Mesenchymal stem cells (MSCs) are an attractive cell type for cartilage tissue engineering in that they can undergo chondrogenesis in a variety of 3D contexts [1]. Focused efforts in MSC-based cartilage tissue engineering have recently culminated in the formation of biologic materials possessing biochemical and functional mechanical properties that match that of the native tissue [2]. These approaches generally involve the continuous or intermittent application of pro-chondrogenic growth factors during in vitro culture. For example, in one recent study, we showed robust construct maturation in MSC-seeded hyaluronic acid (HA) hydrogels transiently exposed to high levels of TGF-β3 [3]. Despite the promise of this approach, MSCs are a multipotent cell type and retain a predilection towards hypertrophic phenotypic conversion (i.e., bone formation) when removed from a pro-chondrogenic environment (e.g., in vivo implantation). Indeed, even in a chondrogenic environment, many MSC-based cultures express pre-hypertrophic markers, including type X collagen, MMP13, and alkaline phosphatase [4]. To address this issue, recent studies have investigated co-culture of human articular chondrocytes and MSCs in both pellet and hydrogel environments. Chondrocytes appear to enhance the initial efficiency of MSC chondrogenic conversion, as well as limit hypertrophic changes in some instances (potentially via secretion of PTHrP and/or other factors) [5–7]. While these findings are intriguing, articular cartilage has a unique depth-dependent morphology including zonal differences in chondrocyte identity. Ng et al. showed that zonal chondrocytes seeded in a bi-layered agarose hydrogel construct can recreate depth-dependent cellular and mechanical heterogeneity, suggesting that these identities are retained with transfer to 3D culture systems [8]. Further, Cheng et al. showed that differences in matrix accumulation and hypertrophy in zonal chondrocytes was controlled by bone morphogenic protein [9]. To determine whether differences in zonal chondrocyte identity influences MSC fate decisions, we evaluated functional properties and phenotypic stability in photocrosslinked hyaluronic acid (HA) hydrogels using distinct, zonal chondrocyte cell fractions co-cultured with bone marrow derived MSCs.

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