Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Larry A Taber
The heart is the first functioning organ in the developing embryo. Initially, the heart is a relatively straight tube created by folding and fusion of the cardiogenic fields, which lie bilaterally within the blastoderm. Shortly after formation, the primitive heart tube (HT) undergoes the morphogenetic process of c-looping as it bends and twists into a c-shaped tube. All these transformations require physical forces, which remain poorly understood.
The aim of this dissertation is to elucidate some of the biophysical mechanisms that create and shape the early HT. Our work involves a combination of ex ovo experiments and computational modeling. Experiments were performed on embryonic chicken hearts, which are morphologically similar to human hearts during development.
First, we explored a somewhat puzzling aspect of early heart development. Previous studies have shown that myosin-II-based cytoskeletal contraction is required for fusion of the heart fields before looping begins, but not as these tissues continue to fuse and extend the length of the HT during subsequent c-looping. To investigate this fundamental change in behavior, we focused on the tissues around the anterior intestinal portal (AIP), where fusion takes place. Our results indicate that stiffness and tangential tension decreased bilaterally with distance from the embryonic midline along the AIP. The stiffness and tension gradients increased to peaks at Hamburger-Hamilton (HH) stage 9 and decreased immediately afterward. Along with experimental results of contraction inhibition, finite-element models indicate that the measured mechanical gradients are consistent with a relatively uniform contraction of the endoderm along the AIP. Taken together, these results suggest that, before looping begins at HH10, cytoskeletal contraction pulls the bilateral cardiogenic fields toward the midline where they begin to fuse to create the HT. By HH10, however, the fusion process is far enough along to enable apposing cardiac progenitor cells to subsequently undergo filopodia-mediated “zippering” without the continuing need for contraction.
Next, in light of recently published data, we examined the possible role of differential hypertrophic growth in driving the bending component of c-looping. Using cultured isolated hearts, which bend without the complicating effects of external loads, we found that myocardial growth patterns correlate with bending. We also developed finite-element models that include previously measured regional changes in myocardial growth during c-looping. The simulations show that differential growth alone can produce results that agree reasonably well with trends in our experimental data, including changes in HT morphology and tissue strains and stresses. Incorporating other mechanisms into the model, such as active changes in myocardial cell shape, provides closer agreement. These results suggest that regional difference in hypertrophic myocardial growth is the primary cause of the bending component of c-looping, with other mechanisms playing lesser roles.
Finally, we extended the model of the previous study to explore the physical plausibility of a hypothesis for the entire process of c-looping. According to our hypothesis, bending is driven primarily by differential hypertrophic growth in the myocardium, torsion is mainly caused by compressive loads exerted by the overlying splanchnopleuric membrane, and looping direction is determined by asymmetric regional growth in the omphalomesenteric veins at the caudal end of the HT. Our model includes both bending and torsion of the HT, realistic 3D geometry, and loads exerted by neighboring tissues. The behavior of the model is in reasonable agreement with available experimental data from control and mechanically perturbed embryos, offering support for our hypothesis. The results also suggest, however, that several other mechanisms contribute secondarily to normal looping, and we speculate that these mechanisms play backup roles when looping is perturbed.
In summary, studies of this dissertation address several important questions during early cardiac development. The results should enrich our understanding of the underlying biophysical mechanisms.
Shi, Yunfei, "Biophysical Mechanisms of Early Heart Morphogenesis" (2014). All Theses and Dissertations (ETDs). 1346.