High-frequency ultrasonographic imaging of avian cardiovascular development

Experimental assessment of cardiovascular physiology in the chick embryo

High resolution assessment of cardiac functional parameters is crucial in translational animal research. The chick embryo is a historically well-used in vivo model for cardiovascular research due to its many practical advantages, and the conserved form and function of the chick and human cardiogenesis programs. This review aims to provide an overview of several different technical approaches for chick embryo cardiac assessment. Doppler echocardiography, optical coherence tomography, micromagnetic resonance imaging, microparticle image velocimetry, real-time pressure monitoring, and associated issues with the techniques will be discussed. Alongside this discussion, we also highlight recent advances in cardiac function measurements in chick embryos.

OCT Meets micro-CT: A Subject-Specific Correlative Multimodal Imaging Workflow for Early Chick Heart Development Modeling

Structural and Doppler velocity data collected from optical coherence tomography have already provided crucial insights into cardiac morphogenesis. X-ray microtomography and other ex vivo methods have elucidated structural details of developing hearts. However, by itself, no single imaging modality can provide comprehensive information allowing to fully decipher the inner workings of an entire developing organ. Hence, we introduce a specimen-specific correlative multimodal imaging workflow combining OCT and micro-CT imaging which is applicable for modeling of early chick heart development—a valuable model organism in cardiovascular development research. The image acquisition and processing employ common reagents, lab-based micro-CT imaging, and software that is free for academic use. Our goal is to provide a step-by-step guide on how to implement this workflow and to demonstrate why those two modalities together have the potential to provide new insight into normal cardiac development and heart malformations leading to congenital heart disease.

High-resolution ultrasound and speckle tracking: a non-invasive approach to assess in vivo gastrointestinal motility during development

Gastrointestinal motor activity has been extensively studied in adults; however, only few studies have investigated fetal motor skills. It is unknown when the gastrointestinal tract starts to contract during the embryonic period and how this function evolves during development. Here, we adapted a non-invasive high-resolution echography technique combined with speckle tracking analysis to examine the gastrointestinal tract motor activity dynamics during chick embryo development. We provided the first recordings of fetal gastrointestinal motility in living embryos without anesthesia. We found that, although gastrointestinal contractions appear very early during development, they become synchronized only at the end of the fetal period. To validate this approach, we used various pharmacological inhibitors and BAPX1 gene overexpression in vivo. We found that the enteric nervous system determines the onset of the synchronized contractions in the stomach. Moreover, alteration of smooth muscle fiber organization led to an impairment of this functional activity. Altogether, our findings show that non-invasive high-resolution echography and speckle tracking analysis allows visualization and quantification of gastrointestinal motility during development and highlight the progressive acquisition of functional and coordinated gastrointestinal motility before birth.

HLHS: Power of the Chick Model

Background: Hypoplastic left heart syndrome (HLHS) is a rare but deadly form of human congenital heart disease, most likely of diverse etiologies. Hemodynamic alterations such as those resulting from premature foramen ovale closure or aortic stenosis are among the possible pathways. Methods: The information gained from studies performed in the chick model of HLHS is reviewed. Altered hemodynamics leads to a decrease in myocyte proliferation causing hypoplasia of the left heart structures and their functional changes. Conclusions: Although the chick phenocopy of HLHS caused by left atrial ligation is certainly not representative of all the possible etiologies, it provides many useful hints regarding the plasticity of the genetically normal developing myocardium under altered hemodynamic loading leading to the HLHS phenotype, and even suggestions on some potential strategies for prenatal repair.

Photoacoustic flow velocity imaging based on complex field decorrelation

Photoacoustic (PA) imaging can be used to monitor flowing blood inside the microvascular and capillary bed. Ultrasound speckle decorrelation based velocimetry imaging was previously shown to accurately estimate blood flow velocity in mouse brain (micro-)vasculature. Translating this method to photoacoustic imaging will allow simultaneous imaging of flow velocity and extracting functional parameters like blood oxygenation. In this study, we use a pulsed laser diode and a quantitative method based on normalized first order field autocorrelation function of PA field fluctuations to estimate flow velocities in an ink tube phantom and in the microvasculature of the chorioallantoic membrane of a chicken embryo. We demonstrate how the decorrelation time of signals acquired over frames are related to the flow speed and show that the PA flow analysis based on this approach is an angle independent flow velocity imaging method.

Effect of left atrial ligation-driven altered inflow hemodynamics on embryonic heart development: clues for prenatal progression of hypoplastic left heart syndrome

Congenital heart defects (CHDs) are abnormalities in the heart structure present at birth. One important condition is hypoplastic left heart syndrome (HLHS) where severely underdeveloped left ventricle (LV) cannot support systemic circulation. HLHS usually initiates as localized tissue malformations with no underlying genetic cause, suggesting that disturbed hemodynamics contribute to the embryonic development of these defects. Left atrial ligation (LAL) is a surgical procedure on embryonic chick resulting in a phenotype resembling clinical HLHS. In this study, we investigated disturbed hemodynamics and deteriorated cardiac growth following LAL to investigate possible mechanobiological mechanisms for the embryonic development of HLHS. We integrated techniques such as echocardiography, micro-CT and computational fluid dynamics (CFD) for these analyses. Specifically, LAL procedure causes an immediate flow disturbance over atrioventricular (AV) cushions. At later stages after the heart septation, it causes hemodynamic disturbances in LV. As a consequence of the LAL procedure, the left-AV canal and LV volume decrease in size, and in the opposite way, the right-AV canal and right ventricle volume increase. According to our CFD analysis, LAL results in an immediate decrease in the left AV canal WSS levels for 3.5-day (HH21) pre-septated hearts. For 7-day post-septated hearts (HH30), LAL leads to further reduction in WSS levels in the left AV canal, and relatively increased WSS levels in the right AV canal. This study demonstrates the critical importance of the disturbed hemodynamics during the heart valve and ventricle development.

Monitoring of tumor growth and vascularization with repetitive ultrasonography in the chicken chorioallantoic-membrane-assay

The chorioallantoic-membrane (CAM)-assay is an established model for in vivo tumor research. Contrary to rodent-xenograft-models, the CAM-assay does not require breeding of immunodeficient strains due to native immunodeficiency. This allows xenografts to grow on the non-innervated CAM without pain or impairment for the embryo. Considering multidirectional tumor growth, limited monitoring capability of tumor size is the main methodological limitation of the CAM-assay for tumor research. Enclosure of the tumor by the radiopaque eggshell and the small structural size only allows monitoring from above and challenges established imaging techniques. We report the eligibility of ultrasonography for repetitive visualization of tumor growth and vascularization in the CAM-assay. After tumor ingrowth, ultrasonography was repetitively performed in ovo using a commercial ultrasonographic scanner. Finally, the tumor was excised and histologically analyzed. Tumor growth and angiogenesis were successfully monitored and findings in ultrasonographic imaging significantly correlated with results obtained in histological analysis. Ultrasonography is cost efficient and widely available. Tumor imaging in ovo enables the longitudinal monitoring of tumoral development, yet allowing high quantitative output due to the CAM-assays simple and cheap methodology. Thus, this methodological novelty improves reproducibility in the field of in vivo tumor experimentation emphasizing the CAM-assay as an alternative to rodent-xenograft-models.

