Traumatized and diseased joint cartilage is the leading cause of disability, afflicting 33% of the adult population. State-of-the-art treatments for destroyed joint cartilage are seriously limited. Tissue engineering has this far failed to regenerate high-quality cartilage and prevent subsequent cartilage hypertrophy and degeneration. We have developed biodegradable nanofibrous hollow microspheres (NF-HMS) as a novel injectable carrier for chondrocytes and successfully regenerated high-quality cartilage. To circumvent both the shortage of autologous chondrocytes and the regulatory barriers to cell-based therapies, we propose to regenerate cartilage and suppress hypertrophy and degeneration by injecting the novel NF-HMS microcarrier that delivers key biological molecules to activate endogenous bone marrow stromal cells (BMSCs) for hyaline cartilage regeneration and suppress. First, we will mechanistically determine whether HIF-1, HIF-2, or both promote chondrogenesis and prevent hypertrophy/degeneration. Second, we will develop TGF- and the HIF stabilizer releasing NF-HMS to facilitate the chondrogenic differentiation of human and rabbit BMSCs using in vitro and a subcutaneous implantation model. Third, we will use the novel injectable NF-HMS carrier to synergize TGF- and HIF activities of endogenous BMSCs in order to regenerate and maintain high-quality cartilage in a rabbit osteochondral repair model. By accomplishing these specific aims, we will substantially deepen our understanding of the key factors that affect chondrogenesis, cartilage regeneration, and prevention of hypertrophy and cartilage degeneration, leading to a novel cartilage repair therapy without using exogenous or exogenously manipulated cells. The understandings and strategies developed in this project can also be utilized to engineer other tissues. 1