CHARACTERIZATION OF PASSIVE EMBRYONIC MYOCARDIUM BY QUASI-LINEAR VISCOELASTICITY THEORY

The mechanical properties of embryonic cardiac tissue, an important li nk between form and function, are largely unknown. This study provides new information on the viscoelastic behavior of the stage-16 and stag e-18 (21/2 and 3 d) chick ventricle. The cylindrical ventricles were r emoved from the embryo, arrested in diastole, and mounted between two small wires in a specially designed experimental workstation. After pr econditioning, ramp-and-hold stress relaxation tests were performed at 10, 20 and 40% stretch. The resulting reduced relaxation functions we re fit by graphical approach with a quasi-linear viscoelastic represen tation. 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 tau(1), 0.037-0.052 s; slow time constant tau(2), 296-486 s; and final relaxation G(infinity), 0.38-0.59. There results agree closely with those adult tissue. The differences in parameters were not significant tither between stretch within stage or between st ages at the same stretch level. An extrapolation/renormalization proce dure increased agreement in slope between stretch levels but decreased agreement in G(infinity). Events occurring on the short time scale ma y represent extracellular fluid filtration while final relaxation may be a function of true tissue viscoelasticity. These results will provi de a baseline for extension to later developmental stages in cases of both normal and altered growth. (C) 1997 Elsevier Science Ltd.