Cardia bifida and the development of pacemaker function in the early chick heart

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

Training biochemistry students in experimental developmental biology: Induction of cardia bifida formation in the chick embryo

A high variety of experimental model organisms have been used in developmental biology practical lectures. The work with developing embryos is crucial to make students aware of the multiple biological phenomena underlying normal animal embryogenesis and morphogenesis and represent a unique experimental platform to analyze the impact of molecular signaling in the regulation of all these processes. In particular, Biochemistry undergraduate students enjoy both practical and theoretical lectures on the molecular mechanisms of embryonic development, as that allows them for the integration of crucial molecular concepts (e.g. signaling and signal transduction mechanisms; molecular patterning of development) into the dynamic and progressive context of animal embryonic ontogenesis. Accordingly, it is important to carefully design practical laboratory lectures in developmental biology, as these are a unique pedagogical tools fostering the interests of the students in this subject. This study describes the design, implementation, and evaluation of a two-session laboratory practical activity performed by Biochemistry undergraduate students at University of Málaga (Spain). In this practical activity, which takes advantage of the unique characteristics of the chick embryo, students learn how the vertebrate heart forms from the fusion of two bilateral-symmetric cardiac progenitor pools under the guidance of the underlying endoderm. This cheap and easy practical laboratory activity provides relevant visual information on how experimental manipulations can severely influence anatomical form during organ development, as well as an excellent experimental setting to test molecular regulation of morphogenesis in an ex vivo (ex ovo) context.

Cardioids reveal self-organizing principles of human cardiogenesis

Organoids capable of forming tissue-like structures have transformed our ability to model human development and disease. With the notable exception of the human heart, lineage-specific self-organizing organoids have been reported for all major organs. Here, we established self-organizing cardioids from human pluripotent stem cells that intrinsically specify, pattern, and morph into chamber-like structures containing a cavity. Cardioid complexity can be controlled by signaling that instructs the separation of cardiomyocyte and endothelial layers and by directing epicardial spreading, inward migration, and differentiation. We find that cavity morphogenesis is governed by a mesodermal WNT-BMP signaling axis and requires its target HAND1, a transcription factor linked to developmental heart chamber defects. Upon cryoinjury, cardioids initiated a cell-type-dependent accumulation of extracellular matrix, an early hallmark of both regeneration and heart disease. Thus, human cardioids represent a powerful platform to mechanistically dissect self-organization, congenital heart defects and serve as a foundation for future translational research.

The Early Stages of Heart Development: Insights from Chicken Embryos

The heart is the first functioning organ in the developing embryo and a detailed understanding of the molecular and cellular mechanisms involved in its formation provides insights into congenital malformations affecting its function and therefore the survival of the organism. Because many developmental mechanisms are highly conserved, it is possible to extrapolate from observations made in invertebrate and vertebrate model organisms to humans. This review will highlight the contributions made through studying heart development in avian embryos, particularly the chicken. The major advantage of chick embryos is their accessibility for surgical manipulation and functional interference approaches, both gain- and loss-of-function. In addition to experiments performed in ovo, the dissection of tissues for ex vivo culture, genomic, or biochemical approaches is straightforward. Furthermore, embryos can be cultured for time-lapse imaging, which enables tracking of fluorescently labeled cells and detailed analysis of tissue morphogenesis. Owing to these features, investigations in chick embryos have led to important discoveries, often complementing genetic studies in mice and zebrafish. As well as including some historical aspects, we cover here some of the crucial advances made in understanding early heart development using the chicken model.

