The embryonic vertebrate heart tube is a dynamic suction pump

Doppler flow velocity waveforms in the embryonic chicken heart at developmental stages corresponding to 5-8 weeks of human gestation

OBJECTIVES

To obtain Doppler velocity waveforms from the early embryonic chicken heart by means of ultrasound biomicroscopy and to compare these waveforms at different stages of embryonic development.

METHODS

We collected cardiac waveforms using high-frequency Doppler ultrasound with a 55-MHz transducer at Hamburger-Hamilton (HH) stages 18, 21 and 23, which are comparable to humans at 5 to 8 weeks of gestation. Waveforms were obtained at the inflow tract, the primitive left ventricle, the primitive right ventricle and at the outflow tract in 10 different embryos per stage. M-mode recordings were collected to study opening and closure of the cushions. By exploring the temporal relationship between the waveforms, using a secondary Doppler device, cardiac cycle events were outlined.

RESULTS

Our results demonstrate that stage- and location-dependent intracardiac blood flow velocity waveforms can be obtained in the chicken embryo. The blood flow profiles assessed at the four locations in the embryonic heart demonstrated an increase in peak velocity with advancing developmental stage. In the primitive ventricle the 'passive' (P) filling peak decreased whereas the 'active' (A) filling peak increased, resulting in a decrease in P to A ratio with advancing developmental stage. M-mode recordings demonstrated that the fractional closure time of the atrioventricular cushions increased from 20% at stage HH 18 to 60% at stage HH 23.

CONCLUSION

High-frequency ultrasound biomicroscopy can be used to define flow velocity waveforms in the embryonic chicken heart. This may contribute to an understanding of Doppler signals derived from valveless embryonic human hearts at 5 to 8 weeks of gestation, prior to septation.

Micro-electrocardiograms to study post-ventricular amputation of zebrafish heart

The zebrafish (Danio rerio) is an emerging model for cardiovascular research. The zebrafish heart regenerates after 20% ventricular amputation. However, assessment of the physiological responses during heart regeneration has been hampered by the small size of the heart and the necessity of conducting experiments in an aqueous environment. We developed a methodology to monitor a real-time surface electrocardiogram (ECG) by the use of micro-electrodes, signal amplification, and a low pass-filter at a sampling rate of 1 kHz. Wavelet transform was used to further remove ambient noises. Rather than paralyzing the fish, we performed mild sedation by placing the fish in a water bath mixed with MS-222 (tricane methanesulfonate). We recorded distinct P waves for atrial contraction, QRS complexes for ventricular depolarization, and QT intervals for ventricular repolarization prior to, and 2 and 4 days post-amputation (dpa). Sedation reduced the mean fish heart rate from 149 +/- 18 to 90 +/- 17 beats/min. The PR and QRS intervals remained unchanged in response to ventricular apical amputation (n = 6, p > 0.05). Corrected QT intervals (QTc) were shortened 4 dpa (n = 6, p < 0.05). In a parallel study, histology revealed that apical thrombi were replaced with fibrin clots and collagen fibers. Atrial arrhythmia was noted in response to prolonged sedation. Unlike the human counterpart, ventricular tachycardia or fibrillation was not observed in response to ventricular amputation 2 and 4 dpa. Taken together, we demonstrated a minimally invasive methodology to monitor zebrafish heart function, electrical activities, and regeneration in real-time.

Direct visualization of hemolymph flow in the heart of a grasshopper (Schistocerca americana)

Background

Hemolymph flow patterns in opaque insects have never been directly visualized due to the lack of an appropriate imaging technique. The required spatial and temporal resolutions, together with the lack of contrast between the hemolymph and the surrounding soft tissue, are major challenges. Previously, indirect techniques have been used to infer insect heart motion and hemolymph flow, but such methods fail to reveal fine-scale kinematics of heartbeat and details of intra-heart flow patterns.

Results

With the use of microbubbles as high contrast tracer particles, we directly visualized hemolymph flow in a grasshopper (Schistocerca americana) using synchrotron x-ray phase-contrast imaging. In-vivo intra-heart flow patterns and the relationship between respiratory (tracheae and air sacs) and circulatory (heart) systems were directly observed for the first time.

Conclusion

Synchrotron x-ray phase contrast imaging is the only generally applicable technique that has the necessary spatial, temporal resolutions and sensitivity to directly visualize heart dynamics and flow patterns inside opaque animals. This technique has the potential to illuminate many long-standing questions regarding small animal circulation, encompassing topics such as retrograde heart flow in some insects and the development of flow in embryonic vertebrates.

