Differentiation to Cartilage and Bone

A Biocompare article 'Signaling For Cartilage' is of particular interest to me having lost much cartilage in my knees. Differentiation is a process wherein the nature of a cell is determined. Prior to cellular differentiation cells have a potential to develop in different ways and become different cell types. The differentiation process entails a commitment to a particular pathway and its associated cell type.

The linked article states that "skeletal progenitor cells differentiate into cartilage cells when one master gene actually suppresses the action of another." Quoting from the article: "Skeletons are made of bone and cartilage cells that are differentiated from the same multipotent stem cell."

So how does this work? What are the regulatory genes and how do they function?

Two proteins, known as transcription factors, have been identified as involved in the bone and cartilage differentiation process. "Transcription factors are protein complexes that help RNA polymerase bind to DNA." These proteins are SOX9 and RUNX2; said to be master transcription factors. SOX9 reportedly directs the differentiation of skeletal progenitor cells into cartilage and RUNX2, directs the same cells to bone.

SOX9 affects RUNX2 by inhibiting its expression so that a cell will differentiate to cartilage. However, it is not known what causes the suppression of SOX9.