Live mechanistic assessment of localized cardiac pumping in mammalian tubular embryonic heart

SIGNIFICANCE

Understanding how the valveless embryonic heart pumps blood is essential to elucidate biomechanical cues regulating cardiogenesis, which is important for the advancement of congenital heart defects research. However, methods capable of embryonic cardiac pumping analysis remain limited, and assessing this highly dynamic process in mammalian embryos is challenging. New approaches are critically needed to address this hurdle.

AIM

We report an imaging-based approach for functional assessment of localized pumping dynamics in the early tubular embryonic mouse heart.

APPROACH

Four-dimensional optical coherence tomography was used to obtain structural and Doppler hemodynamic imaging of the beating heart in live mouse embryos at embryonic day 9.25. The pumping assessment was performed based on the volumetric blood flow rate, flow resistance within the heart tube, and pressure gradient induced by heart wall movements. The relation between the blood flow, the pressure gradient, and the resistance to flow were evaluated through temporal analyses and Granger causality test.

RESULTS

In the ventricles, our method revealed connections between the temporal profiles of pressure gradient and volumetric blood flow rate. Statistically significant causal relation from the pressure gradient to the blood flow was demonstrated. Our analysis also suggests that cardiac pumping in the early ventricles is a combination of suction and pushing. In contrast, in the outflow tract, where the conduction wave is slower than the blood flow, we did not find significant causal relation from pressure to flow, suggesting that, different from ventricular regions, the local active contraction of the outflow tract is unlikely to drive the flow in that region.

CONCLUSIONS

We present an imaging-based approach that enables localized assessment of pumping dynamics in the mouse tubular embryonic heart. This method creates a new opportunity for functional analysis of the pumping mechanism underlying the developing mammalian heart at early stages and could be useful for studying biomechanical changes in mutant embryonic hearts that model congenital heart defects.

Validating the Paradigm That Biomechanical Forces Regulate Embryonic Cardiovascular Morphogenesis and Are Fundamental in the Etiology of Congenital Heart Disease

The goal of this review is to provide a broad overview of the biomechanical maturation and regulation of vertebrate cardiovascular (CV) morphogenesis and the evidence for mechanistic relationships between function and form relevant to the origins of congenital heart disease (CHD). The embryonic heart has been investigated for over a century, initially focusing on the chick embryo due to the opportunity to isolate and investigate myocardial electromechanical maturation, the ability to directly instrument and measure normal cardiac function, intervene to alter ventricular loading conditions, and then investigate changes in functional and structural maturation to deduce mechanism. The paradigm of "Develop and validate quantitative techniques, describe normal, perturb the system, describe abnormal, then deduce mechanisms" was taught to many young investigators by Dr. Edward B. Clark and then validated by a rapidly expanding number of teams dedicated to investigate CV morphogenesis, structure-function relationships, and pathogenic mechanisms of CHD. Pioneering studies using the chick embryo model rapidly expanded into a broad range of model systems, particularly the mouse and zebrafish, to investigate the interdependent genetic and biomechanical regulation of CV morphogenesis. Several central morphogenic themes have emerged. First, CV morphogenesis is inherently dependent upon the biomechanical forces that influence cell and tissue growth and remodeling. Second, embryonic CV systems dynamically adapt to changes in biomechanical loading conditions similar to mature systems. Third, biomechanical loading conditions dynamically impact and are regulated by genetic morphogenic systems. Fourth, advanced imaging techniques coupled with computational modeling provide novel insights to validate regulatory mechanisms. Finally, insights regarding the genetic and biomechanical regulation of CV morphogenesis and adaptation are relevant to current regenerative strategies for patients with CHD.

Follow Me! A Tale of Avian Heart Development with Comparisons to Mammal Heart Development

Avian embryos have been used for centuries to study development due to the ease of access. Because the embryos are sheltered inside the eggshell, a small window in the shell is ideal for visualizing the embryos and performing different interventions. The window can then be covered, and the embryo returned to the incubator for the desired amount of time, and observed during further development. Up to about 4 days of chicken development (out of 21 days of incubation), when the egg is opened the embryo is on top of the yolk, and its heart is on top of its body. This allows easy imaging of heart formation and heart development using non-invasive techniques, including regular optical microscopy. After day 4, the embryo starts sinking into the yolk, but still imaging technologies, such as ultrasound, can tomographically image the embryo and its heart in vivo. Importantly, because like the human heart the avian heart develops into a four-chambered heart with valves, heart malformations and pathologies that human babies suffer can be replicated in avian embryos, allowing a unique developmental window into human congenital heart disease. Here, we review avian heart formation and provide comparisons to the mammalian heart.

The Memory of the Heart

The embryological development of the heart is one of the most fascinating phenomena in nature and so is its final structure and function. The various ontogenetic passages form the evolutive basis of the final configuration of the heart. Each key step can be recognized in the final features, as the heart maintains a kind of “memory” of these passages. We can identify the major lines of development of the heart and trace these lines up to the mature organ. The aim of this review is to identify these key parameters of cardiac structure and function as essential elements of the heart’s proper functioning and bases for its treatment. We aim to track key steps of heart development to identify what it “remembers” and maintains in its final form as positively selected. A new vision based on the whole acquired knowledge must guide an in-depth scientific approach in future papers and guidelines on the topic and a complete, farsighted therapeutic conduct able to ensure the physiological correction of cardiac pathologies. The application of this modern, functional vision of the heart could improve the clinical treatment of heart disease, filling the gaps still present.

Influence of blood flow on cardiac development

The role of hemodynamics in cardiovascular development is not well understood. Indeed, it would be remarkable if it were, given the dauntingly complex array of intricately synchronized genetic, molecular, mechanical, and environmental factors at play. However, with congenital heart defects affecting around 1 in 100 human births, and numerous studies pointing to hemodynamics as a factor in cardiovascular morphogenesis, this is not an area in which we can afford to remain in the dark. This review seeks to present the case for the importance of research into the biomechanics of the developing cardiovascular system. This is accomplished by i) illustrating the basics of some of the highly complex processes involved in heart development, and discussing the known influence of hemodynamics on those processes; ii) demonstrating how altered hemodynamic environments have the potential to bring about morphological anomalies, citing studies in multiple animal models with a variety of perturbation methods; iii) providing examples of widely used technological innovations which allow for accurate measurement of hemodynamic parameters in embryos; iv) detailing the results of studies in avian embryos which point to exciting correlations between various hemodynamic manipulations in early development and phenotypic defect incidence in mature hearts; and finally, v) stressing the relevance of uncovering specific biomechanical pathways involved in cardiovascular formation and remodeling under adverse conditions, to the potential treatment of human patients. The time is ripe to unravel the contributions of hemodynamics to cardiac development, and to recognize their frequently neglected role in the occurrence of heart malformation phenotypes.

Increased Hemodynamic Load in Early Embryonic Stages Alters Myofibril and Mitochondrial Organization in the Myocardium

Normal blood flow is essential for proper heart formation during embryonic development, as abnormal hemodynamic load (blood pressure and shear stress) results in cardiac defects seen in congenital heart disease (CHD). However, the detrimental remodeling processes that relate altered blood flow to cardiac malformation and defects remain unclear. Heart development is a finely orchestrated process with rapid transformations that occur at the tissue, cell, and subcellular levels. Myocardial cells play an essential role in cardiac tissue maturation by aligning in the direction of stretch and increasing the number of contractile units as hemodynamic load increases throughout development. This study elucidates the early effects of altered blood flow on myofibril and mitochondrial configuration in the outflow tract myocardium in vivo. Outflow tract banding was used to increase hemodynamic load in the chicken embryo heart between Hamburger and Hamilton stages 18 and 24 (~24 h during tubular heart stages). 3D focused ion beam scanning electron microscopy analysis determined that increased hemodynamic load induced changes in the developing myocardium, characterized by thicker myofibril bundles that were more disbursed in circumferential orientation, and mitochondria that organized in large clusters around the nucleus. Proteomic mass-spectrometry analysis quantified altered protein composition after banding that is consistent with altered myofibril thin filament assembly and function, and mitochondrial maintenance and organization. Additionally, pathway analysis of the proteomics data identified possible activation of signaling pathways in response to banding, including the renin-angiotensin system (RAS). Imaging and proteomic data combined indicate that myofibril and mitochondrial arrangement in early embryonic stages is a critical developmental process that when disturbed by altered blood flow may contribute to cardiac malformation and defects.