A membrane associated mCherry fluorescent reporter line for studying vascular remodeling and cardiac function during murine embryonic development

The development of the cardiovascular system is a highly dynamic process dependent on multiple signaling pathways regulating proliferation, differentiation, migration, cell-cell and cell-matrix interactions. To characterize cell and tissue dynamics during the formation of the cardiovascular system in mice, we generated a novel transgenic mouse line, Tg(Flk1::myr-mCherry), in which endothelial cell membranes are brightly labeled with mCherry, a red fluorescent protein. Tg(Flk1::myr-mCherry) mice are viable, fertile, and do not exhibit any developmental abnormalities. High levels of mCherry are expressed in the embryonic endothelium and endocardium, and expression is also observed in capillaries in adult animals. Targeting of the fluorescent protein to the cell membrane allows for subcellular imaging and cell tracking. By acquiring confocal time lapses of live embryos cultured on the microscope stage, we demonstrate that the newly generated transgenic model beautifully highlights the sprouting behaviors of endothelial cells during vascular plexus formation. We have also used embryos from this line to imaging the endocardium in the beating embryonic mouse heart, showing that Tg(Flk1::myr-mCherry) mice are suitable for the characterization of cardio dynamics. Furthermore, when combined with the previously described Tg(Flk1::H2B-EYFP) line, cell number in addition to cell architecture is revealed, making it possible to determine how individual endothelial cells contribute to the structure of the vessel.

Cardiac biology and disease in invertebrates

It may seem ridiculous to consider cardiac diseases in sub-vertebrate animals; when on earth is a tarantula, a butterfly or a snail going to be presented as a clinical case with heart failure or a congenital cardiac abnormality? This article examines the work of research groups investigating invertebrates as valuable models of heart disease in man. Examining invertebrates with gene defects similar to those in human patients with heart disease, congenital or acquired, allows us to probe deeply into the aetiopathogenesis of many cardiac conditions.

Live Imaging of Early Developmental Processes in Mammalian Embryos with Optical Coherence Tomography

Early embryonic imaging of cardiovascular development in mammalian models requires a method that can penetrate through and distinguish the many tissue layers with high spatial and temporal resolution. In this paper we evaluate the capability of Optical Coherence Tomography (OCT) technique for structural 3D embryonic imaging in mouse embryos at different stages of the developmental process ranging from 7.5 dpc up to 10.5 dpc. Obtained results suggest that the collected data is suitable for quantitative and qualitative measurements to assess cardiovascular function in mouse models, which is likely to expand our knowledge of the complexity of the embryonic heart, and its development into an adult heart.

Fluid dynamics in developmental biology: moving fluids that shape ontogeny

Human conception, indeed fertilization in general, takes place in a fluid, but what role does fluid dynamics have during the subsequent development of an organism? It is becoming increasingly clear that the number of genes in the genome of a typical organism is not sufficient to specify the minutiae of all features of its ontogeny. Instead, genetics often acts as a choreographer, guiding development but leaving some aspects to be controlled by physical and chemical means. Fluids are ubiquitous in biological systems, so it is not surprising that fluid dynamics should play an important role in the physical and chemical processes shaping ontogeny. However, only in a few cases have the strands been teased apart to see exactly how fluid forces operate to guide development. Here, we review instances in which the hand of fluid dynamics in developmental biology is acknowledged, both in human development and within a wider biological context, together with some in which fluid dynamics is notable but whose workings have yet to be understood, and we provide a fluid dynamicist's perspective on possible avenues for future research.

On the mechanics of cardiac function of Drosophila embryo

The heart is a vital organ that provides essential circulation throughout the body. Malfunction of cardiac pumping, thus, leads to serious and most of the times, to fatal diseases. Mechanics of cardiac pumping is a complex process, and many experimental and theoretical approaches have been undertaken to understand this process. We have taken advantage of the simplicity of the embryonic heart of an invertebrate, Drosophila melanogaster, to understand the fundamental mechanics of the beating heart. We applied a live imaging technique to the beating embryonic heart combined with analytical imaging tools to study the dynamic mechanics of the pumping. Furthermore, we have identified one mutant line that exhibits aberrant pumping mechanics. The Drosophila embryonic heart consists of only 104 cardiac cells forming a simple straight tube that can be easily accessed for real-time imaging. Therefore, combined with the wealth of available genetic tools, the embryonic Drosophila heart may serve as a powerful model system for studies of human heart diseases, such as arrhythmia and congenital heart diseases. We, furthermore, believe our mechanistic data provides important information that is useful for our further understanding of the design of biological structure and function and for engineering the pumps for medical uses.

