For redifferentiation of dedifferentiated HACs, 3D cultures exhibit the most potent chondrogenic potential, whereas a hypertrophic phenotype is best achieved in 2D cultures. This is the first human study that systematically evaluates the differences between proliferation, GAG content, protein expression and mRNA expression of commonly used 2D and 3D chondrocyte culture techniques.

BMP-2 and BMP-7 display opposing actions on the chondrogenic outcome of differentiating progenitor cells: BMP-2 acts a specific inducer of chondrocyte hypertrophy, while BMP-7 appears to increase or maintain chondrogenic potential and prevent chondrocyte hypertrophy. Our results pave the way for an application-dependent differential use of BMP-2 or BMP-7.

Skeletogenesis and bone fracture healing involve endochondral ossification, a process during which cartilaginous primordia are gradually replaced by bone tissue. In line with a role for cyclooxygenase-2 (COX-2) in the endochondral ossifi cation process, non-steroidal anti-inflammatory drugs (NSAIDs) were reported to negatively affect bone fracture healing due to impaired osteogenesis. However, a role for COX-2 activity in the chondrogenic phase of endochondral ossifi cation has not been addressed before. We show that COX-2 activity fulfi ls an important regulatory function in chondrocyte hypertrophic differentiation. Our data reveal essential cross-talk between COX-2 and bone morphogenic protein-2 (BMP-2) during chondrocyte hypertrophic differentiation. BMP-2 mediated chondrocyte hypertrophy is associated with increased COX-2 expression and pharmacological inhibition of COX-2 activity by NSAIDs (e.g., Celecoxib) decreases hypertrophic differentiation in various chondrogenic models in vitro and in vivo, while leaving early chondrogenic development unaltered. Our fi ndings demonstrate that COX-2 activity is a novel factor partaking in chondrocyte hypertrophy in the context of endochondral ossifi cation and these observations provide a novel etiological perspective on the adverse effects of NSAIDs on bone fracture healing and have important implications for the use of NSAIDs during endochondral skeletal development.