The endoderm and myocardium join forces to drive early heart tube assembly

Formation of the muscular layer of the heart, the myocardium, involves the medial movement of bilateral progenitor fields; driven primarily by shortening of the endoderm during foregut formation. Using a combination of time-lapse imaging, microsurgical perturbations and computational modeling, we show that the speed of the medial-ward movement of the myocardial progenitors is similar, but not identical to that of the adjacent endoderm. Further, the extracellular matrix microenvironment separating the two germ layers also moves with the myocardium, indicating that collective tissue motion and not cell migration drives tubular heart assembly. Importantly, as myocardial cells approach the midline, they perform distinct anterior-directed movements relative to the endoderm. Based on the analysis of microincision experiments and computational models, we propose two characteristic, autonomous morphogenetic activities within the early myocardium: 1) an active contraction of the medial portion of the heart field and 2) curling- the tendency of the unconstrained myocardial tissue to form a spherical surface with a concave ventral side. In the intact embryo, these deformations are constrained by the endoderm and the adjacent mesoderm, nevertheless the corresponding mechanical stresses contribute to the proper positioning of myocardial primordia.

Bending and twisting the embryonic heart: a computational model for c-looping based on realistic geometry

The morphogenetic process of cardiac looping transforms the straight heart tube into a curved tube that resembles the shape of the future four-chambered heart. Although great progress has been made in identifying the molecular and genetic factors involved in looping, the physical mechanisms that drive this process have remained poorly understood. Recent work, however, has shed new light on this complicated problem. After briefly reviewing the current state of knowledge, we propose a relatively comprehensive hypothesis for the mechanics of the first phase of looping, termed c-looping, as the straight heart tube deforms into a c-shaped tube. According to this hypothesis, differential hypertrophic growth in the myocardium supplies the main forces that cause the heart tube to bend ventrally, while regional growth and cytoskeletal contraction in the omphalomesenteric veins (primitive atria) and compressive loads exerted by the splanchnopleuric membrane drive rightward torsion. A computational model based on realistic embryonic heart geometry is used to test the physical plausibility of this hypothesis. The behavior of the model is in reasonable agreement with available experimental data from control and perturbed embryos, offering support for our hypothesis. The results also suggest, however, that several other mechanisms contribute secondarily to normal looping, and we speculate that these mechanisms play backup roles when looping is perturbed. Finally, some outstanding questions are discussed for future study.

Cardiac looping may be driven by compressive loads resulting from unequal growth of the heart and pericardial cavity. Observations on a physical simulation model

The transformation of the straight embryonic heart tube into a helically wound loop is named cardiac looping. Such looping is regarded as an essential process in cardiac morphogenesis since it brings the building blocks of the developing heart into an approximation of their definitive topographical relationships. During the past two decades, a large number of genes have been identified which play important roles in cardiac looping. However, how genetic information is physically translated into the dynamic form changes of the looping heart is still poorly understood. The oldest hypothesis of cardiac looping mechanics attributes the form changes of the heart loop (ventral bending → simple helical coiling → complex helical coiling) to compressive loads resulting from growth differences between the heart and the pericardial cavity. In the present study, we have tested the physical plausibility of this hypothesis, which we call the growth-induced buckling hypothesis, for the first time. Using a physical simulation model, we show that growth-induced buckling of a straight elastic rod within the confined space of a hemispherical cavity can generate the same sequence of form changes as observed in the looping embryonic heart. Our simulation experiments have furthermore shown that, under bilaterally symmetric conditions, growth-induced buckling generates left- and right-handed helices (D-/L-loops) in a 1:1 ratio, while even subtle left- or rightward displacements of the caudal end of the elastic rod at the pre-buckling state are sufficient to direct the buckling process toward the generation of only D- or L-loops, respectively. Our data are discussed with respect to observations made in biological “models.” We conclude that compressive loads resulting from unequal growth of the heart and pericardial cavity play important roles in cardiac looping. Asymmetric positioning of the venous heart pole may direct these forces toward a biased generation of D- or L-loops.

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

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

Time-lapse analysis reveals local asymmetrical changes in C-looping heart tube

Heart development has long served as a model system of left-right asymmetrical morphogenesis, and many key laterality genes have been shown to be involved in the process of asymmetrical heart looping. We established a time-lapse imaging system to observe the process of C-looping during chick heart development, and our observations showed that the C-looping is a very complicated process that involves several local changes in shape: the process can be divided into dextral rotation of the rostral and caudal segments with ventral bending in the rostral part and horizontal anti-clockwise rotation with enlargement of the left part in the caudal segment. Further experimental manipulations revealed characteristics of these morphological changes and regional interactions for the events, and we propose that asymmetrical enlargement of the caudal part is one of the targets of the laterality genes in the C-looping process.

