Abstract
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.
Highlights
Entheses, the attachment sites between tendons and bone, are classified into two types according to their histological appearance at the tendon-bone interface: fibrous entheses, which typically insert into the metaphyses or diaphyses of long bones and include a periosteal component, and fibrocartilaginous entheses, which typically insert into epiphyses or bone ridges, such as the rotator cuff tendon or Achilles tendon enthesis [1, 2].Fibrocartilaginous entheses can bear stress concentrations that arise between the tendon and bone, and prevent injury and failure using structural gradients, viz. tendon, uncalcified fibrocartilage, and calcified fibrocartilage [3,4,5]
We investigated the existence of Scx+/SRY-box 9 (Sox9)+ cells during the postnatal formation of fibrocartilaginous entheses of supraspinatus tendon using ScxGFP transgenic (Tg) mice
At 3-weeks of age, a thin layer of fibrocartilaginous tissue without tidemark was observed between the supraspinatus tendon and the bone surface of the humeral head (Fig 2B), and some alkaline phosphatase (ALP)-positive cells were detected near the surface of the fibrocartilage layer (Fig 2F)
Summary
The attachment sites between tendons and bone, are classified into two types according to their histological appearance at the tendon-bone interface: fibrous entheses, which typically insert into the metaphyses or diaphyses of long bones and include a periosteal component, and fibrocartilaginous entheses, which typically insert into epiphyses or bone ridges, such as the rotator cuff tendon or Achilles tendon enthesis [1, 2].Fibrocartilaginous entheses can bear stress concentrations that arise between the tendon and bone, and prevent injury and failure using structural gradients, viz. tendon, uncalcified fibrocartilage, and calcified fibrocartilage [3,4,5]. The attachment sites between tendons and bone, are classified into two types according to their histological appearance at the tendon-bone interface: fibrous entheses, which typically insert into the metaphyses or diaphyses of long bones and include a periosteal component, and fibrocartilaginous entheses, which typically insert into epiphyses or bone ridges, such as the rotator cuff tendon or Achilles tendon enthesis [1, 2]. Fibrocartilaginous entheses can bear stress concentrations that arise between the tendon and bone, and prevent injury and failure using structural gradients, viz. The stress concentrations at enthesis occasionally cause injury, such as rotator cuff tears [6, 7]. It has been demonstrated that the regeneration of functional fibrocartilaginous enthesis is difficult after injury or surgical tendon-bone repair because of poor healing capability, as demonstrated by the weak fibrovascular scar tissue formed between tendon and bone [8,9,10]. Understanding of the natural development of fibrocartilaginous enthesis, which occurs postnatally [12], and of the healing process after injury, are crucial for developing therapeutic strategies to reconstruct functional fibrocartilaginous enthesis in order to improve clinical outcomes [13, 14]
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