Mechanical effects of looping in the embryonic chick heart.

During early embryonic development, the heart bends into a curved tube in a vital morphogenetic process called looping. Since looping involves poorly understood biomechanical forces that are difficult to measure, this paper presents a theoretical model for the tubular chick heart, whose development is similar to that of the human heart. Representing the basic morphology of the looped ventricle, the model is a thick-walled, isotropic, pressurized curved tube composed of three layers representing the myocardium, cardiac jelly, and endocardium. The model is analyzed with nonlinear elasticity theory, modified to include residual strain and muscle activation, and material properties are determined by correlating theoretical and experimental pressure-volume relations. The results show that longitudinal curvature significantly influences the biomechanical behavior of the embryonic heart. As the curvature increases, the compliance of the tube increases, especially at end systole. Stress concentrations, which develop in the endocardium during diastole and in the myocardium during systole, also increase with the curvature. The largest wall stress during the cardiac cycle occurs near the beginning of systolic ejection in the myocardial layer at the inner curvature of the tube. Relative to end diastole, the model predicts epicardial strains that are nearly equal in the circumferential and meridional directions, in agreement with experimental measurements. These results provide insight into the interrelation between biomechanical forces and morphogenesis during cardiac looping.