Characterization of passive embryonic myocardium by quasi-linear viscoelasticity theory

The mechanical properties of embryonic cardiac tissue, an important link between from and function, are largely unknown. This study provides new information on the viscoelastic behavior of the stage-16 and stage-18 (21/2 and 3 d) chick ventricle. The cylindrical ventricles were removed from the embryo, arrested in diastole, and mounted between two small wires in a specially designed experimental workstation. After preconditioning, ramp-and-hold stress relaxation tests were performed at 10, 20 and 40% stretch. The resulting reduced relaxation functions were fit by graphical approach with a quasi-linear viscoelastic representation. All functions were highly linear with natural log time between 1 and 60 s; mean slopes were −0.051 to −0.067. Other mean values were: fast time constant τ1, 0.037 – 0.052 s; slow time constant τ2, 296–486 s; and final relaxation G(∞), 0.38 – 0.59. These results agree closely with those of adult tissue. The differences in parameters were not significant either between stretch levels within stage or between stages at the same stretch level. An extrapolation/renormalization procedure increased agreement in slope between stretch levels but decreased agreement in G(∞). Events occurring on the short time scale may represent extracellular fluid filtration while final relaxation may be a function of true tissue viscoelasticity. These results will provide a baseline for extension to later developmental stages in cases of both normal and altered growth.