Tissue-resident stem and progenitor cells support skeletal healing and regeneration throughout life. The periosteum is a major source of cells involved in fracture healing. We hypothesize that the periosteum is enriched for osteoprogenitor cells compared to other tissue compartments, and therefore periosteal cells more efficiently contribute to bone healing.
Bone marrow, endosteum and periosteum were isolated from mouse hindlimbs using enzymatic digestion. Most stem and progenitor markers examined were enriched in the periosteum, including Sca1 (7.0% ± 1.4% in the periosteum, <1% in the endosteum and bone marrow), CD51, CD90, PDGFRa, and CD200. After sorting of freshly isolated periosteal cells, Sca1+/CD51+ cells showed efficient CFU-F formation (9-fold increase compared with total non-haematopoietic cells) as did Sca1-/CD51+ cells (5-fold increase). Sca1+/CD51+ colonies could undergo both osteogenic and adipogenic differentiation in vitro, while Sca1-/CD51+ cells were osteogenic only.
We also investigated the response of periosteum to a local scratch injury, which causes formation of a callus. Sca1+/CD51+ and Sca1-/CD51+ were significantly expanded 7 days after injury. In addition, CD90+ cells were more abundant by day 3 (34.8% ± 4.4%) and day 7 (42.9% ± 4.2%) after injury compared with uninjured (22.4% ± 2.8%).
We also examined marker expression in human skeletal tissue: bone marrow from the femoral head; endosteum (or bone-associated cells) digested from cleaned trabecular bone; and periosteum dissected from the femoral neck, followed by collagenase digestion. Most markers were enriched in the periosteum, including CD51 (32.3% ± 17.2% in the periosteum and <1% in the endosteum and bone marrow), CD90 (32.8% ± 15.4%, <1% in the endosteum and bone marrow), and CD73 (52.5% ± 19.3%, <0.5% in the endosteum and bone marrow).
Using cell surface markers we identified populations of progenitors with dual lineage potential and osteoprogenitors. Our findings suggest that the periosteum is highly enriched for skeletal stem and progenitor cells, and some of these populations expand in response to injury.