Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Agriculture, Food and Environment


Veterinary Science

First Advisor

Dr. James N. MacLeod


Articular cartilage in mammals has a limited intrinsic capacity to repair structural injuries and defects, a fact that contributes to the chronic and progressive nature of osteoarthritis. Current treatment modalities do not enable articular cartilage to achieve a complete and permanent restoration of normal structure and function with large or partial thickness lesions. In contrast to mammals, Mexican axolotl salamanders (Ambystoma mexicanum) have demonstrated the remarkable ability to spontaneously and completely repair even large joint cartilage lesions, an intrinsic healing process that involves interzone cells in the intraarticular space. Further, when interzone tissue is transplanted into critical sized diaphyseal defects, it forms an entirely new diarthrodial joint in these amphibians, demonstrating a multi-differentiation potential. Cellular and molecular mechanisms of this repair process, however, remain unclear. This thesis examined whether paracrine signals are an important variable in the interaction between interzone cells and the skeletal microenvironment. In vivo experiments in axolotl salamanders compared the outcomes of interzone tissue transplants placed in either a skeletal or non-skeletal site within the same individual. The hypothesis tested was that the interzone-mediated repair of skeletal defects is regulated by mechanisms that are localized to the skeletal microenvironment. Interzone cell proliferation and differentiation was only observed in skeletal transplant sites, suggesting that local signals from the skeletal microenvironment played a vital role in the interzone-mediated repair process. In a second series of experiments, paracrine regulation of the proliferation and chondrogenic differentiation of equine interzone cells was evaluated in an in vitro co-culture system. The results of cellular proliferation studies indicated a mitogenic effect of skeletal paracrine signals on interzone cells. Expression of cartilage biomarker genes, evaluated at both RNA and protein levels, were used to assess chondrogenic differentiation. The in vitro findings suggested that paracrine signals may have a role in the chondrogenic differentiation of interzone cells, but were not compelling. The response may have been limited by levels of paracrine factor accumulation achieved in the co-culture system used for these experiments. Taken together, however, the data support a model that paracrine factors from skeletal tissues are important regulators of interzone cell proliferation and differentiation. This knowledge advances the assessment of interzone cells as a potential cell-based therapy for the repair of articular cartilage injuries.

Digital Object Identifier (DOI)