Chondro progenitor cell response to specifically modified substrate interfaces

Current visions in cartilage repair aim at moving from fibrocartilaginous to a more hyaline like repair tissue by combining autologous cells of different origins (in situ recruited or in vitro precultivated) with repair supporting biomaterials. The role of a chondro-supportive biomaterial within a cartilage defect is seen to support infiltration/recruitment of chnodroprogenitor cells (CPC), accelerate their chondrogenic differentiation and to protect/modulate the newly formed tissue.
Overall, this thesis aimed at studying whether modification of selected substrate interface properties allows for modulating chondroprogenitor cell phenotype & function under expansion or differentiation conditions in vitro. The goal was to contribute to the definition of material characteristics which could be implemented in the design of biomaterials in order to improve current matrix assisted cartilage repair strategies and outcomes.
Substrate composition & architecture (Chapter I) were found to modulate the chondrogenic differentiation of mesenchymal stem cells (MSC). Using a di block copolymer model substrate (Polyactive®) with a more hydrophobic composition better supported MSC chondrogenesis, than a more hydrophilic composition. Moreover, a highly interconnected 3D fibre deposited scaffold architecture allowed for the formation of larger MSC aggregates and was found to considerably better support MSC chondrogenesis than compression molded scaffolds.
Restricting cell/substrate interaction specifically to an RGD-containing peptide ligand (Chapter II) modulated the de-differentiation of proliferating HAC and their subsequent capacity to form cartilaginous matrix. This demonstrated the advantage of small ECM fragments in combination with protein resistant materials to control cell/surface interaction. An important finding was the better maintenance of the HAC chondrocytic phenotype during expansion. It suggests, that an RGD-restricted substrate has the potential to improve the outcome of matrix assisted in situ cartilage repair, which initially requires recruited/infiltrated CPC to proliferate, while keeping/inducing their capacity to form cartilaginous matrix.
Substrate elasticity allowed for modulating the chondrogenic commitment of HAC (Chapter III). The finding, that a soft substrate (0.3kPa) better supported the chondrogenic phenotype of HAC than i.e. a stiffer substrate (75kPa) suggests this parameter to be promising for modulating the outcome of matrix assisted cartilage repair.
Overall, this thesis demonstrates that substrate properties hold substantial potential to modulate CPC behaviour, which could be exploited to improve materials employed in matrix assisted cartilage repair. Yet, although differentially supporting CPC chondrogenesis, none of the substrates was per se chondro-inductive (see chapter I and III) but required for additional, exogenic stimuli as for e.g. transforming growth factor beta (TGF). Thus, modulatory substrate properties as i.e. architecture, composition, ligand presentation and stiffness should be combined with the instructive capacity of soluble stimuli to exploit the full potential of biomaterials in cartilage repair.