Role of Scx(+)/Sox9(+) cells as potential progenitor cells for postnatal supraspinatus enthesis formation and healing after injury in mice

A multipotent cell population co-expressing a basic-helix-loop-helix transcription factor scleraxis (Scx) and SRY-box 9 (Sox9) has been shown to contribute to the establishment of entheses (tendon attachment sites) during mouse embryonic development. The present study aimed to investigate the involvement of Scx(+)/Sox9(+) cells in the postnatal formation of fibrocartilaginous entheses and in the healing process after injury, using ScxGFP transgenic mice. We demonstrate that Scx(+)/Sox9(+) cells are localized in layers at the insertion site during the postnatal formation of fibrocartilaginous entheses of supraspinatus tendon until postnatal 3 weeks. Further, these cells were rarely seen at postnatal 6 weeks, when mature fibrocartilaginous entheses were formed. Furthermore, we investigated the involvement of Scx(+)/Sox9(+) cells in the healing process after supraspinatus tendon enthesis injury, comparing the responses of 20- and 3-week-old mice. In the healing process of 20-week-old mice with disorganized fibrovascular tissue in response to injury, a small number of Scx(+)/Sox9(+) cells transiently appeared from 1 week after injury, but they were rarely seen at 4 weeks after injury. Meanwhile, in 3-week-old mice, a thin layer of fibrocartilaginous tissue with calcification was formed at healing enthesis at 4 weeks after injury. From 1 to 2 weeks after injury, more Scx(+)/Sox9(+) cells, widely distributed at the injured site, were seen compared with the 20-week-old mice. At 4 weeks after injury, these cells were located near the surface of the recreated fibrocartilaginous layer. This spatiotemporal localization pattern of Scx(+)/Sox9(+) cells at the injured enthesis in our 3-week-old mouse model was similar to that in postnatal fibrocartilaginous enthesis formation. These findings indicate that Scx(+)/Sox9(+) cells may have a role as entheseal progenitor-like cells during postnatal maturation of fibrocartilaginous entheses and healing after injury in a manner similar to that seen in embryonic development.