Computational models of heart pumping efficiencies based on contraction waves in spiral elastic bands

We present a framework for modeling biological pumping organs based on coupled spiral elastic band geometries and active wave-propagating excitation mechanisms. Two pumping mechanisms are considered in detail by way of example: one of a simple tube, which represents a embryonic fish heart and another more complicated structure with the potential to model the adult human heart. Through finite element modeling different elastic contractions are induced in the band. For each version the pumping efficiency is measured and the dynamics are evaluated. We show that by combining helical shapes of muscle bands with a contraction wave it is possible not only to achieve efficient pumping, but also to create desired dynamics of the structure. As a result we match the function of the model pumps and their dynamics to physiological observations.

In vivo spectral domain optical coherence tomography volumetric imaging and spectral Doppler velocimetry of early stage embryonic chicken heart development

Progress toward understanding embryonic heart development has been hampered by the inability to image embryonic heart structure and simultaneously measure blood flow dynamics in vivo. We have developed a spectral domain optical coherence tomography system for in vivo volumetric imaging of the chicken embryo heart. We have also developed a technique called spectral Doppler velocimetry (SDV) for quantitative measurement of blood flow dynamics. We present in vivo volume images of the embryonic heart from initial tube formation to development of endocardial cushions of the same embryo over several stages of development. SDV measurements reveal the influence of heart tube structure on blood flow dynamics.

Embryogenesis of the heart muscle

This article concerns the development of myocardial architecture--crucial for contractile performance of the heart and its conduction system, essential for generation and coordinated spread of electrical activity. Topics discussed include molecular determination of cardiac phenotype (contractile and conducting), remodeling of ventricular wall architecture and its blood supply, and relation of trabecular compaction to noncompaction cardiomyopathy. Illustrated are the structure and function of the tubular heart, time course of trabecular compaction, and development of multilayered spiral systems of the compact layer.

High-resolution in vivo imaging of the cross-sectional deformations of contracting embryonic heart loops using optical coherence tomography

The embryonic heart tube consists of an outer myocardial tube, a middle layer of cardiac jelly, and an inner endocardial tube. It is said that tubular hearts pump the blood by peristaltoid contractions. The traditional concept of cardiac peristalsis sees the cyclic deformations of pulsating heart tubes as concentric narrowing and widening of tubes of circular cross-section. We have visualized the cross-sectional deformations of contracting embryonic hearts in chick embryos (HH-stages 9-17) using real-time high-resolution optical coherence tomography. Cardiac contractions are detected from HH-stage 10 onward. During the cardiac cycle, 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. The eccentric deformation of the endocardial tube is the consequence of an uneven distribution of the cardiac jelly. Our data show that the cyclic deformations of pulsating embryonic heart tubes run other than originally thought. There is evidence that heart tubes of elliptic cross-section might pump blood with a higher mechanical efficiency than those of circular-cross section. The uneven distribution of cardiac jelly seems to prefigure the future AV and cono-truncal endocardial cushions.

Advances in comparative physiology from high-speed imaging of animal and fluid motion

Since the time of Muybridge and Marey in the last half of the nineteenth century, studies of animal movement have relied on some form of high-speed or stop-action imaging to permit analysis of appendage and body motion. In the past ten years, the advent of megapixel-resolution high-speed digital imaging with maximal framing rates of 250 to 100,000 images per second has allowed new views of musculoskeletal function in comparative physiology that now extend to imaging flow around moving animals and the calculation of fluid forces produced by animals moving in fluids. In particular, the technique of digital particle image velocimetry (DPIV) has revolutionized our ability to understand how moving animals generate fluid forces and propel themselves through air and water. DPIV algorithms generate a matrix of velocity vectors through the use of image cross-correlation, which can then be used to calculate the force exerted on the fluid as well as locomotor work and power. DPIV algorithms can also be applied to images of moving animals to calculate the velocity of different regions of the moving animal, providing a much more detailed picture of animal motion than can traditional digitizing methods. Although three-dimensional measurement of animal motion is now routine, in the near future model-based kinematic reconstructions and volumetric analyses of animal-generated fluid flow patterns will provide the next step in imaging animal biomechanics and physiology.