Heart field: from mesoderm to heart tube

In this review we discuss the major morphogenetic and regulative events that control myocardial progenitor cells from the time that they delaminate from the epiblast in the primitive streak to their differentiation into cardiomyocytes in the heart tube. During chick and mouse embryogenesis, myocardial progenitor cells go through four specific processes that are sequential but overlapping: specification of the cardiogenic mesoderm, determination of the bilaterally symmetric heart fields, patterning of the heart field, and finally cardiomyocyte differentiation and formation of the heart tube. We describe the morphological and molecular events that play a pivotal role in each of these four processes.

Bidirectional fusion of the heart-forming fields in the developing chick embryo

It is generally thought that the early pre-tubular chick heart is formed by fusion of the anterior or cephalic limits of the paired cardiogenic fields. However, this study shows that the heart fields initially fuse at their midpoint to form a transitory "butterfly"-shaped, cardiogenic structure. Fusion then progresses bi-directionally along the longitudinal axis in both cranial and caudal directions. Using in vivo labeling, we demonstrate that cells along the ventral fusion line are highly motile, crossing future primitive segments. We found that mesoderm cells migrated cephalically from the unfused tips of the anterior/cephalic wings into the head mesenchyme in the region that has been called the secondary heart field. Perturbing the anterior/cranial fusion results in formation of a bi-conal heart. A theoretical role of the ventral fusion line acting as a "heart organizer" and its role in cardia bifida is discussed.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.

Symmetry and asymmetry in the development of inner organs in parabiotic twins of amphibians (Urodela)

Newt embryos of different developmental stages were combined to parabiotic twins in different positions. The exterior appearance and the symmetry relations, particularly of the internal organs (intestinal tract, heart, nuclei habenulae, and vitelline vein) were studied. Experimentally caused organ inversions allowed conclusions with respect to organ asymmetry and unilateral dominance. There was no direct correlation between appearance and symmetry of the exterior and the internal organs. All internal organs showed a continuous transition between normal and ideally inverse situs. The concordance of the organ situs differs greatly. The "left-hand side" or "right-hand side" dominance is not uniform. It depends on the type of fusion, i.e. the relative position of the parabiotic twins, and is often specific for a given organ. In some cases a non-genetic "symmetrisation factor" appears to be strongly active, depending on the fusion type and resulting in a dominant transindividual organ mirror image symmetry in the parabiotic twins. The older twin generally dominates the processes of determination and induction. The "symmetrisation factor" also acts on members of different families, i.e. genetically completely heterogeneous parabiotic twins. The development of organ asymmetry appears to be a process with several phases.

Fashioning the vertebrate heart: earliest embryonic decisions

Our goal here is to set out the types of unitary decisions made by heart progenitor cells, from their appearance in the heart field until they form the simple heart tube. This provides a context to evaluate cell fate, lineage and, finally, morphogenetic decisions that configure global heart form and function. Some paradigms for cellular differentiation and for pattern generation may be borrowed from invertebrates, but neither Drosophila nor Caenorhabditis elegans suffice to unravel higher order decisions. Genetic analyses in mouse and zebrafish may provide one entrance to these pathways.

Zebrafish tinman homolog demarcates the heart field and initiates myocardial differentiation

