Uncovering the Number and Clonal Dynamics of Mesp1 Progenitors during Heart Morphogenesis

Cardiac construction-Recent advances in morphological and transcriptional modeling of early heart development

During human embryonic development the early establishment of a functional heart is vital to support the growing fetus. However, forming the embryonic heart is an extremely complex process, requiring spatiotemporally controlled cell specification and differentiation, tissue organization, and coordination of cardiac function. These complexities, in concert with the early and rapid development of the embryonic heart, mean that understanding the intricate interplay between these processes that help shape the early heart remains highly challenging. In this review I focus on recent insights from animal models that have shed new light on the earliest stages of heart development. This includes specification and organization of cardiac progenitors, cell and tissue movements that make and shape the early heart tube, and the initiation of the first beat in the developing heart. In addition I highlight relevant in vitro models that could support translation of findings from animal models to human heart development. Finally I discuss challenges that are being addressed in the field, along with future considerations that together may help move us towards a deeper understanding of how our hearts are made.

Understanding the cell fate and behavior of progenitors at the origin of the mouse cardiac mitral valve

Congenital heart malformations include mitral valve defects, which remain largely unexplained. During embryogenesis, a restricted population of endocardial cells within the atrioventricular canal undergoes an endothelial-to-mesenchymal transition to give rise to mitral valvular cells. However, the identity and fate decisions of these progenitors as well as the behavior and distribution of their derivatives in valve leaflets remain unknown. We used single-cell RNA sequencing (scRNA-seq) of genetically labeled endocardial cells and microdissected mouse embryonic and postnatal mitral valves to characterize the developmental road. We defined the metabolic processes underlying the specification of the progenitors and their contributions to subtypes of valvular cells. Using retrospective multicolor clonal analysis, we describe specific modes of growth and behavior of endocardial cell-derived clones, which build up, in a proper manner, functional valve leaflets. Our data identify how both genetic and metabolic mechanisms specifically drive the fate of a subset of endocardial cells toward their distinct clonal contribution to the formation of the valve.

Resolving morphogenesis into quantifiable cell behaviours

Morphogenesis is extremely diverse, but its systematic quantification to determine the physical mechanisms that produce different phenotypes is possible by quantifying the underlying cell behaviours. These are limited and definable: they consist of cell proliferation, orientation of cell division, cell rearrangement, directional matrix production, cell addition/subtraction and cell size/shape change. Although minor variations in these categories are possible, in sum they capture all possible morphogenetic behaviours. This article summarises these processes, discusses their measurement, and highlights some salient examples.

Dissecting mechanisms of chamber-specific cardiac differentiation and its perturbation following retinoic acid exposure

The specification of distinct cardiac lineages occurs before chamber formation and acquisition of bona fide atrial or ventricular identity. However, the mechanisms underlying these early specification events remain poorly understood. Here, we performed single cell analysis at the murine cardiac crescent, primitive heart tube and heart tube stages to uncover the transcriptional mechanisms underlying formation of atrial and ventricular cells. We find that progression towards differentiated cardiomyocytes occurs primarily based on heart field progenitor identity, and that progenitors contribute to ventricular or atrial identity through distinct differentiation mechanisms. We identify new candidate markers that define such differentiation processes and examine their expression dynamics using computational lineage trajectory methods. We further show that exposure to exogenous retinoic acid causes defects in ventricular chamber size, dysregulation in FGF signaling and a shunt in differentiation towards orthogonal lineages. Retinoic acid also causes defects in cell-cycle exit resulting in formation of hypomorphic ventricles. Collectively, our data identify, at a single cell level, distinct lineage trajectories during cardiac specification and differentiation, and the precise effects of manipulating cardiac progenitor patterning via retinoic acid signaling.

Branching structure of genealogies in spatially growing populations and its implications for population genetics inference

Spatial models where growth is limited to the population edge have been instrumental to understanding the population dynamics and the clone size distribution in growing cellular populations, such as microbial colonies and avascular tumours. A complete characterization of the coalescence process generated by spatial growth is still lacking, limiting our ability to apply classic population genetics inference to spatially growing populations. Here, we start filling this gap by investigating the statistical properties of the cell lineages generated by the two dimensional Eden model, leveraging their physical analogy with directed polymers. Our analysis provides quantitative estimates for population measurements that can easily be assessed via sequencing, such as the average number of segregating sites and the clone size distribution of a subsample of the population. Our results not only reveal remarkable features of the genealogies generated during growth, but also highlight new properties that can be misinterpreted as signs of selection if non-spatial models are inappropriately applied.