Fast fluorescence microscopy for imaging the dynamics of embryonic development

Live imaging has gained a pivotal role in developmental biology since it increasingly allows real-time observation of cell behavior in intact organisms. Microscopes that can capture the dynamics of ever-faster biological events, fluorescent markers optimal for in vivo imaging, and, finally, adapted reconstruction and analysis programs to complete data flow all contribute to this success. Focusing on temporal resolution, we discuss how fast imaging can be achieved with minimal prejudice to spatial resolution, photon count, or to reliably and automatically analyze images. In particular, we show how integrated approaches to imaging that combine bright fluorescent probes, fast microscopes, and custom post-processing techniques can address the kinetics of biological systems at multiple scales. Finally, we discuss remaining challenges and opportunities for further advances in this field.

In vitro hemodynamic investigation of the embryonic aortic arch at late gestation

This study focuses on the dynamic flow through the fetal aortic arch driven by the concurrent action of right and left ventricles. We created a parametric pulsatile computational fluid dynamics (CFD) model of the fetal aortic junction with physiologic vessel geometries. To gain a better biophysical understanding, an in vitro experimental fetal flow loop for flow visualization was constructed for identical CFD conditions. CFD and in vitro experimental results were comparable. Swirling flow during the acceleration phase of the cardiac cycle and unidirectional flow following mid-deceleration phase were observed in pulmonary arteries (PA), head-neck vessels, and descending aorta. Right-to-left (oxygenated) blood flowed through the ductus arteriosus (DA) posterior relative to the antegrade left ventricular outflow tract (LVOT) stream and resembled jet flow. LVOT and right ventricular outflow tract flow mixing had not completed until approximately 3.5 descending aorta diameters downstream of the DA insertion into the aortic arch. Normal arch model flow patterns were then compared to flow patterns of four common congenital heart malformations that include aortic arch anomalies. Weak oscillatory reversing flow through the DA junction was observed only for the Tetralogy of Fallot configuration. PA and hypoplastic left heart syndrome configurations demonstrated complex, abnormal flow patterns in the PAs and head-neck vessels. Aortic coarctation resulted in large-scale recirculating flow in the aortic arch proximal to the DA. Intravascular flow patterns spatially correlated with abnormal vascular structures consistent with the paradigm that abnormal intravascular flow patterns associated with congenital heart disease influence vascular growth and function.

Fast differential interference contrast imaging combined with autocorrelation treatments to measure the heart rate of embryonic fish

To develop an accurate and convenient method for measuring the heart rate of zebra fish in vivo, a system combining fast differential interference contrast (DIC) imaging with an autocorrelation technique is established. The imaging correlation coefficient corr(i,j) between frame i, selected from the obtained time-lapse imaging series as the reference image, and any other frame j, is calculated as the time-dependent cycle course. Heat rate is determined by the cycle period of the corr with a high temporal resolution of 4 ms, achieved by fast charge-coupled device (CCD) imaging of 250 frames per second. With this high-resolution system, we find that 1-mgL cadmium not only induces the slowing of the heart rate, but also caused signs of arrhythmia in treated fish.

High-frequency ultrasonographic imaging of avian cardiovascular development

The chick embryo has long been a favorite model system for morphologic and physiologic studies of the developing heart, largely because of its easy visualization and amenability to experimental manipulations. However, this advantage is diminished after 5 days of incubation, when rapidly growing chorioallantoic membranes reduce visibility of the embryo. Using high-frequency ultrasound, we show that chick embryonic cardiovascular structures can be readily visualized throughout the period of Stages 9-39. At most stages of development, a simple ex ovo culture technique provided the best imaging opportunities. We have measured cardiac and vascular structures, blood flow velocities, and calculated ventricular volumes as early as Stage 11 with values comparable to those previously obtained using video microscopy. The endocardial and myocardial layers of the pre-septated heart are readily seen as well as the acellular layer of the cardiac jelly. Ventricular inflow in the pre-septated heart is biphasic, just as in the mature heart, and is converted to a monophasic (outflow) wave by ventricular contraction. Although blood has soft-tissue density at the ultrasound resolutions and developmental stages examined, its movement allowed easy discrimination of perfused vascular structures throughout the embryo. The utility of such imaging was demonstrated by documenting changes in blood flow patterns after experimental conotruncal banding.