The fashioning of a vertebrate organ requires integration of decisions of cell fate by individual cells with those that regulate organotypic form. Logical candidates for this role, in an organ such as the heart, are genes that initiate the differentiation process leading to heart muscle and those that define the earliest embryonic heart field, but for neither class are genes defined. We cloned zebrafish Nkx2.5, a homolog of the tinman homeodomain gene needed for visceral and cardiac mesoderm formation in Drosophila. In the zebrafish, its expression is associated with cardiac precursor cells throughout development, even in the early gastrula, where the level of zebrafish Nkx2.5 is in a gradient which spatially matches the regional propensity of ventral-marginal cells to become heart. Overexpression of Nkx2.5 causes formation of disproportionally larger hearts in otherwise apparently normal embryos. Transplanted cell expressing high levels of Nkx2.5 express cardiac genes even in ectopic locales. Fibroblasts transfected with myc-tagged Nkx2.5 express cardiac genes. These effects require the homeodomain. Thus, Nkx2.5 appears to mark the earliest embryonic heart field and to be capable of initiating the cardiogenic differentiation program. Because ectopic cells or transfected fibroblasts do not beat, Nkx2.5 is likely to be but one step in the determination of cardiac myocyte cell fate. Its overexpression increases heart size, perhaps by bringing cells on the edge of the field to a threshold level for initiation of cardiac differentiation.

Gridlock, a localized heritable vascular patterning defect in the zebrafish

We are using the zebrafish, Danio rerio, to identify genes that generate and pattern the vertebrate vasculature. We have isolated a recessive mutation, gridlockm145 (grlm145) in which blood flow to the tail is impeded by a localized vascular defect. Using a novel microangiographic method, we show that the blockade is in the anterior trunk, where the paired lateral dorsal aortae normally merge to form the single midline aorta. Arterial-venous shunts and collateral vessels develop in most mutant embryos, bypassing the lesion and reconstituting caudal blood flow. The grl defect resembles coarctation of the aorta, a human congenital cardiovascular malformation of unknown aetiology, in the location of the lesion and its consequences and in the mutants' dependence on collateral vessels for survival.

Relationship between fibronectin expression during gastrulation and heart formation in the rat embryo

By utilizing myosin immunostaining, we were able to identify early rat myocardium as a thin epithelial sheet and realized that its cohesive movement toward the midline leads to the straight heart tube formation. Localization study of fibronectin mRNA and protein was, therefore, carried out to investigate its tissue origin and possible roles in facilitating mesoderm migration and heart formation. Fibronectin mRNAs were first detected throughout the mesoderm during the early primitive streak stage, suggesting that the mesoderm is the source of fibronectin. By pre-head fold (pre-somite) and head fold (early somite) stages, the mesoderm became largely down-regulated for fibronectin mRNAs, while it was also at these stages when myosin-positive myocardium formed itself into the epithelium and was subsequently folding toward the midline. Thus, there appears to be little fibronectin synthesis during and directly relevant to early heart tube formation. Later, during the early straight heart tube stage (5 somite and older), endocardium became highly positive for fibronectin mRNAs, suggesting that the endocardium is the major source of fibronectin for the cardiac jelly. Based on the results, we present a map for the early mammalian heart in which the heart is a single crescentic band lying in front of the prechordal plate. We also suggest a process for heart tube formation based on the cohesive movement of the myocardial epithelium. During heart tube formation, fibronectin protein had been deposited previously by the mesoderm and was found uniformly in the ECM and not newly produced by any adjacent tissue. The data contradict the endodermal guidance of heart migration by fibronectin gradient and suggest, instead, a permissive role for the fibronectin substrate.

Precardiac mesoderm is specified during gastrulation in quail

The establishment of precardiac mesoderm and the role of anterolateral endoderm and ectoderm in regulating heart muscle cell development have been studied in quail using explant cultures. Mesoderm from precardiac regions of stage 4(+)-6 embryos was explanted alone or in combination with adjacent endoderm or ectoderm, cultured for 12 to 72 hr in several types of culture media, and then assayed by morphological and immunocytochemical criteria for the presence of differentiated cardiac myocytes. Results show that mesoderm from heart forming regions is capable of differentiating into beating cardiac myocytes in a defined medium lacking potential signaling molecules by stage 4+, the earliest time at which we could isolate mesoderm from adjacent cell layers. Although an interaction with anterolateral endoderm from stage 4+ onward is therefore not required for the specification of precardiac mesoderm in quail, explants consisting of mesoderm plus endoderm show an enhanced rate of myocyte differentiation and a shortened delay between expression of myosin heavy chain and the onset of beating. Endoderm also plays a central role in early heart morphogenesis since beating heart tubes form only in explants that contain both mesoderm and endoderm. In contrast, ectoderm from stage 4(+)-5+ embryos does not support development of precardiac mesoderm. These results suggest that early heart muscle cell development involves an initial specification step that occurs prior to or during gastrulation and which leads to the appearance of myocardial precursor cells, and a subsequent differentiation step during which endoderm plays a central role in enhancing the rate of myocyte differentiation and the degree of heart tube morphogenesis.

