RUNX2 and osteoarthritis: In addition, the cluster analogous to the osteoblast-lineage cluster in steady state displayed an osteochondral phenotype after injury (online supplemental figure 17C, D), similar to the hybrid skeletal cells that form the early osteophyte in osteoarthritis.11 Immunostaining confirmed upregulation of Sox9 and Runx2 expression after injury, especially at the joint margin where synovium and periosteum merge and chondrophyte formation is typically observed (figure 3H).