Over the past few years, many of the transcription factors that control chondrogenesis and osteogenesis have been discovered and a pattern of transcription factor expression and activation is beginning to emerge that soon will clearly define the mechanisms that control the phenotype of the cell. As mesenchymal cells differentiate into chondrocytes or osteoblasts, a series of transcription factors is expressed that direct positive expression in the appropriate cells and repress expression in other cells. In chondrocytes, the necessary positive factors include members of the Sox family, Sox 9, L-Sox-5, and Sox 6, and in osteoblasts, the necessary positive factors are Osterix and Cbfa1/Runx2. Negative factors for chondrocytes are deltaEF-1, C/EBP, and CYRBP1. Many other positive and negative factors play accessory roles in development and in response to growth factors and will be discussed.
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