High-speed imaging of developing heart valves reveals interplay of morphogenesis and function

Knowing how mutations disrupt the interplay between atrioventricular valve (AVV) morphogenesis and function is crucial for understanding how congenital valve defects arise. Here, we use high-speed fluorescence microscopy to investigate AVV morphogenesis in zebrafish at cellular resolution. We find that valve leaflets form directly through a process of invagination, rather than first forming endocardial cushions. There are three phases of valve function in embryonic development. First, the atrioventricular canal (AVC) is closed by the mechanical action of the myocardium, rolls together and then relaxes. The growing valve leaflets serve to block the canal during the roll and, depending on the developmental stage, either expand or hang down as a leaflet to block the canal. These steps are disrupted by the subtle morphological changes that result from inhibiting ErbB-, TGFbeta-or Cox2 (Ptgs2)-dependent signaling. Cox2 inhibition affects valve development due to its effect on myocardial cell size and shape, which changes the morphology of the ventricle and alters valve geometry. Thus, different signaling pathways regulate distinct aspects of the behavior of individual cells during valve morphogenesis, thereby influencing specific facets of valve function.

Three-dimensional real-time imaging of cardiac cell motions in living embryos

While quantitative analysis of dynamic biological cell motions in vivo is of great biomedical interest, acquiring 3-D (plus time) information is difficult due to the lack of imaging tools with sufficient spatial and temporal resolution. A novel 3-D high-speed microscopic imaging system is developed to enable 3-D time series data acquisition, based on a defocusing technique (DDPIV). Depth coordinate Z is resolved by the triangular image patterns generated by a mask with three apertures forming an equilateral triangle. Application of this technique to microscale imaging is validated by calibration of targets spread over the image field. 1-microm fluorescent tracer particles are injected into the blood stream of 32 h post-fertilization developing zebrafish embryos to help describe cardiac cell motions. 3-D and velocity fields of cardiovascular blood flow and trajectories of heart-wall motions are obtained.

Satellite and stem cells in muscle growth and repair

The FASEB summer research conference on Skeletal Muscle Satellite and Stem Cells, organized by Thomas Rando, Giulio Cossu and Jeffrey Chamberlain, was held in Indian Wells, California, in July. An international array of researchers gathered to share numerous new insights into the cellular and molecular regulation of stem cells and satellite cells in skeletal muscle biology. The conference is unique in that it brings together investigators from diverse backgrounds, who work on the growth and repair of skeletal muscle in humans and model systems, in health and disease.

Multimodality optical imaging of embryonic heart microstructure

Study of developmental heart defects requires the visualization of the microstructure and function of the embryonic myocardium, ideally with minimal alterations to the specimen. We demonstrate multiple endogenous contrast optical techniques for imaging the Xenopus laevis tadpole heart. Each technique provides distinct and complementary imaging capabilities, including: 1. 3-D coherence microscopy with subcellular (1 to 2 microm) resolution in fixed embryos, 2. real-time reflectance confocal microscopy with large penetration depth in vivo, and 3. ultra-high speed (up to 900 frames per second) that enables real-time 4-D high resolution imaging in vivo. These imaging modalities can provide a comprehensive picture of the morphologic and dynamic phenotype of the embryonic heart. The potential of endogenous-contrast optical microscopy is demonstrated for investigation of the teratogenic effects of ethanol. Microstructural abnormalities associated with high levels of ethanol exposure are observed, including compromised heart looping and loss of ventricular trabecular mass.

Muscular thin films for building actuators and powering devices

We demonstrate the assembly of biohybrid materials from engineered tissues and synthetic polymer thin films. The constructs were built by culturing neonatal rat ventricular cardiomyocytes on polydimethylsiloxane thin films micropatterned with extracellular matrix proteins to promote spatially ordered, two-dimensional myogenesis. The constructs, termed muscular thin films, adopted functional, three-dimensional conformations when released from a thermally sensitive polymer substrate and were designed to perform biomimetic tasks by varying tissue architecture, thin-film shape, and electrical-pacing protocol. These centimeter-scale constructs perform functions as diverse as gripping, pumping, walking, and swimming with fine spatial and temporal control and generating specific forces as high as 4 millinewtons per square millimeter.