Role of Delta-Notch signalling molecules on cell-cell adhesion in determining heterogeneous chemical and cell morphological patterning

Cell mechanics and motility are responsible for collective motion of cells that result in overall deformation of epithelial tissues. On the other hand, contact-dependent cell-cell signalling is responsible for generating a large variety of intricate, self-organized, spatial patterns of the signalling molecules. Moreover, it is becoming increasingly clear that the combined mechanochemical patterns of cell shape/size and signalling molecules in the tissues, for example, in cancerous and sensory epithelium, are governed by mechanochemical coupling between chemical signalling and cell mechanics. However, a clear quantitative picture of how these two aspects of tissue dynamics, i.e., signalling and mechanics, lead to pattern and form is still emerging. Although, a number of recent experiments demonstrate that cell mechanics, cell motility, and cell-cell signalling are tightly coupled in many morphogenetic processes, relatively few modeling efforts have focused on an integrated approach. We extend the vertex model of an epithelial monolayer to account for contact-dependent signalling between adjacent cells and between non-adjacent neighbors through long protrusional contacts with a feedback mechanism wherein the adhesive strength between adjacent cells is controlled by the expression of the signalling molecules in those cells. Local changes in cell-cell adhesion lead to changes in cell shape and size, which in turn drives changes in the levels of signalling molecules. Our simulations show that even this elementary two-way coupling of chemical signalling and cell mechanics is capable of giving rise to a rich variety of mechanochemical patterns in epithelial tissues. In particular, under certain parametric conditions, bimodal distributions in cell size and shape are obtained, which resemble experimental observations in cancerous and sensory tissues.

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.

Emergence of heart and branchiomeric muscles in cardiopharyngeal mesoderm

Branchiomeric muscles of the head and neck originate in a population of cranial mesoderm termed cardiopharyngeal mesoderm that also contains progenitor cells contributing to growth of the embryonic heart. Retrospective lineage analysis has shown that branchiomeric muscles share a clonal origin with parts of the heart, indicating the presence of common heart and head muscle progenitor cells in the early embryo. Genetic lineage tracing and functional studies in the mouse, as well as in Ciona and zebrafish, together with recent experiments using single cell transcriptomics and multipotent stem cells, have provided further support for the existence of bipotent head and heart muscle progenitor cells. Current challenges concern defining where and when such common progenitor cells exist in mammalian embryos and how alternative myogenic derivatives emerge in cardiopharyngeal mesoderm. Addressing these questions will provide insights into mechanisms of cell fate acquisition and the evolution of vertebrate musculature, as well as clinical insights into the origins of muscle restricted myopathies and congenital defects affecting craniofacial and cardiac development.

Direct Reprogramming of Cardiac Fibroblasts to Repair the Injured Heart

Coronary heart disease is a leading cause of mortality and morbidity. Those that survive acute myocardial infarction are at significant risk of subsequent heart failure due to fibrotic remodelling of the infarcted myocardium. By applying knowledge from the study of embryonic cardiovascular development, modern medicine offers hope for treatment of this condition through regeneration of the myocardium by direct reprogramming of fibrotic scar tissue. Here, we will review mechanisms of cell fate specification leading to the generation of cardiovascular cell types in the embryo and use this as a framework in which to understand direct reprogramming. Driving expression of a network of transcription factors, micro RNA or small molecule epigenetic modifiers can reverse epigenetic silencing, reverting differentiated cells to a state of induced pluripotency. The pluripotent state can be bypassed by direct reprogramming in which one differentiated cell type can be transdifferentiated into another. Transdifferentiating cardiac fibroblasts to cardiomyocytes requires a network of transcription factors similar to that observed in embryonic multipotent cardiac progenitors. There is some flexibility in the composition of this network. These studies raise the possibility that the failing heart could one day be regenerated by directly reprogramming cardiac fibroblasts within post-infarct scar tissue.

