Coordinated expression of scleraxis and Sox9 genes during embryonic development of tendons and cartilage.

Embryonic development of tendons is in close association with that of cartilage and bone. Although these tissues are derived from mesenchymal progenitor cells which also give rise to muscle and fat, their fates clearly diverse in early embryonic stages. Transcription factors may play pivotal roles in the process of determination and differentiation of tendon cells as well as other cells in the skeletal system. Scleraxis, a basic helix-loop-helix (bHLH) type transcription factor, is expressed in mesenchymal progenitors that later form connective tissues including tendons. Sox9 is an HMG-box containing transcription factor, which is expressed at high levels in chondrocytes. We hypothesized that the two transcription factors regulate the fate of cells that interact with each other at the interface between the two tissues during divergence of their differentiation pathways. To address this point, we investigated scleraxis and Sox9 mRNA expression during mouse embyogenesis focusing on the coordinated development of tendons and skeletons. In the early stage of mesenchymal tissue development at 10.5 d.p.c. scleraxis and Sox9 transcripts were expressed in the mesenchymal progenitor cells in the appendicular and axial mesenchyme. At 11.5 d.p.c., scleraxis transcripts were observed in the mesenchymal tissue surrounding skeletal primordia which express Sox9. From this stage. seleraxis expression was closely associated with, but distinct from, formation of skeletal primordia. At 13.5 d.p.c., scleraxis was expressed broadly in the interface between muscle and skeletal primordia while Sox9 expression is confined within the early skeletal primordia. Then, at 15.5 d.p.c., scleraxis transcripts were more restricted to tendons. These observations revealed the presence of temporal and spatial association of scleraxis expression during embryonic development of tendon precursor cells in close association with that of Sox9 expression in chondrogenic cells in skeletal tissues.