Abstract
It has previously been demonstrated that dynamic deformational loading of chondrocyte-seeded agarose hydrogels over the course of 1 month can increase construct mechanical and biochemical properties relative to free-swelling controls. The present study examines the manner in which two mediators of matrix biosynthesis, the growth factors TGF-β1 and IGF-I, interact with applied dynamic deformational loading. Under free-swelling conditions in control medium (C), the [proteoglycan content][collagen content][equilibrium aggregate modulus] of cell-laden (10 × 106 cells/mL) 2% agarose constructs reached a peak of [0.54% wet weight (ww)][0.16% ww][13.4 kPa]c, whereas the addition of TGF-β1 or IGF-I to the control medium led to significantly higher peaks of [1.18% ww][0.97% ww][23.6 kPa]C-TGF and [1.00% ww][0.63% ww][19.3 kPa]C-IGF, respectively, by day 28 or 35 (p < 0.01). Under dynamic loading in control medium (L), the measured parameters were [1.10% ww][0.52% ww][24.5 kPa]L, and with the addition of TGF-β1 or IGF-I to the control medium these further increased to [1.49% ww][1.07% ww][50.5 kPa]L-TGF and [1.48% ww][0.81% ww][46.2 kPa]L-IGF, respectively (p < 0.05). Immunohistochemical staining revealed that type II collagen accumulated primarily in the pericellular area under free-swelling conditions, but spanned the entire tissue in dynamically loaded constructs. Applied in concert, dynamic deformational loading and TGF-β1 or IGF-I increased the aggregate modulus of engineered constructs by 277 or 245%, respectively, an increase greater than the sum of either stimulus applied alone. These results support the hypothesis that the combination of chemical and mechanical promoters of matrix biosynthesis can optimize the growth of tissue-engineered cartilage constructs.
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