Mechanisms influencing retrograde flow in the atrioventricular canal during early embryonic cardiogenesis

Effect of Blood Flow on Cardiac Morphogenesis and Formation of Congenital Heart Defects

Congenital heart disease (CHD) affects about 1 in 100 newborns and its causes are multifactorial. In the embryo, blood flow within the heart and vasculature is essential for proper heart development, with abnormal blood flow leading to CHD. Here, we discuss how blood flow (hemodynamics) affects heart development from embryonic to fetal stages, and how abnormal blood flow solely can lead to CHD. We emphasize studies performed using avian models of heart development, because those models allow for hemodynamic interventions, in vivo imaging, and follow up, while they closely recapitulate heart defects observed in humans. We conclude with recommendations on investigations that must be performed to bridge the gaps in understanding how blood flow alone, or together with other factors, contributes to CHD.

Following the Beat: Imaging the Valveless Pumping Function in the Early Embryonic Heart

In vertebrates, the coordinated beat of the early heart tube drives cardiogenesis and supports embryonic growth. How the heart pumps at this valveless stage marks a fascinating problem that is of vital significance for understanding cardiac development and defects. The developing heart achieves its function at the same time as continuous and dramatic morphological changes, which in turn modify its pumping dynamics. The beauty of this muti-time-scale process also highlights its complexity that requires interdisciplinary approaches to study. High-resolution optical imaging, particularly fast, four-dimensional (4D) imaging, plays a critical role in revealing the process of pumping, instructing numerical modeling, and enabling biomechanical analyses. In this review, we aim to connect the investigation of valveless pumping mechanisms with the recent advancements in embryonic cardiodynamic imaging, facilitating interactions between these two areas of study, in hopes of encouraging and motivating innovative work to further understand the early heartbeat.

Myocardial Afterload Is a Key Biomechanical Regulator of Atrioventricular Myocyte Differentiation in Zebrafish

Heart valve development is governed by both genetic and biomechanical inputs. Prior work has demonstrated that oscillating shear stress associated with blood flow is required for normal atrioventricular (AV) valve development. Cardiac afterload is defined as the pressure the ventricle must overcome in order to pump blood throughout the circulatory system. In human patients, conditions of high afterload can cause valve pathology. Whether high afterload adversely affects embryonic valve development remains poorly understood. Here we describe a zebrafish model exhibiting increased myocardial afterload, caused by vasopressin, a vasoconstrictive drug. We show that the application of vasopressin reliably produces an increase in afterload without directly acting on cardiac tissue in zebrafish embryos. We have found that increased afterload alters the rate of growth of the cardiac chambers and causes remodeling of cardiomyocytes. Consistent with pathology seen in patients with clinically high afterload, we see defects in both the form and the function of the valve leaflets. Our results suggest that valve defects are due to changes in atrioventricular myocyte signaling, rather than pressure directly acting on the endothelial valve leaflet cells. Cardiac afterload should therefore be considered a biomechanical factor that particularly impacts embryonic valve development.

Haemodynamic dependence of mechano-genetic evolution of the cardiovascular system in Japanese medaka

The progression of cardiac gene expression-wall shear stress (WSS) interplay is critical to identifying developmental defects during cardiovascular morphogenesis. However, mechano-genetics from the embryonic to larval stages are poorly understood in vertebrates. We quantified peak WSS in the heart and tail vessels of Japanese medaka from 3 days post fertilization (dpf) to 14 dpf using in vivo micro-particle image velocimetry flow measurements, and in parallel analysed the expression of five cardiac genes (fgf8, hoxb6b, bmp4, nkx2.5, smyd1). Here, we report that WSS in the atrioventricular canal (AVC), ventricular outflow tract (OFT), and the caudal vessels in medaka peak with inflection points at 6 dpf and 10-11 dpf instead of a monotonic trend. Retrograde flows are captured at the AVC and OFT of the medaka heart for the first time. In addition, all genes were upregulated at 3 dpf and 7 dpf, indicating a possible correlation between the two, with the cardiac gene upregulation preceding WSS increase in order to facilitate cardiac wall remodelling.

Valveless pumping behavior of the simulated embryonic heart tube as a function of contractile patterns and myocardial stiffness

During development, the heart begins pumping as a valveless multilayered tube capable of driving blood flow throughout the embryonic vasculature. The mechanical properties and how they interface with pumping function are not well-defined at this stage. Here, we evaluate pumping patterns using a fluid-structure interaction computational model, combined with experimental data and an energetic analysis to investigate myocardial mechanical properties. Through this work, we propose that a myocardium modeled as a Neo-Hookean material with a material constant on the order of 10 kPa is necessary for the heart tube to function with an optimal pressure and cardiac output.

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.