These findings demonstrate that the tensile properties, an important and often overlooked metric of cartilage development, increase with time in culture in engineered hydrogel-based cartilage constructs. Under the free-swelling conditions employed in the present study, tensile moduli and toughness did not match that of the native tissue, though significant time-dependent increases were observed with the inclusion of TGF-beta3. Of note, MSC-seeded constructs achieved tensile properties that were comparable to chondrocyte-seeded constructs, confirming the utility of this alternative cell source in cartilage tissue engineering. Further work, including both modulation of the chemical and mechanical culture environment, is required to optimize the deposition of collagen and its remodeling to achieve tensile properties in engineered constructs matching the native tissue.

Tissue engineering is a promising approach for the repair of articular cartilage defects, with engineered constructs emerging that match native tissue properties. However, the inflammatory environment of the damaged joint might compromise outcomes, and this may be impacted by the choice of cell source in terms of their ability to operate anabolically in an inflamed environment. Here, we compared the response of engineered cartilage derived from native chondrocytes and mesenchymal stem cells (MSCs) to challenge by TNFα and IL-1β in order to determine if either cell type possessed an inherent advantage. Compositional (extracellular matrix) and functional (mechanical) characteristics, as well as the release of catabolic mediators (matrix metalloproteinases [MMPs], nitric oxide [NO]) were assessed to determine cell- and tissue-level changes following exposure to IL-1β or TNF-α. Results demonstrated that MSC-derived constructs were more sensitive to inflammatory mediators than chondrocyte-derived constructs, exhibiting a greater loss of proteoglycans and functional properties at lower cytokine concentrations. While MSCs and chondrocytes both have the capacity to form functional engineered cartilage in vitro, this study suggests that the presence of an inflammatory environment is more likely to impair the in vivo success of MSC-derived cartilage repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Articular cartilage lines the surfaces of joints and functions to absorb shock and distribute load. To date, few strategies exist for restoring damaged articular surfaces; therefore, cartilage tissue engineering (TE) has emerged as a means to generate functional replacement tissues. To optimize growth and maturation of TE constructs, various methodologies have been employed, including 3D culture, coupled with mechanical stimulation [1] and growth factor supplementation [2]. Recently, studies have shown that temporal application of TGF-β3 enhances the compressive properties and GAG content of chondrocyte-laden hydrogels to near-native levels after the removal of this morphogen [3, 4]. However, as chondrocytes may prove impractical for clinical use, mesenchymal stem cells (MSCs) have been increasingly utilized in cartilage TE. While MSCs are able to undergo chondrogenesis and deposit cartilaginous extracellular matrix (ECM), they have not demonstrated functional parity with chondrocytes [5]. One recent study showed MSCs were able to maintain a chondrocytic phenotype after brief exposure of TGF-β3, though mechanical and biochemical properties were not assessed [6]. In this study, we evaluated these properties in chondrocyte- and MSC-laden hydrogels with transient exposure of TGF-β3 in a chemically defined medium. In addition, we explored the effects of varying seeding density in MSC-laden constructs on functional properties. We hypothesized that transient application of TGF-β3 would improve functional properties of MSC-laden constructs in a seeding density-dependent manner, and that these changes would be marked by differences in cartilaginous gene expression, particularly of enzymes involved in proteoglycan synthesis.

Degeneration of cartilage resulting from trauma or disease processes is an increasingly prevalent problem in the aging population. Intrinsic repair of cartilage is limited and few methodologies exist, short of prosthetic replacement, for restoring damaged articular surfaces. These realities engender a need for new strategies for extrinsic repair. One strategy, tissue engineering, generates replacement cartilage composed of scaffolds and differentiated chondrocytes [1]. In addition to chondrocytes, recent work has demonstrated that mesenchymal stem cells (MSCs) isolated from bone marrow may be induced to take on a chondrocyte-like phenotype [2]. Tissue engineered constructs of either cell type can yield near-native properties (though those derived from MSCs are typically lower) [3]. While such constructs may be surgically implanted to replace areas denuded of cartilage, one factor to consider is that these defect sites exist in an already inflamed joint [4]. In addition, surgical interventions trigger further inflammatory responses, greatest in the area of intervention [5]. Recent literature has shown that pro-inflammatory cytokines, such as interleukin-1beta (IL-1β), instigate catabolic destruction not only of cartilage explants, but also of chondrocyte-based engineered cartilage constructs [6–9]. Still other studies have shown that un-differentiated MSCs themselves may exert an anti-inflammatory effect on their local environment [10–12]. Thus the current study examined the effect of varying doses of IL-1β exposure on both chondrocyte- and MSC-based engineered cartilage constructs to determine the relative sensitivity of neo-cartilage derived from each cell type to cytokine induced degradation.