Endocardial Fibroelastosis is Secondary to Hemodynamic Alterations in the Chick Embryonic Model of Hypoplastic Left Heart Syndrome

BACKGROUND

Endocardial fibroelastosis (EFE) is a diffuse thickening of the ventricular endocardium, causing myocardial dysfunction and presenting as unexplained heart failure in infants and children. One of the postulated causes is persistent and increased wall tension in the ventricles.

RESULTS

To examine whether reduced ventricular pressure in a chick model of hypoplastic left heart syndrome (HLHS) induced by left atrial ligation (LAL) at embryonic day (ED) 4 is associated with EFE at later stages, myocardial fibrosis was evaluated by histology and immunoconfocal microscopy and mass spectrometry (MS) at ED12. Immunohistochemistry with collagen I antibody clearly showed a significant thickening of the layer of subendocardial fibrous tissue in LAL hearts, and MS proved this significant increase of collagen I. To provide further insight into pathogenesis of this increased fibroproduction, hypoxyprobe staining revealed an increased extent of hypoxic regions, normally limited to the interventricular septum, in the ventricular myocardium of LAL hearts at ED8.

CONCLUSIONS

Abnormal hemodynamic loading during heart development leads to myocardial hypoxia, stimulating collagen production in the subendocardium. Therefore, EFE in this chick embryonic model of HLHS appears to be a secondary effect of abnormal hemodynamics. Developmental Dynamics 247:509-520, 2018. © 2017 Wiley Periodicals, Inc.

Effect of altered haemodynamics on the developing mitral valve in chick embryonic heart

Intracardiac haemodynamics is crucial for normal cardiogenesis, with recent evidence showing valvulogenesis is haemodynamically dependent and inextricably linked with shear stress. Although valve anomalies have been associated with genetic mutations, often the cause is unknown. However, altered haemodynamics have been suggested as a pathogenic contributor to bicuspid aortic valve disease. Conversely, how abnormal haemodynamics impacts mitral valve development is still poorly understood. In order to analyse altered blood flow, the outflow tract of the chick heart was constricted using a ligature to increase cardiac pressure overload. Outflow tract-banding was performed at HH21, with harvesting at crucial valve development stages (HH26, HH29 and HH35). Although normal valve morphology was found in HH26 outflow tract banded hearts, smaller and dysmorphic mitral valve primordia were seen upon altered haemodynamics in histological and stereological analysis at HH29 and HH35. A decrease in apoptosis, and aberrant expression of a shear stress responsive gene and extracellular matrix markers in the endocardial cushions were seen in the chick HH29 outflow tract banded hearts. In addition, dysregulation of extracellular matrix (ECM) proteins fibrillin-2, type III collagen and tenascin were further demonstrated in more mature primordial mitral valve leaflets at HH35, with a concomitant decrease of ECM cross-linking enzyme, transglutaminase-2. These data provide compelling evidence that normal haemodynamics are a prerequisite for normal mitral valve morphogenesis, and abnormal blood flow could be a contributing factor in mitral valve defects, with differentiation as a possible underlying mechanism.

Increased Hemodynamic Load in Early Embryonic Stages Alters Endocardial to Mesenchymal Transition

Normal blood flow is essential for proper heart formation during embryonic development, as abnormal hemodynamic load (blood pressure and shear stress) results in cardiac defects seen in congenital heart disease. However, the progressive detrimental remodeling processes that relate altered blood flow to cardiac defects remain unclear. Endothelial-mesenchymal cell transition is one of the many complex developmental events involved in transforming the early embryonic outflow tract into the aorta, pulmonary trunk, interventricular septum, and semilunar valves. This study elucidated the effects of increased hemodynamic load on endothelial-mesenchymal transition remodeling of the outflow tract cushions in vivo. Outflow tract banding was used to increase hemodynamic load in the chicken embryo heart between Hamburger and Hamilton stages 18 and 24. Increased hemodynamic load induced increased cell density in outflow tract cushions, fewer cells along the endocardial lining, endocardium junction disruption, and altered periostin expression as measured by confocal microscopy analysis. In addition, 3D focused ion beam scanning electron microscopy analysis determined that a portion of endocardial cells adopted a migratory shape after outflow tract banding that is more irregular, elongated, and with extensive cellular projections compared to normal cells. Proteomic mass-spectrometry analysis quantified altered protein composition after banding that is consistent with a more active stage of endothelial-mesenchymal transition. Outflow tract banding enhances the endothelial-mesenchymal transition phenotype during formation of the outflow tract cushions, suggesting that endothelial-mesenchymal transition is a critical developmental process that when disturbed by altered blood flow gives rise to cardiac malformation and defects.

Human embryonic cardiovascular function

INTRODUCTION

This review presents an overview of descriptive knowledge on human embryonic cardiovascular physiology mostly based on noninvasive assessment by Doppler ultrasonography. Our objective was to identify and analyze published studies on embryonic cardiovascular function, and summarize available knowledge in this field.

MATERIAL AND METHODS

Citations related to human embryonic cardiovascular function were searched in PubMed, EMBASE, CINAHL and Web of Science using keywords and MeSH terms without any time limitation. The search was restricted to English language articles. Abstracts were screened and full texts of relevant articles were obtained. All articles that reported on physiological aspects of human embryonic cardiovascular function were included. Studies reporting on cardiovascular function after 10 weeks of gestation were excluded. Data were synthesized and presented narratively.

RESULTS

We identified 10 studies that had evaluated cardiovascular function and/or hemodynamics in human embryos at ≤10 weeks of gestation. All of these reported only certain aspects of embryonic cardiovascular function. Embryonic heart rate is associated significantly with gestational age and increases from 6 to 10 weeks of gestation. Cardiac inflow is monophasic during the embryonic period and atria appear to generate higher force during contraction compared with ventricles. Both ventricular inflow and outflow velocities increase with advancing gestation, whereas the Tei index decreases significantly. During the embryonic period, placental blood flow increases with gestation, but absent umbilical artery diastolic flow and umbilical venous pulsations are normal phenomena.

CONCLUSION

There are important differences in normal cardiovascular function between the embryonic and fetal stages of human in utero development.

The avian embryo to study development of the cardiac conduction system

The avian embryo has long been a popular model system in developmental biology. The easy accessibility of the embryo makes it particularly suitable for in ovo microsurgery and manipulation. Re-incubation of the embryo allows long-term follow-up of these procedures. The current review focuses on the variety of techniques available to study development of the cardiac conduction system in avian embryos. Based on the large amount of relevant data arising from experiments in avian embryos, we conclude that the avian embryo has and will continue to be a powerful model system to study development in general and the developing cardiac conduction system in particular.