Roadmap for the multiscale coupling of biochemical and mechanical signals during development

The way in which interactions between mechanics and biochemistry lead to the emergence of complex cell and tissue organization is an old question that has recently attracted renewed interest from biologists, physicists, mathematicians and computer scientists. Rapid advances in optical physics, microscopy and computational image analysis have greatly enhanced our ability to observe and quantify spatiotemporal patterns of signalling, force generation, deformation, and flow in living cells and tissues. Powerful new tools for genetic, biophysical and optogenetic manipulation are allowing us to perturb the underlying machinery that generates these patterns in increasingly sophisticated ways. Rapid advances in theory and computing have made it possible to construct predictive models that describe how cell and tissue organization and dynamics emerge from the local coupling of biochemistry and mechanics. Together, these advances have opened up a wealth of new opportunities to explore how mechanochemical patterning shapes organismal development. In this roadmap, we present a series of forward-looking case studies on mechanochemical patterning in development, written by scientists working at the interface between the physical and biological sciences, and covering a wide range of spatial and temporal scales, organisms, and modes of development. Together, these contributions highlight the many ways in which the dynamic coupling of mechanics and biochemistry shapes biological dynamics: from mechanoenzymes that sense force to tune their activity and motor output, to collectives of cells in tissues that flow and redistribute biochemical signals during development.

DAND5 Inactivation Enhances Cardiac Differentiation in Mouse Embryonic Stem Cells

Deciphering the clues of a regenerative mechanism for the mammalian adult heart would save millions of lives in the near future. Heart failure due to cardiomyocyte loss is still one of the significant health burdens worldwide. Here, we show the potential of a single molecule, DAND5, in mouse pluripotent stem cell-derived cardiomyocytes specification and proliferation. Dand5 loss-of-function generated the double of cardiac beating foci compared to the wild-type cells. The early formation of cardiac progenitor cells and the increased proliferative capacity of Dand5 KO mESC-derived cardiomyocytes contribute to the observed higher number of derived cardiac cells. Transcriptional profiling sequencing and quantitative RT-PCR assays showed an upregulation of early cardiac gene networks governing cardiomyocyte differentiation, cell cycling, and cardiac regenerative pathways but reduced levels of genes involved in cardiomyocyte maturation. These findings prompt DAND5 as a key driver for the generation and expansion of pluripotent stem cell-derived cardiomyocytes systems with further clinical application purposes.

Nkx2-5 defines distinct scaffold and recruitment phases during formation of the murine cardiac Purkinje fiber network

The ventricular conduction system coordinates heartbeats by rapid propagation of electrical activity through the Purkinje fiber (PF) network. PFs share common progenitors with contractile cardiomyocytes, yet the mechanisms of segregation and network morphogenesis are poorly understood. Here, we apply genetic fate mapping and temporal clonal analysis to identify murine cardiomyocytes committed to the PF lineage as early as E7.5. We find that a polyclonal PF network emerges by progressive recruitment of conductive precursors to this scaffold from a pool of bipotent progenitors. At late fetal stages, the segregation of conductive cells increases during a phase of rapid recruitment to build the definitive PF network through a non-cell autonomous mechanism. We also show that PF differentiation is impaired in Nkx2-5 haploinsufficient embryos leading to failure to extend the scaffold. In particular, late fetal recruitment fails, resulting in PF hypoplasia and persistence of bipotent progenitors. Our results identify how transcription factor dosage regulates cell fate divergence during distinct phases of PF network morphogenesis.

Here, the authors apply genetic fate mapping and temporal clonal analysis to study progenitor recruitment and network morphogenesis of murine cardiac Purkinje fibers. Additionally, they characterize how transcription factor dosage regulates cell fate divergence during distinct phases of this process.

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Increasing Neural Stem Cell Division Asymmetry and Quiescence Are Predicted to Contribute to the Age-Related Decline in Neurogenesis

Adult murine neural stem cells (NSCs) generate neurons in drastically declining numbers with age. How cellular dynamics sustain neurogenesis and how alterations with age may result in this decline are unresolved issues. We therefore clonally traced NSC lineages using confetti reporters in young and middle-aged adult mice. To understand the underlying mechanisms, we derived mathematical models that explain observed clonal cell type abundances. The best models consistently show self-renewal of transit-amplifying progenitors and rapid neuroblast cell cycle exit. In middle-aged mice, we identified an increased probability of asymmetric stem cell divisions at the expense of symmetric differentiation, accompanied by an extended persistence of quiescence between activation phases. Our model explains existing longitudinal population data and identifies particular cellular properties underlying adult NSC homeostasis and the aging of this stem cell compartment.

