Ph.D Thesis


Ph.D StudentLandau Shira
SubjectA Biophysical Understanding of Vascular Morphogenesis
within Engineered Tissues
DepartmentDepartment of Biomedical Engineering
Supervisor PROF. Shulamit Levenberg


Abstract

The robust repair of large wounds and tissue defects relies on blood flow. This vascularization is a significant challenge faced by tissue engineering on the path to forming viable thick, implantable tissue constructs. Without the presence of this vascular network, oxygen and nutrients cannot reach the cells located far from host blood vessels. In-vitro vessel network systems frequently serve as platforms for examining cellular and functional mechanisms underlying angiogenesis. Understanding the signals activating the observed cell migration, organization and differentiation, can advance the design of engineered vascularized tissues. In the first part of the dissertation, we present evidence of the migration of endothelial cells into the depths of the scaffold, where they formed blood vessels. The supporting cells exhibited localization-dependent phenotypes, where cells nearby the blood vessels exhibited a mature phenotype, whereas cells on the scaffold surface showed a pericyte-like phenotype. In the second part of the dissertation we examined how external stimulus can affect and improve the formation and morphogenesis of blood vessels within engineered tissues, we focused on two different stimuli: 1) Biomaterials- We combined synthetic PLLA/PLGA with tropoelastin to create scaffolds for blood vessels. These scaffolds were shown to stimulate vessel formation and to enhance host vessel penetration upon implantation within mice with abdominal wall defects.  The results of this study point to the enormous potential of these combined materials in promoting the vascularization of implanted tissue-engineered constructs. 2) External forces- We show that under cyclic stretch vessel formation is improved, cells form a more flourished and mature blood vessel network. When further investigating cellular behavior under stretch, fibroblasts and endothelial cells show distinctly different sensitivities to mechanical stimulation. The fibroblasts, sense the stress directly and respond by increased alignment, proliferation, differentiation, and migration, facilitated by YAP translocation into the nucleus. In contrast, the endothelial cells form aligned vessels by tracking fibroblast alignment. Moreover, we show that cyclic stretching of the biomaterial orients the newly formed network perpendicularly to the stretching direction, and this is to minimize the elastic stored energy transferred from the stretched substrate. We further show that under static stretch vascular networks orient parallel to the stretching direction due to force-induced anisotropy of the biomaterial polymer network. Additionally, static stretching followed by cyclic stretching reveals a competition between the two mechanosensitive mechanisms. These results demonstrate tissue-level mechanosensitivity and constitute an essential step toward developing enhanced tissue repair capabilities using well-oriented vascular networks.