Blood flow dynamics reflect degree of outflow tract banding in Hamburger-Hamilton stage 18 chicken embryos

Altered blood flow during embryonic development has been shown to cause cardiac defects; however, the mechanisms by which the resulting haemodynamic forces trigger heart malformation are unclear. This study used heart outflow tract banding to alter normal haemodynamics in a chick embryo model at HH18 and characterized the immediate blood flow response versus the degree of band tightness. Optical coherence tomography was used to acquire two-dimensional longitudinal structure and Doppler velocity images from control (n = 16) and banded (n = 25, 6-64% measured band tightness) embryos, from which structural and velocity data were extracted to estimate haemodynamic measures. Peak blood flow velocity and wall shear rate (WSR) initially increased linearly with band tightness (p < 0.01), but then velocity plateaued between 40% and 50% band tightness and started to decrease with constriction greater than 50%, whereas WSR continued to increase up to 60% constriction before it began decreasing with increased band tightness. Time of flow decreased with constriction greater than 20% (p < 0.01), while stroke volume in banded embryos remained comparable to control levels over the entire range of constriction (p > 0.1). The haemodynamic dependence on the degree of banding reveals immediate adaptations of the early embryonic cardiovascular system and could help elucidate a range of cardiac adaptations to gradually increased load.

Using optical coherence tomography to rapidly phenotype and quantify congenital heart defects associated with prenatal alcohol exposure

BACKGROUND

The most commonly used method to analyze congenital heart defects involves serial sectioning and histology. However, this is often a time-consuming process where the quantification of cardiac defects can be difficult due to problems with accurate section registration. Here we demonstrate the advantages of using optical coherence tomography, a comparatively new and rising technology, to phenotype avian embryo hearts in a model of fetal alcohol syndrome where a binge-like quantity of alcohol/ethanol was introduced at gastrulation.

RESULTS

The rapid, consistent imaging protocols allowed for the immediate identification of cardiac anomalies, including ventricular septal defects and misaligned/missing vessels. Interventricular septum thicknesses and vessel diameters for three of the five outflow arteries were also significantly reduced. Outflow and atrioventricular valves were segmented using image processing software and had significantly reduced volumes compared to controls. This is the first study to our knowledge that has 3D reconstructed the late-stage cardiac valves in precise detail to examine their morphology and dimensions.

CONCLUSIONS

We believe, therefore, that optical coherence tomography, with its ability to rapidly image and quantify tiny embryonic structures in high resolution, will serve as an excellent and cost-effective preliminary screening tool for developmental biologists working with a variety of experimental/disease models.

Congenital heart malformations induced by hemodynamic altering surgical interventions

Embryonic heart formation results from a dynamic interplay between genetic and environmental factors. Blood flow during early embryonic stages plays a critical role in heart development, as interactions between flow and cardiac tissues generate biomechanical forces that modulate cardiac growth and remodeling. Normal hemodynamic conditions are essential for proper cardiac development, while altered blood flow induced by surgical manipulations in animal models result in heart defects similar to those seen in humans with congenital heart disease. This review compares the altered hemodynamics, changes in tissue properties, and cardiac defects reported after common surgical interventions that alter hemodynamics in the early chick embryo, and shows that interventions produce a wide spectrum of cardiac defects. Vitelline vein ligation and left atrial ligation decrease blood pressure and flow; and outflow tract banding increases blood pressure and flow velocities. These three surgical interventions result in many of the same cardiac defects, which indicate that the altered hemodynamics interfere with common looping, septation and valve formation processes that occur after intervention and that shape the four-chambered heart. While many similar defects develop after the interventions, the varying degrees of hemodynamic load alteration among the three interventions also result in varying incidence and severity of cardiac defects, indicating that the hemodynamic modulation of cardiac developmental processes is strongly dependent on hemodynamic load.

Congenital heart malformations induced by hemodynamic altering surgical interventions

Embryonic heart formation results from a dynamic interplay between genetic and environmental factors. Blood flow during early embryonic stages plays a critical role in heart development, as interactions between flow and cardiac tissues generate biomechanical forces that modulate cardiac growth and remodeling. Normal hemodynamic conditions are essential for proper cardiac development, while altered blood flow induced by surgical manipulations in animal models result in heart defects similar to those seen in humans with congenital heart disease. This review compares the altered hemodynamics, changes in tissue properties, and cardiac defects reported after common surgical interventions that alter hemodynamics in the early chick embryo, and shows that interventions produce a wide spectrum of cardiac defects. Vitelline vein ligation and left atrial ligation decrease blood pressure and flow; and outflow tract banding increases blood pressure and flow velocities. These three surgical interventions result in many of the same cardiac defects, which indicate that the altered hemodynamics interfere with common looping, septation and valve formation processes that occur after intervention and that shape the four-chambered heart. While many similar defects develop after the interventions, the varying degrees of hemodynamic load alteration among the three interventions also result in varying incidence and severity of cardiac defects, indicating that the hemodynamic modulation of cardiac developmental processes is strongly dependent on hemodynamic load.

Studying dynamic events in the developing myocardium

Differentiation and conduction properties of the cardiomyocytes are critically dependent on physical conditioning both in vitro and in vivo. Historically, various techniques were introduced to study dynamic events such as electrical currents and changes in ionic concentrations in live cells, multicellular preparations, or entire hearts. Here we review this technological progress demonstrating how each improvement in spatial or temporal resolution provided answers to old and provoked new questions. We further demonstrate how high-speed optical mapping of voltage and calcium can uncover pacemaking potential within the outflow tract myocardium, providing a developmental explanation of ectopic beats originating from this region in the clinical settings.

Ethanol exposure alters early cardiac function in the looping heart: a mechanism for congenital heart defects?

Alcohol-induced congenital heart defects are frequently among the most life threatening and require surgical correction in newborns. The etiology of these defects, collectively known as fetal alcohol syndrome, has been the focus of much study, particularly involving cellular and molecular mechanisms. Few studies have addressed the influential role of altered cardiac function in early embryogenesis because of a lack of tools with the capability to assay tiny beating hearts. To overcome this gap in our understanding, we used optical coherence tomography (OCT), a nondestructive imaging modality capable of micrometer-scale resolution imaging, to rapidly and accurately map cardiovascular structure and hemodynamics in real time under physiological conditions. In this study, we exposed avian embryos to a single dose of alcohol/ethanol at gastrulation when the embryo is sensitive to the induction of birth defects. Late-stage hearts were analyzed using standard histological analysis with a focus on the atrio-ventricular valves. Early cardiac function was assayed using Doppler OCT, and structural analysis of the cardiac cushions was performed using OCT imaging. Our results indicated that ethanol-exposed embryos developed late-stage valvuloseptal defects. At early stages, they exhibited increased regurgitant flow and developed smaller atrio-ventricular cardiac cushions, compared with controls (uninjected and saline-injected embryos). The embryos also exhibited abnormal flexion/torsion of the body. Our evidence suggests that ethanol-induced alterations in early cardiac function have the potential to contribute to late-stage valve and septal defects, thus demonstrating that functional parameters may serve as early and sensitive gauges of cardiac normalcy and abnormalities.