Transitions in Early Embryonic Atrioventricular Valvular Function Correspond With Changes in Cushion Biomechanics That Are Predictable by Tissue Composition

Endocardial cushions are critical to maintain unidirectional blood flow under constantly increasing hemodynamic forces, but the interrelationship between endocardial cushion structure and the mechanics of atrioventricular junction function is poorly understood. Atrioventricular (AV) canal motions and blood velocities of embryonic chicks at Hamburger and Hamilton (HH) stages 17, 21, and 25 were quantified using ultrasonography. Similar to the embryonic zebrafish heart, the HH17 AV segment functions like a suction pump, with the cushions expanding in a wave during peak myocardial contraction and becoming undetectable during the relaxation phase. By HH25, the AV canal contributes almost nothing to the piston-like propulsion of blood, but the cushions function as stoppers apposing blood flow with near constant thickness. Using a custom built mesomechanical testing system, we quantified the nonlinear pseudoelastic biomechanics of developing AV cushions, and found that both AV cushions increased in effective modulus between HH17 and HH25. Enzymatic digestion of major structural constituent collagens or glycosaminoglycans resulted in distinctly different stress-strain curves suggestive of their individual contributions. Mixture theory using histologically determined volume fractions of cells, collagen, and glycosaminoglycans showed good prediction of cushion material properties regardless of stage and cushion position. These results have important implications in valvular development, as biomechanics may play a larger role in stimulating valvulogenic events than previously thought.

Cardiomyocytes re-enter the cell cycle and contribute to heart development after differentiation from cardiac progenitors expressing Isl1 in chick embryo

Cardiomyocytes are generated from the precardiac mesoderm and the size of the heart increases dramatically during embryogenesis. However, it is unclear how differentiation and proliferation correlate in the cardiac cell line during development. Here, we show that cardiomyocytes re-entered into a proliferative state after differentiation with a concomitant cell cycle arrest in chick embryo. The cells in the course of differentiation from Isl1-positive cardiac precursors to cardiomyocytes did not proliferate, but differentiated cardiomyocytes proliferated even after the acquisition of contractile function. After differentiation, cardiomyocytes developed a proliferative potential to contribute to the increase in cell numbers during heart development. Almost all differentiated cardiomyocytes (82.8%) incorporated bromodeoxyuridine (BrdU) in vitro, indicating the ability of DNA replication. Furthermore, mitotic chromosomes were observed in the cardiomyocytes in which a sarcomeric structure was sustained in the cytoplasm. We conclude that the sequential events of the differentiation to contractile myocytes and the re-entry into the cell cycle are strictly regulated during cardiac cell maturation. These results provide an insight into the maturation mechanism of the cardiac cell line.

Cardiovascular developmental insights from embryos

We investigate cardiovascular (CV) developmental physiology and biomechanics in order to understand the dramatic acquisition of form and function during normal development and to identify the adaptive mechanisms that allow embryos to survive adverse genetic and epigenetic events. Cardiovascular patterning, morphogenesis, and growth occur via highly conserved genetic mechanisms. Structural and functional maturation of the embryonic heart is also conserved across a broad range of species with evidence for load dependence from onset of the heartbeat. The embryonic heart dynamically adapts to changes in biomechanical loading conditions and for reasons not yet clear, adapts better to increased than to decreased mechanical load. In mammals, maternal cardiovascular function dynamically impacts embryonic/fetal growth and hemodynamics and these interactions can now be studied longitudinally using high-resolution noninvasive techniques. Maternal exposure to hypoxia and to bioactive chemicals, such as caffeine, can rapidly impact embryonic/fetal cardiovascular function, growth, and outcome. Finally, tissue engineering approaches can be applied to investigate basic developmental aspects of the embryonic myocardium. We use isolated embryonic and fetal chick, mouse, or rat cardiac cells to generate 3D engineered early embryonic cardiac tissues (EEECT). EEECT retains the morphologic and proliferative features of embryonic myocardium, responds to increased mechanical load with myocyte hyperplasia, and may be an excellent future material for use in cardiac repair and regeneration. These insights into cardiovascular embryogenesis are relevant to identifying mechanisms for congenital cardiovascular malformations and for developing cell- and tissue-based strategies for myocardial repair.

