Scx+/Sox9+ progenitors contribute to the establishment of the junction between cartilage and tendon/ligament

SRY-box containing gene 9 (Sox9) and scleraxis (Scx) regulate cartilage and tendon formation, respectively. Here we report that murine Scx(+)/Sox9(+) progenitors differentiate into chondrocytes and tenocytes/ligamentocytes to form the junction between cartilage and tendon/ligament. Sox9 lineage tracing in the Scx(+) domain revealed that Scx(+) progenitors can be subdivided into two distinct populations with regard to their Sox9 expression history: Scx(+)/Sox9(+) and Scx(+)/Sox9(+)-progenitors. Tenocytes are derived from Scx(+)/Sox9(+) and Scx(+)/Sox9(-) progenitors. The closer the tendon is to the cartilaginous primordium, the more tenocytes arise from Scx(+)/Sox9(+) progenitors. Ligamentocytes as well as the annulus fibrosus cells of the intervertebral discs are descendants of Scx(+)/Sox9(+) progenitors. Conditional inactivation of Sox9 in Scx(+)/Sox9(+) cells causes defective formation in the attachment sites of tendons/ligaments into the cartilage, and in the annulus fibrosus of the intervertebral discs. Thus, the Scx(+)/Sox9(+) progenitor pool is a unique multipotent cell population that gives rise to tenocytes, ligamentocytes and chondrocytes for the establishment of the chondro-tendinous/ligamentous junction.