Mechanical Development and Functional Mechanosensitivity During Early Cardiogenesis

This thesis addresses the questions of when and how mechanical stiffness arises during embryonic heart development and how mechanics affects early cardiomyocyte and myocardium contractile function and cytoskeletal organization. Previous studies addressing how mechanics influence the contractile and electrochemical capacity of mature cardiomyocytes on compliant substrates are reviewed in light of theory explaining how contractile striated fibers might optimally align on intermediate substrates. Embryonic heart and brain tissue stiffness through early development are measured by micropipette aspiration, and the earliest functional heart is found to be three-fold stiffer than early embryonic tissue while brain remains soft. Contraction strain in intact embryonic day 4 (E4) heart tubes shows an optimum relative to hearts with softened or stiffened extracellular matrices. Contraction wave velocity, however, goes linearly with softening or stiffening of tissue, consistent with a theory. Isolated E4 cardiomyocytes cultured on collagen-coated substrates of various stiffnesses show optimal contraction on substrates that match the stiffness of E4 tissue. Sarcomere organization shows optimal organization in intact tissue relative to soft and on intermediate substrates relative to soft or very stiff. The feedback between matrix stiffness and contractile capacity of cardiomyocytes in developing heart tissue is modeled and extended to include interactions with nuclear structural proteins, Lamins. A method for perturbing and imaging nuclear lamina in vivo is discussed and preliminary measurements indicate that the nucleus could act as a measure for intracellular stresses.