Quantitative volumetric analysis of cardiac morphogenesis assessed through micro-computed tomography

We present a method to generate quantitative embryonic cardiovascular volumes at extremely high resolution without tissue shrinkage using micro-computed tomography (Micro-CT). A CT dense polymer (Microfil, Flow Tech, Inc.) was used to perfuse avian embryonic hearts from Hamburger and Hamilton stage (HH) 15 through HH36, which solidified to create a cast within the luminal space. Hearts were then scanned at 10.5 mum(3) voxel resolution using a VivaCT scanner, digital slices were contoured for regions of interest, and computational analysis was conducted to quantify morphogenetic parameters. The three-dimensional morphology was compared with that of scanning electron microscopy (SEM) images and serial section reconstruction of similarly staged hearts. We report that Microfil-perfused hearts swelled to maximum end-diastolic volume with negligible shrinking after polymerization. Comparison to SEM revealed good agreement of cardiac chamber proportions and intracardiac tissue structures (i.e., valves and septa) at the stages of development assessed. Quantification of changes in chamber volume over development revealed several notable results that confirm earlier hypotheses. Heart chamber volumes grow over two orders of magnitude during the 1-week developmental period analyzed. The atrioventricular canal comprised a significant proportion of the early heart volume. While left atrium/left ventricular volume ratios approached 1 in later development, right atrium/right ventricle ratios increase to over 2.5. Quantification of trabeculation patterns confirmed that the right and left ventricles are similarly trabeculated before HH27, after which the right ventricle became quantitatively coarser than that of the left ventricle. These results demonstrate that Micro-CT can be used to image and quantify cardiovascular structures during development.

Illuminating cardiac development: Advances in imaging add new dimensions to the utility of zebrafish genetics

The use of the zebrafish as a model organism for the analysis of cardiac development is no longer proof-of-principle science. Over the last decade, the identification of a variety of zebrafish mutations and the subsequent cloning of mutated genes have revealed many critical regulators of cardiogenesis. More recently, increasingly sophisticated techniques for phenotypic characterization have facilitated analysis of the specific mechanisms by which key genes drive cardiac specification, morphogenesis, and function. Future enrichment of the arsenal of experimental strategies available for zebrafish should continue the yield of high returns from such a small source.

Ciona intestinalis as a model for cardiac development

The primitive chordate Ciona intestinalis has emerged as a significant model system for the study of heart development. The Ciona embryo employs a conserved heart gene network in the context of extremely low cell numbers and reduced genetic redundancy. Here, I review recent studies on the molecular genetics of Ciona cardiogenesis as well as classic work on heart anatomy and physiology. I also discuss the potential of employing Ciona to decipher a comprehensive chordate gene network and to determine how this network controls heart morphogenesis.

Modelling data across labs, genomes, space and time

Logical models and physical specifications provide the foundation for storage, management and analysis of complex sets of data, and describe the relationships between measured data elements and metadata - the contextual descriptors that define the primary data. Here, we use imaging applications to illustrate the purpose of the various implementations of data specifications and the requirement for open, standardized, data formats to facilitate the sharing of critical digital data and metadata.

Rapid three-dimensional imaging and analysis of the beating embryonic heart reveals functional changes during development

We report an accurate method for studying the functional dynamics of the beating embryonic zebrafish heart. The fast cardiac contraction rate and the high velocity of blood cells have made it difficult to study cellular and subcellular events relating to heart function in vivo. We have devised a dynamic three-dimensional acquisition, reconstruction, and analysis procedure by combining (1) a newly developed confocal slit-scanning microscope, (2) novel strategies for collecting and synchronizing cyclic image sequences to build volumes with high temporal and spatial resolution over the entire depth of the beating heart, and (3) data analysis and reduction protocols for the systematic extraction of quantitative information to describe phenotype and function. We have used this approach to characterize blood flow and heart efficiency by imaging fluorescent protein-expressing blood and endocardial cells as the heart develops from a tube to a multichambered organ. The methods are sufficiently robust to image tissues within the heart at cellular resolution over a wide range of ages, even when motion patterns are only quasiperiodic. These tools are generalizable to imaging and analyzing other cyclically moving structures at microscopic scales.

High-speed confocal fluorescence imaging with a novel line scanning microscope

Research in the life sciences increasingly involves the investigation of fast dynamic processes at the cellular and subcellular level. It requires tools to image complex systems with high temporal resolution in three-dimensional space. For this task, we introduce the concept of a fast fluorescence line scanner providing image acquisition speeds in excess of 100 frames per second at 512 x 512 pixels. Because the system preserves the capability for optical sectioning of confocal systems, it allows us to observe processes with three-dimensional resolution. We describe the principle of operation, the optical characteristics of the microscope, and cover several applications in particular from the field of cell and developmental biology. A commercial system based on the line scanning concept has been realized by Carl Zeiss (LSM 5 LIVE).

