Osteocalcin expressing cells from tendon sheaths in mice contribute to tendon repair by activating Hedgehog signaling.

Both extrinsic and intrinsic tissues contribute to tendon repair, but the origin and molecular functions of extrinsic tissues in tendon repair are not fully understood. Here we show that tendon sheath cells harbor stem/progenitor cell properties and contribute to tendon repair by activating Hedgehog signaling. We found that Osteocalcin (Bglap) can be used as an adult tendon sheath-specific marker in mice. Lineage tracing experiments show that Bglap-expressing cells in adult sheath tissues possess clonogenic and multipotent properties comparable to those of stem/progenitor cells isolated from tendon fibers. Transplantation of sheath tissues improves tendon repair. Mechanistically, Hh signaling in sheath tissues is necessary and sufficient to promote the proliferation of Mkx-expressing cells in sheath tissues, and its action is mediated through TGF beta/Smad3 signaling. Furthermore, co-localization of GLI1(+) and MKX+ cells is also found in human tendinopathy specimens. Our work reveals the molecular function of Hh signaling in extrinsic sheath tissues for tendon repair.