Date of Award
Doctor of Philosophy (PhD)
For decades, biologists have worked to identify many of the genetic and molecular factors involved in heart and eye development. Over the years, these efforts have helped elucidate the vast biochemical signaling networks that regulate specification and differentiation in the embryo. Although mechanical forces play an essential role in morphogenesis, the shaping of embryonic structures, the biophysical mechanisms that link these molecular factors to physical changes in morphology remain unclear.
The aim of this thesis is to identify some of the mechanical forces which drive heart tube and eye assembly in the early chick embryo. A distinctive feature of this work is the combination of mathematical modeling and laboratory experiments. Experiments were performed on chick embryos, in which the heart and eyes are morphologically similar to those in human embryos.
The heart is the first functioning organ in the developing embryo. For decades, it was commonly thought that the bilateral heart fields in the early embryo fold directly toward the midline, where they meet and fuse to create the primitive heart tube. Recent studies have challenged this view, however, suggesting that the heart fields fold diagonally. As early foregut and heart tube morphogenesis are intimately related, this finding also raises questions concerning the traditional view of foregut formation. Here, we combine experiments on chick embryos with computational modeling to explore a new hypothesis for the physical mechanisms of heart tube and foregut formation. According to our hypothesis, differential anisotropic growth between mesoderm and endoderm drives diagonal folding. Then, active contraction along the anterior intestinal portal generates tension to elongate the foregut and heart tube. We test this hypothesis using biochemical perturbations of cell proliferation and contractility, as well as computational modeling based on nonlinear elasticity theory including growth and contraction. The present results generally support the view that differential growth and actomyosin contraction drive formation of the foregut and heart tube in the early chick embryo.
Precise shaping of the eye is crucial for proper vision. The second part of this work focuses on development of eye from a biomechanical perspective. The embryonic eyes begin as bilateral protrusions called optic vesicles (OVs) that grow outward from the anterior end of the brain tube. The optic vesicles contact and adhere to the overlying surface ectoderm (SE) via extracellular matrix (ECM). Then, both layers thicken in the region of contact to form the retinal and lens placodes, which bend inward (invaginate) to form the roughly spherical optic cup (OC, primitive retina) and lens pit, respectively. These two structures then separate, and the lens pit continues to fold until it closes to create the lens vesicle (LV).
First, we explored the mechanisms that create the OVs. Mechanical dissections were used to remove the surface ectoderm (SE), a membrane that contacts the outer surfaces of the OVs. Principal components analysis of OV shapes suggests that the SE exerts asymmetric loads that cause the OVs to flatten and shear caudally during the earliest stages of OV development and later to bend in the caudal and dorsal directions. These deformations cause the initially spherical OVs to become pear-shaped. Exposure to the myosin II inhibitor blebbistatin reduced these effects, suggesting that cytoskeletal contraction controls OV shape by regulating tension in the SE. To test the physical plausibility of these interpretations, we developed 2-D finite-element models for frontal and transverse cross-sections of the forebrain, including frictionless contact between the SE and OVs. With geometric data used to specify differential growth in the OVs, these models were used to simulate each experiment (control, SE removed, no contraction). For each case, the predicted shape of the OV agrees reasonably well with experiments. The results of this study indicate that differential growth in the OV and external pressure exerted by the SE are sufficient to cause the global changes in OV shape observed during the earliest stages of eye development.
Next, we created a 3-D finite element model to study the forces that create the OC. Once the OV forms, previous studies have shown that extracellular matrix (ECM) locally constrains the OV as it grows, forcing it to thicken and invaginate. Experimental observations have suggested that the ECM is required for the early stages but not the late stages of OV invagination. Our finite element model consisting of a growing spherical OV in contact with SE reproduced the observed behavior, as well as experimental measurements of OV curvature, wall thickness, and invagination depth reasonably well. These results support our hypothesis for invagination and formation of OC.
Lastly, we also studied the forces that create the LV. Previous studies have shown that the lens placode is produced by growth of the SE constrained locally by ECM, while actomyosin contraction at the apical surface of the lens placode plays a major role in its subsequent invagination. Programmed cell death, or apoptosis, which has been observed near the opening in the lens pit, supplies the required force for closure by causing tissue surrounding the opening to shorten circumferentially. Results from our model support this view.
In summary, studies of this dissertation address several important questions during early heart and eye development. The results should enrich our understanding of the underlying biophysical mechanisms.
Chair and Committee
Larry Anders A. Taber carlsson
Larry A. Taber, Anders E. carlsson, Philip V. Bayly, Ralf Wessel,
Hosseini, Seyedhadi, "Biomechanics of Early Heart and Eye Morphogenesis" (2017). Arts & Sciences Electronic Theses and Dissertations. 1256.
Available for download on Wednesday, December 15, 2117