Heart rate changes mediate the embryotoxic effect of antiarrhythmic drugs in the chick embryo

A significant increase in cardiovascular medication use during pregnancy occurred in recent years. Only limited evidence on safety profiles is available, and little is known about the mechanisms of adverse effect on the fetus. We hypothesized that drug-induced bradycardia is the leading mechanism of developmental toxicity. Embryotoxicity was tested in ovo after administration of various doses of metoprolol, carvedilol, or ivabradine. Embryonic day (ED) 4 and 8 chick embryos were studied by video microscopy and ultrasound biomicroscopy ex ovo after intraamniotic injection of the drug for a period of 30 min. Stroke volume was calculated by the Simpson method and prolate ellipsoid formula. Significant dose-dependent mortality was achieved in embryos injected with carvedilol and ivabradine. In ED4 embryos, metoprolol, carvedilol, and ivabradine reduced the heart rate by 33%, 27%, and 55%, respectively, compared with controls (6%). In ED8 embryos this effect was more pronounced with a heart rate reduction by 71%, 54%, and 53%, respectively (controls, 36%). Cardiac output decreased in all tested groups but only proved significant in the metoprolol group in ED8 embryos. The number of β-adrenergic receptors showed a downward tendency during embryonic development. A negative chronotropic effect of metoprolol, carvedilol, and ivabradine was increasingly pronounced with embryonic maturity despite a downward trend in the number of β-adrenergic receptors. This effect was associated with reduced cardiac output in chick embryos, probably leading to premature death. Although standard doses of these drugs appear relatively safe, high doses have a potentially adverse effect on the fetus through reduced heart rate.

Stress and strain adaptation in load-dependent remodeling of the embryonic left ventricle

Altered pressure in the developing left ventricle (LV) results in altered morphology and tissue material properties. Mechanical stress and strain may play a role in the regulating process. This study showed that confocal microscopy, three-dimensional reconstruction, and finite element analysis can provide a detailed model of stress and strain in the trabeculated embryonic heart. The method was used to test the hypothesis that end-diastolic strains are normalized after altered loading of the LV during the stages of trabecular compaction and chamber formation. Stage-29 chick LVs subjected to pressure overload and underload at stage 21 were reconstructed with full trabecular morphology from confocal images and analyzed with finite element techniques. Measured material properties and intraventricular pressures were specified in the models. The results show volume-weighted end-diastolic von Mises stress and strain averaging 50-82 % higher in the trabecular tissue than in the compact wall. The volume-weighted-average stresses for the entire LV were 115, 64, and 147 Pa in control, underloaded, and overloaded models, while strains were 11, 7, and 4 %; thus, neither was normalized in a volume-weighted sense. Localized epicardial strains at mid-longitudinal level were similar among the three groups and to strains measured from high-resolution ultrasound images. Sensitivity analysis showed changes in material properties are more significant than changes in geometry in the overloaded strain adaptation, although resulting stress was similar in both types of adaptation. These results emphasize the importance of appropriate metrics and the role of trabecular tissue in evaluating the evolution of stress and strain in relation to pressure-induced adaptation.

Quantitative in vivo imaging of embryonic development: opportunities and challenges

Animal models are critically important for a mechanistic understanding of embryonic morphogenesis. For decades, visualizing these rapid and complex multidimensional events has relied on projection images and thin section reconstructions. While much insight has been gained, fixed tissue specimens offer limited information on dynamic processes that are essential for tissue assembly and organ patterning. Quantitative imaging is required to unlock the important basic science and clinically relevant secrets that remain hidden. Recent advances in live imaging technology have enabled quantitative longitudinal analysis of embryonic morphogenesis at multiple length and time scales. Four different imaging modalities are currently being used to monitor embryonic morphogenesis: optical, ultrasound, magnetic resonance imaging (MRI), and micro-computed tomography (micro-CT). Each has its advantages and limitations with respect to spatial resolution, depth of field, scanning speed, and tissue contrast. In addition, new processing tools have been developed to enhance live imaging capabilities. In this review, we analyze each type of imaging source and its use in quantitative study of embryonic morphogenesis in small animal models. We describe the physics behind their function, identify some examples in which the modality has revealed new quantitative insights, and then conclude with a discussion of new research directions with live imaging.

Computational fluid dynamics of developing avian outflow tract heart valves

Hemodynamic forces play an important role in sculpting the embryonic heart and its valves. Alteration of blood flow patterns through the hearts of embryonic animal models lead to malformations that resemble some clinical congenital heart defects, but the precise mechanisms are poorly understood. Quantitative understanding of the local fluid forces acting in the heart has been elusive because of the extremely small and rapidly changing anatomy. In this study, we combine multiple imaging modalities with computational simulation to rigorously quantify the hemodynamic environment within the developing outflow tract (OFT) and its eventual aortic and pulmonary valves. In vivo Doppler ultrasound generated velocity profiles were applied to Micro-Computed Tomography generated 3D OFT lumen geometries from Hamburger-Hamilton (HH) stage 16-30 chick embryos. Computational fluid dynamics simulation initial conditions were iterated until local flow profiles converged with in vivo Doppler flow measurements. Results suggested that flow in the early tubular OFT (HH16 and HH23) was best approximated by Poiseuille flow, while later embryonic OFT septation (HH27, HH30) was mimicked by plug flow conditions. Peak wall shear stress (WSS) values increased from 18.16 dynes/cm(2) at HH16 to 671.24 dynes/cm(2) at HH30. Spatiotemporally averaged WSS values also showed a monotonic increase from 3.03 dynes/cm(2) at HH16 to 136.50 dynes/cm(2) at HH30. Simulated velocity streamlines in the early heart suggest a lack of mixing, which differed from classical ink injections. Changes in local flow patterns preceded and correlated with key morphogenetic events such as OFT septation and valve formation. This novel method to quantify local dynamic hemodynamics parameters affords insight into sculpting role of blood flow in the embryonic heart and provides a quantitative baseline dataset for future research.

Homocysteine exposure affects early hemodynamic parameters of embryonic chicken heart function

Maternal hyperhomocysteinemia has been associated with an increased risk of newborns with a congenital heart defect. This has been substantiated in the chicken embryo, as congenital heart defects have been induced after homocysteine treatment. Comparable heart defects are observed in venous clipping studies, a model of altered embryonic blood flow. Because of this overlap in heart defects, our aim was to test the hypothesis that homocysteine would cause alterations in embryonic heart function that precede the structural malformations previously described. Therefore, Doppler flow velocity waveforms were recorded in both primitive ventricles and the outflow tract of the embryonic heart of homocysteine treated and control chicken embryos at embryonic day 3.5. Homocysteine treatment consisted of 50 μL 0.05 M L-homocysteine thiolactone at 24, 48, and 72 hr. Homocysteine-treated embryos displayed significantly lower mean heart rates of 134 (SD 22) bpm, compared to 150 (14) bpm in control embryos. Homocysteine treatment caused an inhibiting effect on hemodynamic parameters, and altered heart function was presented by a shift in the proportions of the different wave times in percentage of total cycle time. Homocysteine induces changes in hemodynamic parameters of early embryonic chicken heart function. These changes may precede morphological changes and contribute to the development of CHD defects through alterations in shear stress and shear stress related genes, as seen before in venous clipping studies.

Microbubble contrast imaging of the cardiovascular system of the chick embyro

Ultrasound imaging of the chick embryo cardiovascular system is limited to B-scan and Doppler technologies. This study demonstrates microbubble contrast imaging of the embryonic cardiovascular anatomy and physiology. Day 8-19 (Hamburger & Hamilton Stage 34-43) chick embryos are examined in ovo using high-frequency ultrasound imaging through an opening in the blunt end (air cell) of the egg. A chorioallantoic vein is cannulated, and small boluses of octofluoropropane lipid microspheres (Definity®) are injected to visualize the chick embryo cardiovascular system. The entire chick embryo cardiovascular system including the two embryologic arteriovenous (AV) shunts can be visualized. More accurate physiologic measurements of ejection fractions and cardiac output measurements can be obtained using this technology. Microbubble contrast ultrasound imaging in the chick embryo greatly expands the ability to study cardiovascular development. Also, the two natural embryonic A-V shunts provide an excellent model to study the bioeffects of microbubbles in the arterial system.

