Functional Tissue Engineering of Cartilage Using Adipose-derived Stem Cells
Articular cartilage is the thin, load-bearing connective tissue that lines the ends of long bones in diarthroidal joints, providing predominantly a mechanical function. Because cartilage is avascular and aneural, it has little capacity for self-repair if damaged. One repair strategy is through a functional tissue engineering approach using adipose-derived stem cells (ASCs). ASCs are an abundant progenitor cell source easily obtained through a minimally invasive liposuction procedure. When appropriately stimulated, ASCs have demonstrated significant potential for chondrogenic differentiation. Though studies have demonstrated the ability of ASCs to synthesize cartilage-specific macromolecules, a more thorough understanding of factors that modulate ASC chondrogenesis is required. Accordingly, the central aim of this dissertation was to study the chondrogenic response of ASCs to biochemical, biomechanical, and biomaterial factors.
We hypothesized that factors, other than TGF-beta and dexamethasone, would improve ASC chondrogenesis. BMP-6 emerged as a potent regulator of ASC chondrogenesis, particularly in early culture, as noted by significant upregulation of cartilage-specific extracellular matrix (ECM) genes and downregulation of cartilage hypertrophy markers.
Hypothesizing that biomechanical factors would accelerate the formation of cartilage-specific macromolecules, we designed and manufactured an instrument to apply dynamic deformational loading to ASC seeded constructs. Dynamic loading significantly inhibited ASC metabolism and downregulated cartilage-specific ECM genes. However, 21 days of dynamic loading induced the production of type II collagen, a principal component of articular cartilage.
We hypothesized that a biomaterial derived from cartilage would serve as a bioactive scaffold and induce chondrogenic differentiation. The novel, ECM-derived scaffold promoted the most robust differentiation of ASCs relative to both biochemical and biomechanical factors, particularly noted by a type II collagen-rich matrix after 28 days of culture. After 42 days of culture, biphasic mechanical testing revealed an aggregate modulus of 150 kPa, approaching that of native cartilage. These data suggest that the ECM-derived scaffold may retain important signaling molecules to drive differentiation or that ASC differentiation is dependent on proper cell anchorage.
In summary, we have shown that biochemical, biomechanical, and biomaterial factors have strong influences on the chondrogenic potential of ASCs. Optimization of these factors will ultimately be required to successfully engineer a functional tissue.
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