Bone formation occurs through two ossification processes, intramembranous and endochondral. Intramembranous ossification is characterized by the direct differentiation of stem cells into osteoblasts, which then create bone. Endochondral ossification involves an intermediate step, as stem cells first differentiate into chondrocytes and produce a cartilage anlage. The chondrocytes mature into hypertrophic chondrocytes, which transform the cartilage anlage into bone. Bone tissue engineering has predominantly mimicked intramembranous ossification, creating osteoblast-based grafts through the direct differentiation of stem cells. Though successful in specific applications, greater adoption of osteoblast-based grafts has failed due to incomplete integration, limited regeneration, and poor mechanical maintenance. To overcome these obstacles, inspiration was drawn from native bone fracture repair, creating tissue engineered bone grafts replicating endochondral ossification. Hypertrophic chondrocytes, the key cell in endochondral ossification, were differentiated from mesenchymal stem cell sources by first generating chondrocytes and then instigating maturation to hypertrophic chondrocytes. Conditions influencing this differentiation were investigated, indicating the necessity of prolonged chondrogenic cultivation and elevated oxygen concentrations to ensure widespread hypertrophic maturation. Comparing the bone production performance of differentiated hypertrophic chondrocytes to differentiated osteoblasts revealed that hypertrophic chondrocytes deposit significantly greater volume of bone mineral at a higher density than osteoblasts, albeit in a more juvenile form. When implanted subcutaneously, the hypertrophic chondrocytes stimulated turnover of this juvenile template into compact-like bone, whereas osteoblasts proceeded with processes similar to bone remodeling, generating spongy-like bone. Implanting these tissue engineered constructs into an orthotopic, critical-sized femoral defect saw hypertrophic chondrocyte-based constructs integrate quickly with the femur and facilitate the creation of significantly more bone, resulting in a successful bridging of the defect. The success of hypertrophic chondrocyte-based grafts in overcoming the failures of tissue engineered bone grafts demonstrates the potential of endochondral ossification inspired bone strategies and prompts its further investigation towards clinical utilization.

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (M.S.)--Mississippi State University. Department of Agricultural and Biological Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves [202]-230). Also available in print.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves [202]-230). Also available online.

Thesis (Ph.D)--Biomedical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Boyan, Barbara; Committee Co-Chair: Schwartz, Zvi; Committee Member: Bonewald, Lynda; Committee Member: Jo, Hanjoong; Committee Member: Sambanis, Athanassios. Part of the SMARTech Electronic Thesis and Dissertation Collection.

published_or_final_version / Biochemistry / Master / Master of Philosophy

Endoplasmic Reticulum (ER) stress signal is a cellular response to various insults including abnormal protein folding load, activating the unfolded protein response. Under severe ER stress, apoptosis will occur in most cell types. Interestingly, this does not happen in a disease model for Metaphyseal chondrodysplasia type Schmid (MCDS), where ER stress was activated in the hypertrophic zone of the growth plate where mutant collagen X proteins that cannot be folded correctly is expressed. Instead of normal progression from proliferating chondrocytes (PCs) to hypertrophic chondrocytes (HCs) and conversion to bone, HCs in MCDS mice undergo re-differentiation to PCs as a survival strategy due to an activation of ER stress. Transcription factors are known to be important in regulating differentiation. p53 family members, as transcription factors, are known to play important roles in developmental processes including cellular reprogramming, thus, we hypothesize that the ectopic expression of key transcription factors, p53 and TAp63, which are activated by ER stress is involved in HC re-differentiation. p53 is normally expressed in late PCs, Pre-HCs, and upper HCs, while TAp63 is expressed in PCs and Pre-HCs suggesting they may have roles in chondrocyte differentiation. p53 activated under ER stress in HCs are nuclear localized in MCDS mice, but did not invoke the apoptotic programme. In this project, using quantitative analyse to study the expression level of p53 and p63 isoforms, it was confirmed that p53 and TAp63γ are in part transcriptionally activated upon ER stress. From functional study by inactivating p53 in MCDS mice, it was shown that p53 alone was not sufficient to mediate re-differentiation. Given that TAp63γ isoforms is also highly upregulated upon ER stress, and the negative regulator, ΔNp63, is downregulated, this combination of change in gene expression also need to be considered. Furthermore, known regulators of p53 and p63 activity such as ASPP1 and iASPP are also differentially expressed in HCs, and are altered upon activation of ER stress favouring cell survival. Thus, it would be important to evaluate the combination of TAp63 in the re-differentiation process from conditional inactivation of p63 or in combination with p53 to gain a clearer understanding of the contribution and relationship of these transcription factors in the survival strategy of stressed HCs. / published_or_final_version / Biochemistry / Master / Master of Philosophy

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Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.