(122f) The Effects of Substrate Elasticity On Myofibrillogenesis in Spontaneously Contracting Embryonic Cardiomyocytes

Formation of a functional, beating heart is one of the first crucial steps in embryogenesis. Cardiac myocytes begin contracting long before the organ has completely developed. As such, embryonic cardiac myocytes are constantly ?doing work? during differentiation. While it is known how the expression of contractile proteins affects the cell's ability to contract, whether the mechanical act of contraction may feed back to the cell's continued cardiac development is not known or understood. We hypothesize that cardiomyocytes contracting in the embryo do work at the optimal efficiency, and lowering this efficiency would perturb the process of myofibrillogenesis. To test this, avian embryonic cardiomyocytes and mouse stem cell derived cardiomyocytes were cultured on polyacrylamide substrates of stiffness ranging from soft, intermediate, and hard. When cultured on polyacrylamide gels as single cells, cardiomyocytes spontaneously contract with an efficiency directly related to the elasticity, or stiffness, of the substrate. The contraction velocity of spontaneously beating cells was measured in E5 to E7 avian embryonic and 7+2 to 7+4 stem cell derived cardiomyocytes. Whether contraction velocity followed the expected force-velocity relationship depended on the cardiomyocyte's stage in development. These cells were later fixed and immunostained for actin and sarcomeric alpha-actinin to assess the degree of myofibril assembly. Overall, it was found that cardiomyocytes from some, but not all, developmental stages follow the expected stiffness-shape relationships found in non-muscle cells, and the degree of sarcomere maturation depended on the cell's ability to contract.