PASSIVE STRESS-STRAIN MEASUREMENTS IN THE STAGE-16 AND STAGE-18 EMBRYONIC CHICK HEART

The first stress-strain measurements on embryonic cardiovascular tissu e are described here, obtained from cyclic uniaxial loading of the pri mitive ventricle. An excised ventricular segment from Hamburger/Hamilt on stage-16 or stage-18 chicks (2-1/2 and 3 days of a 21-day incubatio n period) was mounted longitudinally between two small wires in oxygen ated Krebs-Henseleit cardioplegia solution. One wire was attached to a rt ultrasensitive force transducer and the other to a Huxley micromani pulator controlled by remote motor drive. A real-time video tracking s ystem calculated three myocardial surface strains based on the positio ns of three surface markers while the heal? was deformed in a triangul ar wave pattern. Force transducer output was filtered, digitally sampl ed, and stored with strains and time. Results were plotted as strain ( longitudinal, circumferential, shear, and principal) versus time, stre ss versus time, and stress versus longitudinal strain. The stress-stra in curves were nonlinear, even at low strain levels. The hysteresis lo ops were large; mean hysteresis energy as a proportion of total cycle stored strain energy was 36 percent (stage 16) and 41 percent (stage 1 8). We created a finite element model of the ventricle and fit the mod el behavior to the experimental behavior to determine parameters for a stage-18 pseudoelastic strain-energy function of exponential form. Th e calculated exponential parameter is significantly lower than that fo und in corresponding uniaxial studies of mature myocardium, possibly i ndicating the lower fiber content of the immature tissue. The results of this study are the first step in characterizing material properties for comparisons with later developmental stages and with impaired and altered myocardium. The long-term goal is to aid in identifying the b iomechanical factors regulating growth and morphogenesis.