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

Abstract

Abstract-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. Key Words: chick Ⅲ development Ⅲ modeling Ⅲ ultrasound Ⅲ flow Ⅲ aspiration T he development of the atrioventricular (AV) and semilunar valves of the heart from the endocardial cushions occurs concomitantly with a constant barrage of hemodynamic and mechanical forces. Several studies have demonstrated that both blood pressure and velocities increase during morphological development in the heart, implying that the stresses on the endocardial cushions are also increasing. [1][2][3] Early investigations highlighted the motions of cushions in concert with the contracting myocardium, suggesting that they serve a valve-like function before valves form. 4 However, a recent study showed that the atrioventricular canal in the tubular early zebrafish heart functions like a suction pump, in contrast to the peristaltic mechanism previously described. 5 These observations raise no controversy with respect to current understanding of transitions that occur structurally and molecularly in the myocardium during early tube heart development, but raise major questions with respect to the mechanism through which endocardial cushions function in promoting unidirectional blood flow during the transition from tubular heart to a septated structure. In this study we examined the mechanical properties and myocardial/endocardial cushion mechanical interaction in 3 stages of cardiac development in the chick embryo to better understand how the mechanical properties of the cushions contribute to their functional roles.Various mutant models demonstrate that genetic defects compromising valve structural maturation result in s...

Quantitative Three‐Dimensional Analysis of Embryonic Chick Morphogenesis Via Microcomputed Tomography

“…Finally, we identified chamber specific myocardial growth curves for embryonic chick heart, and found novel trends between atrial and ventricular chambers and between left and right sided chambers. It is well known that hemodynamics is an important regulator of cardiac growth and remodeling (Reckova et al, 2003), and that hemodynamic forces are continually changing during cardiogenesis (Hu and Clark, 1989;Phoon, 2001;Butcher et al, 2007a). Our data shows that right ventricular myocardium volume is larger at Day 4, but the left side dominates by Day 10.…”

“…We and others have used pre-clinical ultrasound to visualize cardiac dynamics in developing chick and mouse embryos (Broekhuizen et al, 1999;Phoon, 2001;Butcher et al, 2007a). Ultrasound has the advantage of high ungated temporal resolution (over 250 frames/ sec), but can also be gated for better resolution.…”

Tgfβ/Alk5 signaling is required for shear stress induced klf2 expression in embryonic endothelial cells

“…Shear stress modulates endothelial structure and function and results in an accurate regulation of endothelial gene expression in vitro (Dekker et al, 2002) and in vivo (Groenendijk et al, 2004;Dekker et al, 2005). Several animal models show that congenital malformations can result from disturbed blood flow (Hogers et al, 1997;Sedmera et al, 1999;Tobita et al, 2002;Hove et al, 2003;Dealmeida et al, 2007;Butcher et al, 2007). The large and rapid changes in volume and geometry of cardiac compartments during development are translated into local changes in shear stress and concomitant gene expression patterns of, e.g., Krü ppel like factor 2 (KLF2) (Groenendijk et al, 2004).…”

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

“…During the past 20 years, however, hemodynamic data have accumulated that seem to cast doubt on this view (Hu and Clark, 1989;Hu et al, 1991;Forouhar et al, 2006;Butcher et al, 2007) and, thereby, show that, at the present time, we have only relatively scant knowledge about the pumping mechanism that generates unidirectional blood flow in the valveless embryonic heart tube.…”

“…Therefore, it was impossible to gain exact information about the cyclic changes in shape of the pulsating embryonic heart. During the past decade, however, new imaging techniques such as optical coherence tomography (OCT) and high-frequency ultrasonography were found to be well-suited for noninvasive visualization of various morphological and functional aspects of the embryonic vertebrate heart at high spatial and temporal resolutions (Boppart et al, 1997;Yelbuz et al, 2002;Luo et al, 2006;Jenkins et al, 2007a,b;McQuinn et al, 2007;Butcher et al, 2007;Norozi et al, 2008;Davis et al, 2008Davis et al, , 2009Rugonyi et al, 2008;Thrane et al, 2009;Damon et al, 2009).…”