In vitro analysis of cardiac progenitor cell differentiation

Cardiac myoblast commitment and differentiation were studied in the developing avian embryo. Single cell analysis of isolated cardiogenic cells grown in vitro established that stage 4 (newly gastrulated) mesodermal cells are capable of myocyte differentiation in the absence of intercellular contact or short range cellular interactions. While cardiac myocytes derived from single isolated progenitors expressed muscle-specific myosin heavy chains (MHC), atrial and ventricular MHCs characteristic of in vivo development were not detected. When the same progenitors were grown at high density or in organ cultures, cell-specific, expression of atrial and ventricular MHCs was observed, suggesting a role of cell density-dependent processes for differential MHC expression. Cardiogenic mesoderm (stages 4-8) was treated with the cocarcinogen 12-O-tetradecanoylphorbol-13-acetate (TPA), maintained as organ cultures, and assayed for muscle differentiation in an attempt to identify possible stage-specific variations in cardiac progenitors. TPA irreversibly blocked the differentiation of early (stages 4-7) progenitors. When exposed to TPA, stages 4-7 cardiogenic cells failed to synthesize several muscle-specific proteins as determined by immunochemical analysis of myosin synthesis and two-dimensional gel electrophoresis of 35S-labeled proteins isolated from cardiogenic cultures. In addition, stages 4-7, TPA-treated cells did not differentiate after the withdrawal of TPA. In contrast, TPA had no effect on the expression of several muscle-specific proteins in late (stage 8) cells including the cell-specific expression of atrial and ventricular MHCs.

Isomyosin expression pattern during formation of the tubular chicken heart: a three-dimensional immunohistochemical analysis

Three-dimensional (3-D) distribution of atrial and ventricular isomyosins is analysed immunohistochemically during the formation of the tubular chicken heart (stage 7 to 12 [H/H]) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively. This analysis revealed that both types of isomyosins can be first detected at stage 8 (H/H, possessing four pairs of somites), i.e., when the heart primordium still exists as two separate cardiogenic plates. The ventricular type of isomyosin is initially expressed in those areas of cardiogenic plates in the vicinity of the anterior intestinal portal. The atrial type of isomyosin is initially expressed in zones caudal and lateral to the areas of ventricular isomyosin expression. Medial to the atrial isomyosin-expressing areas, cardiogenic plate areas exist that initially lack myosin expression. Those parts of the cardiogenic plates that fuse in front of the anterior intestinal portal, thereby forming the heart tube, are characterized by the expression of both isomyosins; however, the caudolateral parts of the heart primordium maintain their single atrial isomyosin expression during further development. Cardiac contractions are therefore first observed at stage 10 (H/H, possessing ten pairs of somites) in myocardium that coexpresses both isomyosins.

Development of cardiac beat rate in early chick embryos is regulated by regional cues

