Pulse propagation by a capacitive mechanism drives embryonic blood flow.

Pulsatile flow is a universal feature of the blood circulatory system in vertebrates and can lead to diseases when abnormal. In the embryo, blood flow forces stimulate vessel remodeling and stem cell proliferation. At these early stages, when vessels lack muscle cells, the heart is valveless and the Reynolds number (Re) is low, few details are available regarding the mechanisms controlling pulses propagation in the developing vascular network. Making use of the recent advances in optical-tweezing flow probing approaches, fast imaging and elastic-network viscous flow modeling, we investigated the blood-flow mechanics in the zebrafish main artery and show how it modifies the heart pumping input to the network. The movement of blood cells in the embryonic artery suggests that elasticity of the network is an essential factor mediating the flow. Based on these observations, we propose a model for embryonic blood flow where arteries act like a capacitor in a way that reduces heart effort. These results demonstrate that biomechanics is key in controlling early flow propagation and argue that intravascular elasticity has a role in determining embryonic vascular function.

Multiple holographic optical tweezers parallel calibration with optical potential well characterization.

We report the extension to a multi-axes exploration of the potential well reconstruction method against drag force to simultaneously characterize the potential wells of several trapping sites generated with holographic optical tweezers. The final result is a robust, fast and automatic procedure we use to characterize holographic tweezers. We mainly focus on the reliability of the method and its application to address the dependence of the diffraction efficiency with the trap position in a given holographic traps pattern.

Using correlative light and electron microscopy to study zebrafish vascular morphogenesis.

Live imaging is extremely useful to characterize the dynamics of cellular events in vivo, yet it is limited in terms of spatial resolution. Correlative light and electron microscopy (CLEM) allows combining live confocal microscopy with electron microscopy (EM) for the characterization of biological samples at high temporal and spatial resolution. Here we describe a protocol allowing extracting endothelial cell ultrastructure after having imaged the same cell in its in vivo context through live confocal imaging during zebrafish embryonic development.

Heartbeat-driven pericardiac fluid forces contribute to epicardium morphogenesis.

BACKGROUND: Hydrodynamic forces play a central role in organ morphogenesis. The role of blood flow in shaping the developing heart is well established, but the role of fluid forces generated in the pericardial cavity surrounding the heart is unknown. Mesothelial cells lining the pericardium generate the proepicardium (PE), the precursor cell population of the epicardium, the outer layer covering the myocardium, which is essential for its maturation and the formation of the heart valves and coronary vasculature. However, there is no evidence from in vivo studies showing how epicardial precursor cells reach and attach to the heart. RESULTS: Using optical tools for real-time analysis in the zebrafish, including high-speed imaging and optical tweezing, we show that the heartbeat generates pericardiac fluid advections that drive epicardium formation. These flow forces trigger PE formation and epicardial progenitor cell release and motion. The pericardial flow also influences the site of PE cell adhesion to the myocardium. We additionally identify novel mesothelial sources of epicardial precursors and show that precursor release and adhesion occur both through pericardiac fluid advections and through direct contact with the myocardium. CONCLUSIONS: Two hydrodynamic forces couple cardiac development and function: first, blood flow inside the heart, and second, the pericardial fluid advections outside the heart identified here. This pericardiac fluid flow is essential for epicardium formation and represents the first example of hydrodynamic flow forces controlling organogenesis through an action on mesothelial cells.