Left ventricular structure and function: basic science for cardiac imaging

The myofiber geometry of the left ventricle (LV) changes gradually from a right-handed helix in the subendocardium to a left-handed helix in the subepicardium. In this review, we associate the LV myofiber architecture with emerging concepts of the electromechanical sequence in a beating heart. We discuss: 1) the morphogenesis and anatomical arrangement of muscle fibers in the adult LV; 2) the sequence of depolarization and repolarization; 3) the physiological inhomogeneity of transmural myocardial mechanics and the apex-to-base sequence of longitudinal and circumferential deformation; 4) the sequence of LV rotation; and 5) the link between LV deformation and the intracavitary flow direction observed during each phase of the cardiac cycle. Integrating the LV structure with electrical activation and motion sequences observed in vivo provides an understanding about the spatiotemporal sequence of regional myocardial performance that is essential for noninvasive cardiac imaging.

From imaging to understanding: Frontiers in Live Cell Imaging, Bethesda, MD, April 19-21, 2006

A recent meeting entitled Frontiers in Live Cell Imaging was attended by more than 400 cell biologists, physicists, chemists, mathematicians, and engineers. Unlike typical special topics meetings, which bring together investigators in a defined field primarily to review recent progress, the purpose of this meeting was to promote cross-disciplinary interactions by introducing emerging methods on the one hand and important biological applications on the other. The goal was to turn live cell imaging from a "technique" used in cell biology into a new exploratory science that combines a number of research fields.

Computational Model for the Transition From Peristaltic to Pulsatile Flow in the Embryonic Heart Tube

Early in development, the heart is a single muscle-wrapped tube without formed valves. Yet survival of the embryo depends on the ability of this tube to pump blood at steadily increasing rates and pressures. Developmental biologists historically have speculated that the heart tube pumps via a peristaltic mechanism, with a wave of contraction propagating from the inflow to the outflow end. Physiological measurements, however, have shown that the flow becomes pulsatile in character quite early in development, before the valves form. Here, we use a computational model for flow though the embryonic heart to explore the pumping mechanism. Results from the model show that endocardial cushions, which are valve primordia arising near the ends of the tube, induce a transition from peristaltic to pulsatile flow. Comparison of numerical results with published experimental data shows reasonably good agreement for various pressure and flow parameters. This study illustrates the interrelationship between form and function in the early embryonic heart.

Confocal quantification of cis-regulatory reporter gene expression in living sea urchin

Quantification of GFP reporter gene expression at single cell level in living sea urchin embryos can now be accomplished by a new method of confocal laser scanning microscopy (CLSM). Eggs injected with a tissue-specific GFP reporter DNA construct were grown to gastrula stage and their fluorescence recorded as a series of contiguous Z-section slices that spanned the entire embryo. To measure the depth-dependent signal decay seen in the successive slices of an image stack, the eggs were coinjected with a freely diffusible internal fluorescent standard, rhodamine dextran. The measured rhodamine fluorescence was used to generate a computational correction for the depth-dependent loss of GFP fluorescence per slice. The intensity of GFP fluorescence was converted to the number of GFP molecules using a conversion constant derived from CLSM imaging of eggs injected with a measured quantity of GFP protein. The outcome is a validated method for accurately counting GFP molecules in given cells in reporter gene transfer experiments, as we demonstrate by use of an expression construct expressed exclusively in skeletogenic cells.

Multimodal imaging of mouse development: Tools for the postgenomic era

With the sequence of the mouse genome known, it is now possible to create or identify mutations in every gene to determine the molecules necessary for normal development. Consequently, there is a growing need for advanced phenotyping tools to best understand defects produced by altering gene function. Perhaps nothing is more satisfying than to directly observe a process in action; to disturb it and see for ourselves how the process changes before our very eyes. No doubt, this desire is what drove the invention of the very first microscopes and continues to this day to fuel progress in the field of biological imaging. Because mouse embryos are small and develop embedded within many tissue layers within the nurturing environment of the mother, directly observing the dynamic, micro- and nanoscopic events of early mammalian development has proven to be one of the greater challenges for imaging scientists. Here, I will review some of the imaging methods being used to study mouse development, highlighting the results obtained from imaging.