The discovery of the multipotent lineage of mesenchymal stem cells has dawned a new age in tissue engineering, where an autologous cell-seeded scaffold can be implanted into different therapeutic sites. Mesenchymal stem cells have been reported to differentiate into numerous anchorage-dependent cell phenotypes, including neurons, adipocytes, myoblasts, chondrocytes, tenocytes, and osteoblasts. A seminal work detailing that mesenchymal stem cells can be directed towards differentiation of different cell types by substrate stiffness alone [1] has led to numerous studies attempting to understand how cells can sense the stiffness of their substrate [2–3] Substrate stiffness has been shown to be an inducer of stem cell differentiation. MSCs on extremely soft substrates (250 Pa), similar to the stiffness of bone marrow, became quiescent but still retained their multipotency [4]. Elastic substrates in the stiffness range of 34 kPa revealed MSCs with osteoblast morphology, and osteocalcin along with other osteoblast markers were expressed [1]. However, osteogenesis has been found to increase on much stiffer (20–80 kPa) [5–6] (400 kPa) [7] as well as much softer substrates (75 Pa) [8]. Overall, cells have increased projected cell area and proliferation on stiffer substrates, leading to higher stress fiber formation. This study seeks to understand if the stiffness of the substrate has any effect on the differentiation potential of osteochondral progenitor cells into bone cells, using an in vitro dual fluorescent mouse model.

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