Cell-Size Pleomorphism Drives Aberrant Clone Dispersal in Proliferating Epithelia

As epithelial tissues develop, groups of cells related by descent tend to associate in clonal populations rather than dispersing within the cell layer. While this is frequently assumed to be a result of differential adhesion, precise mechanisms controlling clonal cohesiveness remain unknown. Here we employ computational simulations to modulate epithelial cell size in silico and show that junctions between small cells frequently collapse, resulting in clone-cell dispersal among larger neighbors. Consistent with similar dynamics in vivo, we further demonstrate that mosaic disruption of Drosophila Tor generates small cells and results in aberrant clone dispersal in developing wing disc epithelia. We propose a geometric basis for this phenomenon, supported in part by the observation that soap-foam cells exhibit similar size-dependent junctional rearrangements. Combined, these results establish a link between cell-size pleomorphism and the control of epithelial cell packing, with potential implications for understanding tumor cell dispersal in human 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.

The Functions of Long Non-Coding RNA during Embryonic Cardiovascular Development and Its Potential for Diagnosis and Treatment of Congenital Heart Disease

Congenital heart disease (CHD) arises due to errors during the embryonic development of the heart, a highly regulated process involving an interplay between cell-intrinsic transcription factor expression and intercellular signalling mediated by morphogens. Emerging evidence indicates that expression of these protein-coding genes is controlled by a plethora of previously unappreciated non-coding RNAs operating in complex feedback-control circuits. In this review, we consider the contribution of long non-coding RNA (lncRNA) to embryonic cardiovascular development before discussing applications to CHD diagnostics and therapeutics. We discuss the process of lineage restriction during cardiovascular progenitor cell differentiation, as well as the subsequent patterning of the cardiogenic progenitor fields, taking as an example the regulation of NODAL signalling in left-right patterning of the heart. lncRNA are a highly versatile group. Nuclear lncRNA can target specific genomic sequences and recruit chromatin remodelling complexes. Some nuclear lncRNA are transcribed from enhancers and regulate chromatin looping. Cytoplasmic lncRNA act as endogenous competitors for micro RNA, as well as binding and sequestering signalling proteins. We discuss features of lncRNA that limit their study by conventional methodology and suggest solutions to these problems.

In vivo analysis of cardiomyocyte proliferation during trabeculation

Cardiomyocyte proliferation is crucial for cardiac growth, patterning and regeneration; however, few studies have investigated the behavior of dividing cardiomyocytes in vivo Here, we use time-lapse imaging of beating hearts in combination with the FUCCI system to monitor the behavior of proliferating cardiomyocytes in developing zebrafish. Confirming in vitro observations, sarcomere disassembly, as well as changes in cell shape and volume, precede cardiomyocyte cytokinesis. Notably, cardiomyocytes in zebrafish embryos and young larvae mostly divide parallel to the myocardial wall in both the compact and trabecular layers, and cardiomyocyte proliferation is more frequent in the trabecular layer. While analyzing known regulators of cardiomyocyte proliferation, we observed that the Nrg/ErbB2 and TGFβ signaling pathways differentially affect compact and trabecular layer cardiomyocytes, indicating that distinct mechanisms drive proliferation in these two layers. In summary, our data indicate that, in zebrafish, cardiomyocyte proliferation is essential for trabecular growth, but not initiation, and set the stage to further investigate the cellular and molecular mechanisms driving cardiomyocyte proliferation in vivo.

Universality of clone dynamics during tissue development

The emergence of complex organs is driven by the coordinated proliferation, migration and differentiation of precursor cells. The fate behaviour of these cells is reflected in the time evolution their progeny, termed clones, which serve as a key experimental observable. In adult tissues, where cell dynamics is constrained by the condition of homeostasis, clonal tracing studies based on transgenic animal models have advanced our understanding of cell fate behaviour and its dysregulation in disease (1, 2). But what can be learned from clonal dynamics in development, where the spatial cohesiveness of clones is impaired by tissue deformations during tissue growth? Drawing on the results of clonal tracing studies, we show that, despite the complexity of organ development, clonal dynamics may converge to a critical state characterized by universal scaling behaviour of clone sizes. By mapping clonal dynamics onto a generalization of the classical theory of aerosols, we elucidate the origin and range of scaling behaviours and show how the identification of universal scaling dependences may allow lineage-specific information to be distilled from experiments. Our study shows the emergence of core concepts of statistical physics in an unexpected context, identifying cellular systems as a laboratory to study non-equilibrium statistical physics.

