Synovial joints are essential for skeletal function, and much is known about their structure, distinct tissues and susceptibility to congenital and acquired diseases, including osteoarthritis (OA). In contrast, little is known about their developmental biology. Were such information available, it could be used directly or together with bioengineering tools to create new joint repair strategies. This project started several years ago to fill these important gaps. In the limb, joint formation initiates with appearance of a mesenchymal cell interzone at each prospective joint site within the cartilaginous long bone anlagen. However, it had long remained unclear whether the interzone cells serve as a transient joint demarcation point, whether they actually produce joint tissues over time and if so, how they perform that key task. To address these questions, we genetically tracked interzone cells using Gdf5 and found that Gdf5-Cre/R26-lineage cells persisted and produced joint tissues over time, thus representing specialized progenitors with joint formation capacity. We found also that Wnt/?- catenin signaling was important for surface zone cells expressing Prg4/lubricin. Though novel, the data left much unanswered, including how the Gdf5-lineage cells produce different joint tissues, how articular cartilage grows and acquires its important cell columnar organization, whether joint progenitors persist postnatally, and what roles they could have in repair. To address these issues, we created new Gdf5-CreER and Prg4-CreER lines and used others including Axin 2-CreER. Our preliminary data now indicate that: (a) progenitors within the prenatal joint are sequentially specified for contribution to different tissues; (b) lateral expanson of incipient articular cartilage over the growing long bone epiphysis involves progenitors near the groove of Ranvier with an Axin 2-lineage; (c) articular cartilage maturation and columnar organization involve cell intercalation mechanisms rather than appositional growth as suggested by others; and (iv) joint progenitor performance and Prg4 expression are enhanced by Kartogenin. These data lead to our central hypothesis that joint formation is a modular process brought about by progenitors specified sequentially during embryogenesis, with Wnt/?-catenin representing a major regulatory pathway. We also hypothesize that postnatal articular cartilage growth and maturation require specific changes in chondrocyte cytostructure and phenotype, and that joint cells' repair capacity is amenable to pharmacological or genetic stimulation. Our Aims are: (1) To fate-map joint progenitors during prenatal development and postnatal growth; (2) To determine mechanisms by which articular cartilage thickens and attains its columnar organization; and (3) To assess the repair capacity of joint progenitors and their progenies and means to boost it pharmacologically and/or genetically. The project will produce fundamentally new insights into the mechanisms of joint formation and repair and will thus have considerable basic and translational medicine relevance and significance.