Multiple influences of blood flow on cardiomyocyte hypertrophy in the embryonic zebrafish heart

Cardiomyocyte hypertrophy is a complex cellular behavior involving coordination of cell size expansion and myofibril content increase. Here, we investigate the contribution of cardiomyocyte hypertrophy to cardiac chamber emergence, the process during which the primitive heart tube transforms into morphologically distinct chambers and increases its contractile strength. Focusing on the emergence of the zebrafish ventricle, we observed trends toward increased cell surface area and myofibril content. To examine the extent to which these trends reflect coordinated hypertrophy of individual ventricular cardiomyocytes, we developed a method for tracking cell surface area changes and myofibril dynamics in live embryos. Our data reveal a previously unappreciated heterogeneity of ventricular cardiomyocyte behavior during chamber emergence: although cardiomyocyte hypertrophy was prevalent, many cells did not increase their surface area or myofibril content during the observed timeframe. Despite the heterogeneity of cell behavior, we often found hypertrophic cells neighboring each other. Next, we examined the impact of blood flow on the regulation of cardiomyocyte behavior during this phase of development. When blood flow through the ventricle was reduced, cell surface area expansion and myofibril content increase were both dampened, and the behavior of neighboring cells did not seem coordinated. Together, our studies suggest a model in which hemodynamic forces have multiple influences on cardiac chamber emergence: promoting both cardiomyocyte enlargement and myofibril maturation, enhancing the extent of cardiomyocyte hypertrophy, and facilitating the coordination of neighboring cell behaviors.

Optical coherence tomography captures rapid hemodynamic responses to acute hypoxia in the cardiovascular system of early embryos

BACKGROUND

The trajectory to heart defects may start in tubular and looping heart stages when detailed analysis of form and function is difficult by currently available methods. We used a novel method, Doppler optical coherence tomography (OCT), to follow changes in cardiovascular function in quail embryos during acute hypoxic stress. Chronic fetal hypoxia is a known risk factor for congenital heart diseases (CHDs). Decreased fetal heart rates during maternal obstructive sleep apnea suggest that studying fetal heart responses under acute hypoxia is warranted.

RESULTS

We captured responses to hypoxia at the critical looping heart stages. Doppler OCT revealed detailed vitelline arterial pulsed Doppler waveforms. Embryos tolerated 1 hr of hypoxia (5%, 10%, or 15% O(2) ), but exhibited changes including decreased systolic and increased diastolic duration in 5 min. After 5 min, slower heart rates, arrhythmic events and an increase in retrograde blood flow were observed. These changes suggested slower filling of the heart, which was confirmed by four-dimensional Doppler imaging of the heart itself.

CONCLUSIONS

Doppler OCT is well suited for rapid noninvasive screening for functional changes in avian embryos under near physiological conditions. Analysis of the accessible vitelline artery sensitively reflected changes in heart function and can be used for rapid screening. Acute hypoxia caused rapid hemodynamic changes in looping hearts and may be a concern for increased CHD risk.

Microarray analysis of normal and abnormal chick ventricular myocardial development

The left and right ventricle originate from distinct parts of the cardiac tube, and several genes are known to be differentially expressed in these compartments. The aims of this study were to determine developmental differences in gene expression between the left and right ventricle, and to assess the effect of altered hemodynamic loading. RNA was extracted from isolated left and right normal chick embryonic ventricles at embryonic day 6, 8, and 10, and from day 8 left atrial ligated hearts with hypoplastic left and dilated right ventricles. cRNA was hybridized to Affymetrix Chicken Genome array according to manufacturer protocols. Microarray analysis identified 302 transcripts that were differentially expressed between the left and right ventricle. Comparative analysis detected 91 genes that were different in left ventricles of ligated hearts compared to age-matched ventricles, while 66 were different in the right ones. A large number of the changes could be interpreted as a delay of normal maturation. The approach described in this study could be used as one of the measures to gauge success of surgical procedures for congenital heart disease and help in determining the optimal time frame for intervention to prevent onset of irreversible changes.

Avian models in teratology and developmental toxicology

The avian embryo is a long-standing model for developmental biology research. It also has proven utility for toxicology research both in ovo and in explant culture. Like mammals, avian embryos have an allantois and their developmental pathways are highly conserved with those of mammals, thus avian models have biomedical relevance. Fertile eggs are inexpensive and the embryo develops rapidly, allowing for high-throughput. The chick genome is sequenced and significant molecular resources are available for study, including the ability for genetic manipulation. The absence of a placenta permits the direct study of an agent's embryotoxic effects. Here, we present protocols for using avian embryos in toxicology research, including egg husbandry and hatch, toxicant delivery, and assessment of proliferation, apoptosis, and cardiac structure and function.

Quantitative three-dimensional imaging of live avian embryonic morphogenesis via micro-computed tomography

Many clinically relevant congenital malformations arise during mid to late embryonic stages. This period is challenging to image quantitatively in live embryos, necessitating the use of multiple specimens with increased experimental variability. Here we establish X-ray and blood-pool computed tomography (CT) contrast agent toxicity and teratogenesis thresholds for 3D Micro-CT imaging of live avian embryos. Day 4 chick embryos micro-injected with Visipaque™ (VP) developed for an additional 6 days without defect. X-ray radiation up to 798 mGy was nontoxic. Peak average contrast of 1,060 HU occurred within 1 hr of imaging at 50 μm resolution. VP-enhanced contrast persisted past 24 hr with delayed accumulation in the allantois. Regional volumes of VP-injected embryos were statistically identical to those of fixed embryos perfused with osmium tetroxide. We further quantified longitudinal volumetric morphogenesis of the allantois over 30 hr. These results demonstrate the safety and efficacy of contrast enhanced quantitative micro-CT imaging for live embryos.

Cardiac outflow and wall motion in hypothermic chick embryos

Cardiac outflow in the early developmental stage of a chick embryo is known to be highly variable depending on environmental temperature. To investigate the effects of environmental hypothermia on the blood flow in the outflow tract (OFT) of chick embryonic hearts, microscopic flow images were consecutively captured from chick embryos at HH stage 17 (2.5 days of incubation) at room temperature. Instantaneous velocity field information of blood flow in OFT was obtained using a micro-particle image velocimetry technique. The cyclic variations of the OFT vessel diameter and wall thickness were simultaneously measured. The experimental results show that environmental hypothermia causes bradycardia with a decrease in peak velocity during systole and the occurrence of backflow during diastole in the OFT. These abnormal phenomena seem to be attributed to the suppression of myocardial wall motion under hypothermic conditions.

Micro-ultrasound for preclinical imaging

Over the past decade, non-invasive preclinical imaging has emerged as an important tool to facilitate biomedical discovery. Not only have the markets for these tools accelerated, but the numbers of peer-reviewed papers in which imaging end points and biomarkers have been used have grown dramatically. High frequency 'micro-ultrasound' has steadily evolved in the post-genomic era as a rapid, comparatively inexpensive imaging tool for studying normal development and models of human disease in small animals. One of the fundamental barriers to this development was the technological hurdle associated with high-frequency array transducers. Recently, new approaches have enabled the upper limits of linear and phased arrays to be pushed from about 20 to over 50 MHz enabling a broad range of new applications. The innovations leading to the new transducer technology and scanner architecture are reviewed. Applications of preclinical micro-ultrasound are explored for developmental biology, cancer, and cardiovascular disease. With respect to the future, the latest developments in high-frequency ultrasound imaging are described.

