Advisor: Evan A. Zamir, DSc., George W. Woodruff  School of Mechanical Engineering, Georgia Institute of Technology
Co-Advisor: Andrés J. García, PhD, George W. Woodruff  School of Mechanical Engineering, Georgia Institute of Technology

Committee:

Thomas Barker, PhD, Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology
Andrew Kowalczyk, PhD, Departments of Cell Biology and Dermatology, Emory University
Todd Sulchek, PhD, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology

Large-scale morphogenetic movements during early embryo development are driven by complex changes in biochemical and biophysical factors. Current models for amniote primitive streak morphogenesis and gastrulation take into account numerous genetic pathways but largely ignore the role of mechanical forces. Here, we used atomic force microscopy (AFM) to obtain for the first time precise biomechanical properties of the early avian embryo. Our data reveal that the primitive streak is significantly stiffer than neighboring regions of the epiblast, and that it is stiffer than the pre-primitive streak epiblast. To test our hypothesis that these changes in mechanical properties are due to a localized increase of actomyosin contractility, we inhibited actomyosin contractility via the Rho kinase (ROCK) pathway using the small-molecule inhibitor Y-27632. Our results using several different assays show the following: 1) primitive streak formation was blocked; 2) the time-dependent increase in primitive streak stiff ness was abolished; and 3) convergence of epiblast cells to the midline was inhibited. Taken together, our data suggest that actomyosin contractility is necessary for primitive streak morphogenesis, and specially, ROCK plays a critical role. To better understand the underlying mechanisms of this fundamental process, future models should account for the findings presented in this study.

 As presumptive cardiac cells traverse the course of differentiation into cardiac myocytes during cardiogenesis, the sequence, magnitude, and spatiotemporal map of biomechanical and biochemical signals has not been fully explored. There have been many studies detailing the induction of cardiogenesis on a variety of substrates and ECM proteins, but none have completed a rigorous study of the effects of substrate stiffness on the induction of precardiac cells prior to the onset of cardiac gene expression (smooth muscle alpha actin [SMAA] at stage 5.) We investigate the effects of the mechanical environment on precardiac cell behaviors in an in vitro setting to elucidate the effect of substrate stiff ness on precardiac tissue. The cells in the anterior portion of the primitive streak are fated to form the heart, and we show differing levels of SMAA expression on substrates of differing moduli, whereas 3-day-old embryonic cardiomyocytes do not show differing levels of SMAA on the same substrates.  This finding suggests that substrate stiff ness effects the behavior of undifferentiated embryonic primitive streak cells, an effect which appears to lose its "potency" after heart formation. We cannot determine the physical mechanisms during morphogenesis without understanding the response of precardiac cells to changes in their mechanical environment.