Revealing age-related changes of adult hippocampal neurogenesis using mathematical models

New neurons are continuously generated in the dentate gyrus of the adult hippocampus. This continuous supply of newborn neurons is important to modulate cognitive functions. Yet the number of newborn neurons declines with age. Increasing Wnt activity upon loss of dickkopf 1 can counteract both the decline of newborn neurons and the age-related cognitive decline. However, the precise cellular changes underlying the age-related decline or its rescue are fundamentally not understood. The present study combines a mathematical model and experimental data to address features controlling neural stem cell (NSC) dynamics. We show that available experimental data fit a model in which quiescent NSCs may either become activated to divide or may undergo depletion events, such as astrocytic transformation and apoptosis. Additionally, we demonstrate that old NSCs remain quiescent longer and have a higher probability of becoming re-activated than depleted. Finally, our model explains that high NSC-Wnt activity leads to longer time in quiescence while enhancing the probability of activation. Altogether, our study shows that modulation of the quiescent state is crucial to regulate the pool of stem cells throughout the life of an animal.

Summary: New deterministic and stochastic mathematical models are proposed to investigate adult neurogenesis in young, old and perturbed hippocampus, and quantified using population-level and clonal experimental data.

Chronic exposure to rapamycin and episodic serum starvation modulate ageing of human fibroblasts in vitro

Mild stress-induced activation of stress response (SR) pathways, such as autophagy, heat shock response, oxidative SR, DNA damage response, and inflammatory response, can be potentially health beneficial. Using the model system of cellular ageing and replicative senescence in vitro, we have studied the ageing modulatory effects of the two conditions, rapamycin and serum starvation. Chronic exposure to 0.1, 1 and 10 nM rapamycin positively modulated the survival, growth, morphology, telomere length, DNA methylation levels, 8-oxo-dG level in DNA, N⁶-methyl-adenosine level in RNA, and ethanol stress tolerance of serially passaged normal human skin fibroblasts. Furthermore, episodic (once a week) serum starvation of human skin fibroblasts extended their replicative lifespan by about 22%, along with the maintenance of early passage youthful morphology even in late passage cultures. Although the results of this study may be considered preliminary, it can be inferred that intermittent and episodic induction of SR, rather than chronic up-regulation of SR, is more effective and applicable in the practice of hormesis for healthy ageing and longevity.

Defining stem cell dynamics and migration during wound healing in mouse skin epidermis

Wound healing is essential to repair the skin after injury. In the epidermis, distinct stem cells (SCs) populations contribute to wound healing. However, how SCs balance proliferation, differentiation and migration to repair a wound remains poorly understood. Here, we show the cellular and molecular mechanisms that regulate wound healing in mouse tail epidermis. Using a combination of proliferation kinetics experiments and molecular profiling, we identify the gene signatures associated with proliferation, differentiation and migration in different regions surrounding the wound. Functional experiments show that SC proliferation, migration and differentiation can be uncoupled during wound healing. Lineage tracing and quantitative clonal analysis reveal that, following wounding, progenitors divide more rapidly, but conserve their homoeostatic mode of division, leading to their rapid depletion, whereas SCs become active, giving rise to new progenitors that expand and repair the wound. These results have important implications for tissue regeneration, acute and chronic wound disorders.

Wound healing is essential to repair the skin after injury and distinct stem cells in the epidermis are known to contribute to the process. Here the authors perform molecular, functional and clonal analysis and reveal the individual contribution of stem cells coming from different epidermal compartments to the wound-healing process in mice.

Cellular plasticity in cardiovascular development and disease

Knowledge on cellular differentiation pathways is critical to understanding organ development, homeostasis, and disease. Studying cell differentiation and cell fate restrictions in these contexts can be done through lineage tracing experiments, which entail permanent labeling of a cell and all its progeny. Recent lineage experiments within the cardiovascular system have uncovered unexpected findings on cellular origins during organogenesis and cell plasticity during disease. For example, there is increasing evidence that multiple progenitor sources exist for a single cell type, and that cells have remarkable expansive capacities under disease settings. Here, we summarize some recent findings in the cardiovascular system and highlight where there is evidence that the underlying concepts are a widespread phenomenon used by other organ systems. Developmental Dynamics 246:328-335, 2017. © 2016 Wiley Periodicals, Inc.

