's conversion to mesoderm. A unique CASPASE-3-positive cell separates streak- and non-streak-associated domains in visceral endoderm. Based on these new insights at the posterior embryonic-extraembryonic interface, we conclude by asking whether so-called primordial germ cells are truly antecedents to the germ line that segregate within the allantois, or whether they are placental progenitor cells. Incorporating these new working hypotheses into mutational analyses in which the placentae are affected will aid understanding a spectrum of disorders, including orphan diseases, which often include abnormalities of the umbilical cord, yolk sac, and hindgut, whose developmental relationship to each other has, until now, been poorly understood. This article is categorized under: Birth Defects > Associated with Preimplantation and Gastrulation Early Embryonic Development > Gastrulation and Neurulation.

Stem Cell Therapy for Hypoplastic Left Heart Syndrome: Mechanism, Clinical Application, and Future Directions

Hypoplastic left heart syndrome is a type of congenital heart disease characterized by underdevelopment of the left ventricle, outflow tract, and aorta. The condition is fatal if aggressive palliative operations are not undertaken, but even after the complete 3-staged surgical palliation, there is significant morbidity because of progressive and ultimately intractable right ventricular failure. For this reason, there is interest in developing novel therapies for the management of right ventricular dysfunction in patients with hypoplastic left heart syndrome. Stem cell therapy may represent one such innovative approach. The field has identified numerous stem cell populations from different tissues (cardiac or bone marrow or umbilical cord blood), different age groups (adult versus neonate-derived), and different donors (autologous versus allogeneic), with preclinical and clinical experience demonstrating the potential utility of each cell type. Preclinical trials in small and large animal models have elucidated several mechanisms by which stem cells affect the injured myocardium. Our current understanding of stem cell activity is undergoing a shift from a paradigm based on cellular engraftment and differentiation to one recognizing a primarily paracrine effect. Recent studies have comprehensively evaluated the individual components of the stem cells' secretomes, shedding new light on the intracellular and extracellular pathways at the center of their therapeutic effects. This research has laid the groundwork for clinical application, and there are now several trials of stem cell therapies in pediatric populations that will provide important insights into the value of this therapeutic strategy in the management of hypoplastic left heart syndrome and other forms of congenital heart disease. This article reviews the many stem cell types applied to congenital heart disease, their preclinical investigation and the mechanisms by which they might affect right ventricular dysfunction in patients with hypoplastic left heart syndrome, and finally, the completed and ongoing clinical trials of stem cell therapy in patients with congenital heart disease.

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.

Mesoderm specification and diversification: from single cells to emergent tissues

The three germ layers - mesoderm, endoderm and ectoderm - constituting the cellular blueprint for the tissues and organs that will form during embryonic development, are specified at gastrulation. Cells of mesodermal origin are the most abundant in the human body, representing a great variety of cell types, including the musculoskeletal system (bone, cartilage and muscle), cardiovascular system (heart, blood and blood vessels), as well as the connective tissues found throughout our bodies. A long-standing question pertains how this panoply of mesodermal cell types arises in a stereotypical fashion in time and space. This review discusses the events associated with mesoderm specification, highlighting the reconstruction of putative developmental trajectories facilitated by recent single-cell 'omic' data. We will also discuss the potential of emergent organoid systems to emulate and interrogate the dynamics of lineage specification at cellular resolution.

Role of Epigenetics in Cardiac Development and Congenital Diseases

The heart is the first organ to be functional in the fetus. Heart formation is a complex morphogenetic process regulated by both genetic and epigenetic mechanisms. Congenital heart diseases (CHD) are the most prominent congenital diseases. Genetics is not sufficient to explain these diseases or the impact of them on patients. Epigenetics is more and more emerging as a basis for cardiac malformations. This review brings the essential knowledge on cardiac biology of development. It further provides a broad background on epigenetics with a focus on three-dimensional conformation of chromatin. Then, we summarize the current knowledge of the impact of epigenetics on cardiac cell fate decision. We further provide an update on the epigenetic anomalies in the genesis of CHD.

SCL/TAL1 cooperates with Polycomb RYBP-PRC1 to suppress alternative lineages in blood-fated cells

During development, it is unclear if lineage-fated cells derive from multilineage-primed progenitors and whether active mechanisms operate to restrict cell fate. Here we investigate how mesoderm specifies into blood-fated cells. We document temporally restricted co-expression of blood (Scl/Tal1), cardiac (Mesp1) and paraxial (Tbx6) lineage-affiliated transcription factors in single cells, at the onset of blood specification, supporting the existence of common progenitors. At the same time-restricted stage, absence of SCL results in expansion of cardiac/paraxial cell populations and increased cardiac/paraxial gene expression, suggesting active suppression of alternative fates. Indeed, SCL normally activates expression of co-repressor ETO2 and Polycomb-PRC1 subunits (RYBP, PCGF5) and maintains levels of Polycomb-associated histone marks (H2AK119ub/H3K27me3). Genome-wide analyses reveal ETO2 and RYBP co-occupy most SCL target genes, including cardiac/paraxial loci. Reduction of Eto2 or Rybp expression mimics Scl-null cardiac phenotype. Therefore, SCL-mediated transcriptional repression prevents mis-specification of blood-fated cells, establishing active repression as central to fate determination processes.

