Stress, structure, and function of the embryonic heart
Gendernalik, Alex L., author
Bark, David, advisor
Garrity, Deborah, committee member
Puttlitz, Christian, committee member
Heyliger, Paul, committee member
Embryonic heart development is a complex process that requires the coordination of hemodynamic stress and tissue morphogenesis. Improperly timed or distorted mechanical cues can cause reverberating malformations that result in congenital heart defects (CHDs) or embryo death. CHDs occur in ~1% of live births. Only ~20% have a known genetic origin. Altered mechanical signaling or hemodynamics is likely a common contributor to CHD prevalence. Significant research using animal models has shown that altered hemodynamics causes varied malformations. Mechanical properties describe the underlying tissue architecture, which dictates how the heart transmits and reacts to stress. This work aims to quantify and map the mechanical properties to understand how they direct the formation of specific heart structures and function. Furthermore, we seek to understand how mechanical properties change in response to altered hemodynamics. We hypothesize that mechanical properties indicate regions of eventual structure formation, are sensitive to altered hemodynamics, and dictate the pumping method that the heart uses to drive blood flow. We test this hypothesis through three aims. In aim 1, we describe a novel technique in which we use controlled pressurization to deform the embryonic zebrafish heart. We measure deformation in two-dimensions and identify constitutive models of the embryonic heart tissue. Finite element analysis is used to validate our findings in three dimensions. In this aim, we establish that controlled pressurization is a valid technique for inducing measured deformation of the heart. Through this, we determine that the zebrafish myocardial stiffness is on the order of 10 kPa. In aim 2, we further develop our pressurization technique by measuring deformation in three dimensions using confocal microscopy. Furthermore, we use a morpholino antisense oligonucleotide (MO), gata1, to alter embryonic zebrafish heart hemodynamics by blocking development of red blood cells, thus decreasing the viscosity and arterial pressure based on Poiseuille's law. Upon mapping strain in three dimensions, we find that strain throughout the heart is variable, with specific regions of low and high strain from 2 to 3 days post-fertilization (dpf). Low arterial pressure in gata1 MO embryos resulted in significantly increased strain compared to controls, indicating that altered hemodynamics cause altered mechanical properties in the developing embryonic heart. In aim 3, we seek to determine if the zebrafish early embryonic heart tube drives blood flow through peristalsis or impedance-type pumping. We attempt to directly induce impedance pumping by cannulating the atrial inlet of the heart tube after halting contractions and applying a controlled pressure pulse. Additionally, we use precisely controlled temperatures to increase heart rate, thus increasing arterial pressure. As temperature is increased, we use high speed imaging to analyze the contractile motion and resulting blood flow in the tube heart. Furthermore, we describe a previously unknown response whereby the traveling endocardial closure shortens with increased arterial pressure. In this aim, we fail to find evidence of impedance-type pumping but cannot preclude it contributes to blood flow. In summary, our pressurization technique can be used to map strain in the zebrafish embryonic heart; altering hemodynamics by reducing arterial pressure results in decreased stiffness of the embryonic heart myocardium, and endocardial closure length in the embryonic zebrafish heart tube shortens as arterial pressure and heart rate increase.
Includes bibliographical references.