Foxa2 identifies a cardiac progenitor population with ventricular differentiation potential

The recent identification of progenitor populations that contribute to the developing heart in a distinct spatial and temporal manner has fundamentally improved our understanding of cardiac development. However, the mechanisms that direct atrial versus ventricular specification remain largely unknown. Here we report the identification of a progenitor population that gives rise primarily to cardiovascular cells of the ventricles and only to few atrial cells (<5%) of the differentiated heart. These progenitors are specified during gastrulation, when they transiently express Foxa2, a gene not previously implicated in cardiac development. Importantly, Foxa2+ cells contribute to previously identified progenitor populations in a defined pattern and ratio. Lastly, we describe an analogous Foxa2+ population during differentiation of embryonic stem cells. Together, these findings provide insight into the developmental origin of ventricular and atrial cells, and may lead to the establishment of new strategies for generating chamber-specific cell types from pluripotent stem cells.

A beginner's guide to tissue clearing

The last decade has seen a proliferation of tissue clearing methods that render large biological samples transparent and allow unprecedented three-dimensional views of enormous volumes of tissue. For a scientist wondering whether these methods will be useful to address their research problems, it can be bewildering to sort through the ever-increasing number of papers introducing new clearing methods. Here, I provide a concise summary for the novice describing what tissue clearing is, which research problems it can be applied to, how to decide on a clearing method, and where the field is headed in the future.

The Role of Aneuploidy in Cancer Evolution

Chromosomal aberrations during cell division represent one of the first recognized features of human cancer cells, and modern detection methods have revealed the pervasiveness of aneuploidy in cancer. The ongoing karyotypic changes brought about by chromosomal instability (CIN) contribute to tumor heterogeneity, drug resistance, and treatment failure. Whole-chromosome and segmental aneuploidies resulting from CIN have been proposed to allow "macroevolutionary" leaps that may contribute to profound phenotypic change. In this review, we will outline evidence indicating that aneuploidy and CIN contribute to cancer evolution.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

Ciona as a Simple Chordate Model for Heart Development and Regeneration

Cardiac cell specification and the genetic determinants that govern this process are highly conserved among Chordates. Recent studies have established the importance of evolutionarily-conserved mechanisms in the study of congenital heart defects and disease, as well as cardiac regeneration. As a basal Chordate, the Ciona model system presents a simple scaffold that recapitulates the basic blueprint of cardiac development in Chordates. Here we will focus on the development and cellular structure of the heart of the ascidian Ciona as compared to other Chordates, principally vertebrates. Comparison of the Ciona model system to heart development in other Chordates presents great potential for dissecting the genetic mechanisms that underlie congenital heart defects and disease at the cellular level and might provide additional insight into potential pathways for therapeutic cardiac regeneration.

Tracing cellular dynamics in tissue development, maintenance and disease

The coordination of cell proliferation and differentiation is central to the development and maintenance of tissues, while its dysregulation underlies the transition to diseased states. By combining lineage tracing with transcriptional profiling and marker-based assays, statistical methods are delivering insights into the dynamics of stem cells and their developmental precursors. These studies have provided evidence for molecular heterogeneity and fate priming, and have revealed a role for stochasticity in stem cell fate, refocusing the search for regulatory mechanisms. At the same time, they present a quantitative platform to study the initiation and progression of disease. Here, we review how quantitative lineage tracing strategies are shaping our understanding of the cellular mechanisms of tissue development, maintenance and disease.

Single-Cell Lineage Tracing Reveals that Oriented Cell Division Contributes to Trabecular Morphogenesis and Regional Specification

The cardiac trabeculae are sheet-like structures extending from the myocardium that function to increase surface area. A lack of trabeculation causes embryonic lethality due to compromised cardiac function. To understand the cellular and molecular mechanisms of trabecular formation, we genetically labeled individual cardiomyocytes prior to trabeculation via the brainbow multicolor system and traced and analyzed the labeled cells during trabeculation by whole-embryo clearing and imaging. The clones derived from labeled single cells displayed four different geometric patterns that are derived from different patterns of oriented cell division (OCD) and migration. Of the four types of clones, the inner, transmural, and mixed clones contributed to trabecular cardiomyocytes. Further studies showed that perpendicular OCD is an extrinsic asymmetric cell division that putatively contributes to trabecular regional specification. Furthermore, N-Cadherin deletion in labeled clones disrupted the clonal patterns. In summary, our data demonstrate that OCD contributes to trabecular morphogenesis and specification.