In Toto Imaging and Reconstruction of Post-Implantation Mouse Development at the Single-Cell Level

The mouse embryo has long been central to the study of mammalian development; however, elucidating the cell behaviors governing gastrulation and the formation of tissues and organs remains a fundamental challenge. A major obstacle is the lack of live imaging and image analysis technologies capable of systematically following cellular dynamics across the developing embryo. We developed a light-sheet microscope that adapts itself to the dramatic changes in size, shape, and optical properties of the post-implantation mouse embryo and captures its development from gastrulation to early organogenesis at the cellular level. We furthermore developed a computational framework for reconstructing long-term cell tracks, cell divisions, dynamic fate maps, and maps of tissue morphogenesis across the entire embryo. By jointly analyzing cellular dynamics in multiple embryos registered in space and time, we built a dynamic atlas of post-implantation mouse development that, together with our microscopy and computational methods, is provided as a resource. VIDEO ABSTRACT.

Hand Factors in Cardiac Development

Congenital heart defects account for 1% of infant mortality and 10% of in utero deaths. As the vertebrate embryo develops, multiple tissue types develop in tandem to morphologically pattern the functional heart. Underlying cardiac development is a network of transcription factors known to tightly control these morphological events. Members of the Twist family of basic helix-loop-helix transcription factors, Hand1 and Hand2, are essential to this process. The expression patterns and functional role of Hand factors in neural crest cells, endocardium, myocardium, and epicardium is indicative of their importance during cardiogenesis; however, to date, an extensive understanding of the transcriptional targets of Hand proteins and their overall mechanism of action remain unclear. In this review, we summarize the recent findings that further outline the crucial functions of Hand factors during heart development and in post-natal heart function. Anat Rec, 302:101-107, 2019. © 2018 Wiley Periodicals, Inc.

Notch signaling in the division of germ layers in bilaterian embryos

Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/β-catenin pathway.

Inducing hair follicle neogenesis with secreted proteins enriched in embryonic skin

Organ development is a sophisticated process of self-organization. However, despite growing understanding of the developmental mechanisms, little is known about how to reactivate them postnatally for regeneration. We found that treatment of adult non-hair fibroblasts with cell-free extract from embryonic skin conferred upon them the competency to regenerate hair follicles. Proteomics analysis identified three secreted proteins enriched in the embryonic skin, apolipoprotein-A1, galectin-1 and lumican that together were essential and sufficient to induce new hair follicles. These 3 proteins show a stage-specific co-enrichment in the perifolliculogenetic embryonic dermis. Mechanistically, exposure to embryonic skin extract or to the combination of the 3 proteins altered the gene expression to an inductive hair follicle dermal papilla fibroblast-like profile and activated Igf and Wnt signaling, which are crucial for the regeneration process. Therefore, a cocktail of organ-specific extracellular proteins from the embryonic environment can render adult cells competent to re-engage in developmental interactions for organ neogenesis. Identification of factors that recreate the extracellular context of respective developing tissues can become an important strategy to promote regeneration in adult organs.

The canonical way to make a heart: β-catenin and plakoglobin in heart development and remodeling

The main mediator of the canonical Wnt pathway, β-catenin, is a major effector of embryonic development, postnatal tissue homeostasis, and adult tissue regeneration. The requirement for β-catenin in cardiogenesis and embryogenesis has been well established. However, many questions regarding the molecular mechanisms by which β-catenin and canonical Wnt signaling regulate these developmental processes remain unanswered. An interesting question that emerged from our studies concerns how β-catenin signaling is modulated through interaction with other factors. Recent experimental data implicate new players in canonical Wnt signaling, particularly those which modulate β-catenin function in many its biological processes, including cardiogenesis. One of the interesting candidates is plakoglobin, a little-studied member of the catenin family which shares several mechanistic and functional features with its close relative, β-catenin. Here we have focused on the function of β-catenin in cardiogenesis. We also summarize findings on plakoglobin signaling function and discuss possible interplays between β-catenin and plakoglobin in the regulation of embryonic heart development. Impact statement Heart development, function, and remodeling are complex processes orchestrated by multiple signaling networks. This review examines our current knowledge of the role of canonical Wnt signaling in cardiogenesis and heart remodeling, focusing primarily on the mechanistic action of its effector β-catenin. We summarize the generally accepted understanding of the field based on experimental in vitro and in vivo data, and address unresolved questions in the field, specifically relating to the role of canonical Wnt signaling in heart maturation and regeneration. What are the modulators of canonical Wnt, and particularly what are the potential roles of plakoglobin, a close relative of β-catenin, in regulating Wnt signaling?Answers to these questions will enhance our understanding of the mechanism by which the canonical Wnt signaling regulates development of the heart and its regeneration after damage.

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.