Integration of an optical coherence tomography (OCT) system into an examination incubator to facilitate in vivo imaging of cardiovascular development in higher vertebrate embryos under stable physiological conditions

High-resolution in vivo imaging of higher vertebrate embryos over short or long time periods under constant physiological conditions is a technically challenging task for researchers working on cardiovascular development. In chick embryos, for example, various studies have shown that without appropriate maintenance of temperature, as one of the main environmental factors, the embryonic heart rate drops rapidly and often results in an increase in regurgitant flow. Hemodynamic parameters are critical stimuli for cardiovascular development that, for a correct evaluation of their developmental significance, should be documented under physiological conditions. However, previous studies were mostly carried out outside of an incubator or under suboptimal environmental conditions. Here we present, to the best of our knowledge, the first detailed description of an optical coherence tomography (OCT) system integrated into an examination incubator to facilitate real-time in vivo imaging of cardiovascular development under physiological environmental conditions. We demonstrate the suitability of this OCT examination incubator unit for use in cardiovascular development studies by examples of proof of principle experiments. We, furthermore, point out the need for use of examination incubators for physiological OCT examinations by documenting the effects of room climate (22°C) on the performance of the cardiovascular system of chick embryos (HH-stages 16/17). Upon exposure to room climate, chick embryos showed a fast drop in the heart rate and striking changes in the cardiac contraction behaviour and the blood flow through the vitelline circulation. We have documented these changes for the first time by M-mode OCT and Doppler M-mode OCT.

Hemodynamic patterning of the avian atrioventricular valve

In this study, we develop an innovative approach to rigorously quantify the evolving hemodynamic environment of the atrioventricular (AV) canal of avian embryos. Ultrasound generated velocity profiles were imported into Micro-Computed Tomography generated anatomically precise cardiac geometries between Hamburger-Hamilton (HH) stages 17 and 30. Computational fluid dynamic simulations were then conducted and iterated until results mimicked in vivo observations. Blood flow in tubular hearts (HH17) was laminar with parallel streamlines, but strong vortices developed simultaneous with expansion of the cushions and septal walls. For all investigated stages, highest wall shear stresses (WSS) are localized to AV canal valve-forming regions. Peak WSS increased from 19.34 dynes/cm(2) at HH17 to 287.18 dynes/cm(2) at HH30, but spatiotemporally averaged WSS became 3.62 dynes/cm(2) for HH17 to 9.11 dynes/cm(2) for HH30. Hemodynamic changes often preceded and correlated with morphological changes. These results establish a quantitative baseline supporting future hemodynamic analyses and interpretations.

Blood flow dynamics of one cardiac cycle and relationship to mechanotransduction and trabeculation during heart looping

Analyses of form-function relationships during heart looping are directly related to technological advances. Recent advances in four-dimensional optical coherence tomography (OCT) permit observations of cardiac dynamics at high-speed acquisition rates and high resolution. Real-time observation of the avian stage 13 looping heart reveals that interactions between the endocardial and myocardial compartments are more complex than previously depicted. Here we applied four-dimensional OCT to elucidate the relationships of the endocardium, myocardium, and cardiac jelly compartments in a single cardiac cycle during looping. Six cardiac levels along the longitudinal heart tube were each analyzed at 15 time points from diastole to systole. Using image analyses, the organization of mechanotransducing molecules, fibronectin, tenascin C, α-tubulin, and nonmuscle myosin II was correlated with specific cardiac regions defined by OCT data. Optical coherence microscopy helped to visualize details of cardiac architectural development in the embryonic mouse heart. Throughout the cardiac cycle, the endocardium was consistently oriented between the midline of the ventral floor of the foregut and the outer curvature of the myocardial wall, with multiple endocardial folds allowing high-volume capacities during filling. The cardiac area fractional shortening is much higher than previously published. The in vivo profile captured by OCT revealed an interaction of the looping heart with the extra-embryonic splanchnopleural membrane providing outside-in information. In summary, the combined dynamic and imaging data show the developing structural capacity to accommodate increasing flow and the mechanotransducing networks that organize to effectively facilitate formation of the trabeculated four-chambered heart.

High-speed optical coherence tomography imaging of the beating avian embryonic heart

Congenital heart defects (CHDs) affect thousands of newborns each year in the United States. Recent research using animal model systems indicates that the abnormal function of the early tubular heart precedes structural defects such as septal defects. Optical coherence tomography (OCT) is an imaging modality that can provide high spatial and temporal resolution to study both the structure and the function of the tubular heart. With technical advances in OCT imaging speed, especially with frequency domain OCT and image-based retrospective gating, it is now possible to image a beating avian embryonic heart in three dimensions under physiological conditions and follow morphogenesis over critical periods of developmental time. These technological advances have already revealed novel aspects of heart development. By expanding our understanding of heart development, research using OCT technology combined with other imaging modalities may eventually lead to strategies to predict, treat, and even prevent CHDs.

An ex-ovo chicken embryo culture system suitable for imaging and microsurgery applications

Understanding the relationships between genetic and microenvironmental factors that drive normal and malformed embryonic development is fundamental for discovering new therapeutic strategies. Advancements in imaging technology have enabled quantitative investigation of the organization and maturing of the body plan, but later stage embryonic morphogenesis is less clear. Chicken embryos are an attractive vertebrate animal model system for this application because of its ease of culture and surgical manipulation. Early embryos can be cultured for a short time on filter paper rings, which enables complete optical access for cell patterning and fate studies. Studying advanced developmental processes such as cardiac morphogenesis are traditionally performed through a window of the eggshell, but this technique limits optical access due to window size. We previously developed a simple method to culture whole embryos ex-ovo on hexagonal weigh boats for up to 10 days, which enabled high resolution imaging via ultrasonography. These cultures were difficult to transport, limiting the types of imaging tools available for live experiments. We here present an improved shell-less culture system with a cost-effective, portable environmental chamber. Eggs were cracked onto a hammock created by a polyurethane membrane (cling wrap) affixed circumferentially to a plastic cup partially filled with sterile water. The dimensions of the circumference and depth of the hammock were both critical to maintain surface tension, while the mechanics of the hammock and water beneath helped dampen vibrations induced by transportation. A small footprint circulating water bath was also developed to enable continuous temperature control during experimentation. We demonstrate the ability to culture embryos in this way for at least 14 days without morphogenic defect or delay and employ this system in several microsurgical and imaging applications.

Measurement of absolute blood flow velocity in outflow tract of HH18 chicken embryo based on 4D reconstruction using spectral domain optical coherence tomography

The measurement of blood-plasma absolute velocity distributions with high spatial and temporal resolution in vivo is important for the investigation of embryonic heart at its early stage of development. We introduce a novel method to measure absolute blood flow velocity based on high speed spectral domain optical coherence tomography (OCT) and apply it to measure velocities across the heart outflow tract (OFT) of a chicken embryo (stage HH18). First, we use the OCT system to acquire 4D 
[(x,y,z) + t] images of the OFT in vivo. Second, we reconstruct the 4D microstructural images and obtain the orientation of the OFT at its maximum expansion, from which the centerline of the OFT is calculated based on the OFT boundary segmentation. Assuming flow is parallel to the vessel orientation, the obtained centerline indicates the flow direction. Finally, the absolute flow velocity is evaluated based on the direction given by the centerline and the axial velocity obtained from Doppler OCT. Using this method, we compare flow velocity profiles at various positions along the chicken embryo OFT.

