Tissue-engineered strategies to repair chondrogenic and osteogenic defects

Abstract

Disorders in the musculoskeletal (MSD) system affect hundreds of millions of people around the globe which suffer of physical disability accompanied with severe long-term pain. Increased life expectancy combined with a change of unprocessed food to a diet high on glucose and fat as well as limited physical activity at work and home continuously increases the prevalence of MSD worldwide. These diseases and syndromes have reached dimensions that result in a heavy economic burden for the society. Estimations of total cost range from 240 billion in the EU to $213 billion in the US and thus demand sophisticated solutions to prevent a crippled health care system. Therefore, current treatment options need to be revised and replaced with superior approaches to reduce costs. These procedures still involve transplantation of autologous or allogeneic tissue which can result in donor-site morbidity, trigger immune reactions, or transfer diseases. Thus, the emerging field of tissue engineering and regenerative medicine tries to address the shortcoming of current therapies. In doing so, new technologies and techniques are developed to produce a functional tissue able to be implanted into the patient and instantly perform its designated task. In this thesis different concepts for bone and cartilage repair are investigated. Although bone has an innate ability to regenerate, large complex fractures often exceed the natural healing capacity of bone and will result in non-unions. For effective treatment, these non-unions require external intervention with the aid of autografts or allografts which cause aforementioned disadvantages such as donor site morbidity, immune reaction and transfer of diseases. In chapter I, we provide an alternative for the current clinical approach in using a decellularized bone graft of xenogenic origin which was seeded with adipose-derived stem cells (ASCs) and stimulated in a perfusion bioreactor system. We could show that hypertrophic chondrocytes that recapitulated endochondral ossification could bridge 7 out of 8 defects, while osteoblasts were able to bridge just 1out of 8. However, although medium perfusion stimulated the formation of bone template in vitro, it failed to enhance bone regeneration in vivo. Contrary to bone, cartilage innate healing ability is heavily compromised by an overall hardly metabolic active tissue. Thus, cartilage damage induced via trauma or degenerative processes outperforms the limited regenerative potential and necessitates external intervention. An integral part in the functionality of articular cartilage plays the specialized architecture and structure a composition which has not been met by any commercially available biomaterial in clinical use. In chapter II, we introduce AuriScaff as a novel biomaterial from xenogenic origin that could be successfully repopulated with ASC and chondrocytes in vitro. We further demonstrated that it could outperform a clinically used scaffold in mechanical stability as well as production of high-quality hyaline-like cartilage in vivo. Utilizing a biomimetic bioreactor, we could show that AuriScaff is able to not only withstand initial dynamic loading but also provides housing for cells to mature and reorganize the extracellular matrix. While cartilage defects in adolescent and young people have a chance to regenerate, osteoarthritis (OA), a degenerative disease of the joint presumably triggered by acute trauma or chronic overload, affecting a 8 vast portion of the adult population, heavily diminishes this potential. Historically classified as a simple wear-and-tear disease, recent findings identified reactive oxygen species (ROS) as a leading factor in the evolution of OA. With the application of hydrostatic pressure via compressed air, we could induce the production of elevated levels of superoxide and other ROS species that generated an osteoarthritic cartilage model. This model was used to investigate signaling pathways that are known to be involved in OA and could be further employed as a versatile tool for anti-oxidative drug testing.