The mesoderm of each of the paired lateral heart-forming regions (HFRs) in the stage 5-7 chick embryo includes prospective conus (pre-C), ventricle (pre-V), and sinoatrial (pre-SA) cells, arranged in a rostrocaudal sequence (C-V-SA). With microsurgery we divided each HFR into three rostrocaudally arranged segments. After 24 hr of further incubation, each segment differentiated into a spontaneously beating vesicle of heart tissue to form a multiheart embryo. The cardiac vesicles in these embryos expressed left-right and rostrocaudal beat rate gradients: the left caudal pre-SA mesoderm produced tissue with the fastest beat rate of the six while the rostral vesicle formed from right pre-C was the slowest. In another operation, we prevented the HFRs from fusing in the midline by cutting through the anterior intestinal portal at stage 8, to produce cardia bifida (CB) embryos with an independently beating half-heart on each side. In these cases, the left half-heart of 87.2% of CB embryos beat faster than the right, confirming the left-right difference in intrinsic beat rate. To assess whether the future beat rate of each region is already determined in the st 5-7 HFR, we exchanged rectangular fragments of left pre-SA mesoderm and attached endoderm with right pre-C fragments to yield a left HFR with the sequence C-V-C and a right HFR with the sequence SA-V-SA. A CB operation was subsequently performed on these exchange embryos to prevent fusion of the lateral HFRs. Preconus mesoderm, transplanted to the pre-SA region, differentiated into tissue with a rapid beat rate, while pre-SA mesoderm relocated to the preconus region formed heart tissue with a slow spontaneous rate typical of the conus. In 73% of the exchange CB embryos, the left half-heart beat faster than the right, despite the origins of its mesoderm. The exchanged mesoderm developed a rate that was appropriate for its new location rather than the site of origin of the mesodermal fragment. In a third set of operations, we implanted a fragment of st 15 differentiated conus tissue into a site lateral to the left caudal HFR in st 5, 6, and 7 embryos, and subsequently performed CB operations on them. The implant caused the adjacent half-heart to develop with a slower beat rate than in unoperated or sham-operated controls.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of retinoid deficiency on the development of the heart and vascular system of the quail embryo

The regulatory role of retinoids in growth and differentiation has been examined in vitro and in vivo by light and scanning electron microscopy using retinoid-deficient and control quail embryos between the 5-15 somite stage, as well as 2- and 2.5-day-old embryos. Fertile, retinoid-deficient eggs were obtained from flocks of quail maintained on a retinoid- and carotenoid-deficient diet, supplemented only with small amounts of retinoic acid methyl ester as described by Thompson et al. 1969. As described previously, retinoid deprivation during embryonal development causes abnormalities in organs of epithelial and mesenchymal origin, most dramatically preventing the formation of the extraembryonal circulatory system in the avian embryo. Our in vivo studies show that the basis for the latter defect is the failure of the primitive heart tubes to open at their posterior end, thus preventing the formation of omphalomesenteric veins normally connecting the embryonal with the extraembryonal circulatory system. Early manifestation of the retinoid-deficient defect may result also in formation of a cardia bifida, late manifestation in development of a single dilated ventricle. In contrast, the extraembryonal vascular system of blood islands is well developed. Heart function as shown by the rate of heart beat is reduced in deficient embryos. Our in vitro studies demonstrate similar defects in the development of the circulatory system by culture of normal 24-h embryos on retinoid-deficient agar medium; conversely, normal development is observed upon culture of retinoid-deficient embryos on retinoid-containing agar medium.

Development of electrical rhythmic activity in early embryonic cultured chick double-heart monitored optically with a voltage-sensitive dye

Double-hearted embryos were produced by whole-embryo culture of chick embryos which were microsurgically cut through the tissue of the anterior intestinal portal at the 1- to 6-somite developmental stage, at the time when the cardiac primordia have not yet fused in the bulboventricular region. The cultured embryos were removed from an incubator usually at the 7- to 10-somite stages of development, and then spontaneous electrical action potentials and/or contractions were optically recorded simultaneously from both the right and left half-hearts, using a 10 X 10- element photodiode matrix array together with a voltage-sensitive merocyanine-rhodanine dye (NK 2761). At the 7- to 8-somite stages, spontaneous action potentials were detected from bilateral prebeating half-hearts or sometimes from one half-heart. In each half-heart, the first spontaneous beating was often observed in the half-heart of the 9 somite embryos. In the beating half-hearts regular activity was always observed, while in the prebeating half-hearts at the 7- to 8-somite stages, both the regular and irregular rhythms of action potentials were detected, and the incidence of occurrence of regular activity significantly outnumbered that of the irregular rhythm. The heart rate in the left half-heart was faster than that in the right half-heart in the great majority of the prebeating and beating double-hearted embryos.