How does the tubular embryonic heart work? Looking for the physical mechanism generating unidirectional blood flow in the valveless embryonic heart tube

The heart is the first organ to function in vertebrate embryos. The human heart, for example, starts beating around the 21st embryonic day. During the initial phase of its pumping action, the embryonic heart is seen as a pulsating blood vessel that is built up by (1) an inner endothelial tube lacking valves, (2) a middle layer of extracellular matrix, and (3) an outer myocardial tube. Despite the absence of valves, this tubular heart generates unidirectional blood flow. This fact poses the question how it works. Visual examination of the pulsating embryonic heart tube shows that its pumping action is characterized by traveling mechanical waves sweeping from its venous to its arterial end. These traveling waves were traditionally described as myocardial peristaltic waves. It has, therefore, been speculated that the tubular embryonic heart works as a technical peristaltic pump. Recent hemodynamic data from living embryos, however, have shown that the pumping function of the embryonic heart tube differs in several respects from that of a technical peristaltic pump. Some of these data suggest that embryonic heart tubes work as valveless "Liebau pumps." In the present study, a review is given on the evolution of the two above-mentioned theories of early cardiac pumping mechanics. We discuss pros and cons for both of these theories. We show that the tubular embryonic heart works neither as a technical peristaltic pump nor as a classic Liebau pump. The question regarding how the embryonic heart tube works still awaits an answer.

Two-photon microscopy-guided femtosecond-laser photoablation of avian cardiogenesis: noninvasive creation of localized heart defects

Embryonic heart formation is driven by complex feedback between genetic and hemodynamic stimuli. Clinical congenital heart defects (CHD), however, often manifest as localized microtissue malformations with no underlying genetic mutation, suggesting that altered hemodynamics during embryonic development may play a role. An investigation of this relationship has been impaired by a lack of experimental tools that can create locally targeted cardiac perturbations. Here we have developed noninvasive optical techniques that can modulate avian cardiogenesis to dissect relationships between alterations in mechanical signaling and CHD. We used two-photon excited fluorescence microscopy to monitor cushion and ventricular dynamics and femtosecond pulsed laser photoablation to target micrometer-sized volumes inside the beating chick hearts. We selectively photoablated a small (∼100 μm radius) region of the superior atrioventricular (AV) cushion in Hamburger-Hamilton 24 chick embryos. We quantified via ultrasound that the disruption causes AV regurgitation, which resulted in a venous pooling of blood and severe arterial constriction. At 48 h postablation, quantitative X-ray microcomputed tomography imaging demonstrated stunted ventricular growth and pronounced left atrial dilation. A histological analysis demonstrated that the laser ablation produced defects localized to the superior AV cushion: a small quasispherical region of cushion tissue was completely obliterated, and the area adjacent to the myocardial wall was less cellularized. Both cushions and myocardium were significantly smaller than sham-operated controls. Our results highlight that two-photon excited fluorescence coupled with femtosecond pulsed laser photoablation should be considered a powerful tool for studying hemodynamic signaling in cardiac morphogenesis through the creation of localized microscale defects that may mimic clinical CHD.

In vivo imaging of the cyclic changes in cross-sectional shape of the ventricular segment of pulsating embryonic chick hearts at stages 14 to 17: a contribution to the understanding of the ontogenesis of cardiac pumping function

The cardiac cycle-related deformations of tubular embryonic hearts were traditionally described as concentric narrowing and widening of a tube of circular cross-section. Using optical coherence tomography (OCT), we have recently shown that, during the cardiac cycle, only the myocardial tube undergoes concentric narrowing and widening while the endocardial tube undergoes eccentric narrowing and widening, having an elliptic cross-section at end-diastole and a slit-shaped cross-section at end-systole. Due to technical limitations, these analyses were confined to early stages of ventricular development (chick embryos, stages 10-13). Using a modified OCT-system, we now document, for the first time, the cyclic changes in cross-sectional shape of beating embryonic ventricles at stages 14 to 17. We show that during these stages (1) a large area of diminished cardiac jelly appears at the outer curvature of the ventricular region associated with formation of endocardial pouches; (2) the ventricular endocardial lumen acquires a bell-shaped cross-section at end-diastole and becomes compressed like a fireplace bellows during systole; (3) the contracting portions of the embryonic ventricles display stretching along its baso-apical axis at end-systole. The functional significance of our data is discussed with respect to early cardiac pumping function.

Altered cardiac function and ventricular septal defect in avian embryos exposed to low-dose trichloroethylene

Trichloroethylene (TCE) is the most frequently reported organic groundwater contaminant in the United States. It is controversial whether gestational TCE exposure causes congenital heart defects. The basis for TCE's proposed cardiac teratogenicity is not well understood. We previously showed that chick embryos exposed to 8 ppb TCE during cardiac morphogenesis have reduced cardiac output and increased mortality. To further investigate TCE's cardioteratogenic potential, we exposed in ovo chick embryos to TCE and evaluated the heart thereafter. Significant mortality was observed following TCE exposures of 8-400 ppb during a narrow developmental period (Hamburger-Hamilton [HH] stages 15-20, embryo day ED2.3-3.5) that is characterized by myocardial expansion, secondary heart looping, and endocardial cushion formation. Of the embryos that died, most did so between ED5.5 and ED6.5. Echocardiography of embryos at ED5.5 found that TCE-exposed hearts displayed significant functional and morphological heterogeneity affecting heart rate, left ventricular mass, and wall thickness. Individual embryos were identified with cardiac hypertrophy as well as with hypoplasia. Chick embryos exposed to 8 ppb TCE at HH17 that survived to hatch exhibited a high incidence (38%, p < 0.01, n = 16) of muscular ventricular septal defects (VSDs) as detected by echocardiography and confirmed by gross dissection; no VSDs were found in controls (n = 14). The TCE-induced VSDs may be secondary to functional impairments that alter cardiac hemodynamics and subsequent ventricular foramen closure, an interpretation consistent with recent demonstrations that TCE impairs calcium handling in cardiomyocytes. These data demonstrate that TCE is a cardiac teratogen for chick.

Mathematical modeling of flow-generated forces in an in vitro system of cardiac valve development

Heart valve defects are the most common cardiac defects. Therefore, defining the mechanisms of cardiac valve development is critical to our understanding and treatment of these disorders. At early stages of embryonic cardiac development, the heart begins as a simple tube that then becomes constricted into separate atrial and ventricular regions by the formation of small, mound-like structures, called atrioventricular (AV) cushions. As valve development continues, these mounds fuse and then elongate into valve leaflets. A longstanding hypothesis proposes that blood flow-generated shear stress and pressure are critical in shaping the cushions into leaflets. Here we show results from a two-dimensional mathematical model that simulates the forces created by blood flow present in a developing chick heart and in our in vitro, tubular model system. The model was then used to predict flow patterns and the resulting forces in the in vitro system. The model indicated that forces associated with shear stress and pressure have comparable orders of magnitude and collectively produce a rotational profile around the cushion in the direction of flow and leaflet growth. Further, it was concluded that the replication of these forces on a cushion implanted in our tubular in vitro system is possible. Overall, the two-dimensional, mathematical model provides insight into the forces that occur during early cardiac valve elongation.