Material properties of the embryonic chick heart during cardiac looping morphogenesis

Cardiac looping is an essential morphogenetic process that occurs early in vertebrate development. During looping the initially straight heart tube bends ventrally and rotates dextrally. Errors in looping, due to ambiguous or reversed laterality, or the absence of bending, can cause deleterious congenital defects of the cardiovascular system, if not embryonic lethality. While many of the molecular factors involved in looping have now been uncovered, the underlying biophysical mechanism that drives looping remains largely unknown. Several biomechanical models for looping have been developed, but the material properties of the heart have not been directly measured.; In this work, a new microindentation device, called the “heart poker”, was developed to perform mechanical tests on the embryonic chick heart. The heart poker was first used to measure regional stiffness of the stage 12 heart, which is a curved tube consisting primarily of a two-cell thick outer layer of myocardium that surrounds a large extracellular matrix compartment, known as cardiac jelly. It was found that the inner curvature of the intact heart is significantly stiffer than either the outer curvature or sides of the heart. Additionally, the stiffness of the intact heart was found to be an order of magnitude greater than isolated cardiac jelly stiffness. The results of these stiffness measurements provide some insight into mechanisms for looping.; To determine material properties for the myocardium and cardiac jelly, a new microindentation method was developed. The method uses experimental force-displacement data from indentation tests, as well as the shape of the indentation profile. It is shown that residual stress affects the indentation shape for biological tissue structures that behave as a plate (myocardium) on a foundation (cardiac jelly). With a novel inverse finite element method, experimental data from indentation tests and residual stress cutting experiments were used to determine nonlinear elastic material properties and residual stress in the heart. The method developed here can be applied to a wide variety of tissues, especially during development.