Localization of pacemaking activity in early embryonic heart monitored using voltage-sensitive dye

Early in cardiogenesis, heart primordia are brought together at the midline and fuse with each other progressively caudally-- this results in the formation of the primitive tubular heart which begins beating spontaneously at the middle period of the 9-somite developmental stage in the chick embryo. However, in these very early stages of development, the myocardial cells are small and technically difficult to impale with microelectrodes; thus electrophysiological studies on the very early embryonic heart are rare. Recently, potential sensitive dye-related absorption signals have provided a new method for monitoring spontaneous action potential activity in the early embryonic heart. This technique is based on the observation that changes in potential across membrane(s) stained with certain voltage-sensitive dyes are accompanied by changes in their optical properties (absorption, fluorescence, and/or birefringence). Using absorption signals, we have already demonstrated in embryonic pre-beating chick heart in the 7-8-somite stages, the occurrence of action potential activity, development of pacemaker potential and cardiac rhythm generation. With this method, originally introduced to record neuronal activity in invertebrate ganglia, many cells or portions of the preparation can be monitored simultaneously. Accordingly we have expanded the optical recording apparatus to monitor simultaneously spontaneous action potentials from five portions of an early embryonic heart, and report here experiments carried out on the embryonic hearts of chicks (white Leghorn) at the 7-11-somite developmental stages, corresponding to 25-35 h of incubation. The hearts attached to the embryo were stained with a merocyanine-rhodamine dye (NK2761) as a potentiometric probe. This dye is analogue of Dye XVII or Dye XXIII.

Development of form in the embryonic heart. An experimental approach

It is no longer necessary to guess at why embryonic malformations occur. Techniques are now available to test almost any hypothesis concerning developmental phenomena. These include the entire armamentarium of modern cell biology: organ and tissue culture, microsurgery, time-lapse cinematography, autoradiography, and many other related methodologies. The developmental events which lead to the formation of a functional heart in the early embryo can be analyzed with these techniques. Of the pertinent questions concerning developmental processes related to congenital heart disease, those of dextrocardia and differential growth, ventricular septal defects, and cell migration and adhesion in cardiogenesis are discussed. Several experimental systems useful for attacking these questions are described. Relative growth of specific segments of the primitive tubular heart contributed by the right and left wings of the cardiogenic crescent has been examined to elucidate the mechanism of dextral looping of the tube. Migration of precardiac cells to form the heart has been traced with cinematography in the intact embryo, and by following movements of cells labeled in situ with tritium using autoradiographic techniques. To analyze the mechanisms of guidance of such cells, endoderm plus adherent precardiac mesoderm can be explanted and manipulated in tissue culture.

Influence of the endoderm on heart differentiation during the early stages of development of the chicken embryo

1. In stages +4, 5 and +5 in the chick embryo, the bilateral extirpation of the endoderm in the entire extension of the P.H.M. region detains heart differentiation, giving rise to acardiac embryos. 2. The unilateral extirpation of the endoderm in the entire extension of the P.H.M. region detains heart differentiation of the primordium of the side operated on, forming a typical beating heart from the heterolateral primordium. 3. If the extirpation of the endoderm is partial, a small beating vesicle is formed on the side operated on, which has the characters of the ventricle bulb or atrium, according to which region of the endoderm was extirpated. 4. It is not possible to demonstrate a difference between right side and left side extirpations of the endoderm. 5. The formation of the fore-gut is detained completely or unilateraly due to the absence of the morphogenetic movements of the endoderm in the extirpated region. Neither are the fore-gut fissures formed. 6. The embryos which form are abnormal in the encephalic extremity, observing platyneuria and vascular dilatations with great frequency. 7. In the stages in which these experiments have been carried out, the P.H.M. is not found to be determined as yet and still needs the influence or induction of the endoderm in order to initiate its differentiation.