Dmp1 Lineage Cells Contribute Significantly to Periosteal Lamellar Bone Formation Induced by Mechanical Loading But Are Depleted from the Bone Surface During Rapid Bone Formation.

Previous work has shown that osteoprogenitor cells (Prx1+) and pre-osteoblasts (Osx+) contribute to mechanical loading-induced bone formation. However, the role of mature Dmp1-expressing osteoblasts has not been reported. In this study we assessed the contribution of osteoblast lineage cells to bone formation at an early time point following mechanical loading (day 8 from onset of loading). We labeled Osx-expressing and Dmp1-expressing cells in inducible Osx and Dmp1 reporter mice (iOsx-Ai9, iDmp1-Ai9), respectively, 3 weeks before loading. Mice were then loaded daily for 5 days (days 1-5) and were dosed with 5-ethynyl-2'-deoxyuridine (EdU) in their drinking water until euthanasia on day 8. Mice were loaded to lamellar and woven bone inducing stimulation (-7 N/1400 με, -10 N/2000 με) to assess differences in these processes. We found varied responses in males and females to the loading stimuli, inducing modest lamellar (females, -7 N), moderate lamellar (males, -10 N), and robust woven (females, -10 N) bone. Overall, we found that preexisting (ie, lineage positive) Osx-expressing and Dmp1-expressing cells contribute largely to the bone formation response, especially during modest bone formation, while our results stuggest that other (non-lineage-positive) cells support the sustained bone formation response during rapid bone formation. With moderate or robust levels of bone formation, a decrease in preexisting Osx-expressing and Dmp1-expressing cells at the bone surface occurred, with a near depletion of Dmp1-expressing cells from the surface in female mice loaded to -10 N (from 52% to 11%). These cells appeared to be replaced by lineage-negative cells from the periosteum. We also found a dose response in proliferation, with 17% to 18% of bone surface cells arising via proliferation in modest lamellar, 38% to 53% in moderate lamellar, and 59% to 81% in robust woven bone formation. In summary, our results show predominant contributions by preexisting Osx and Dmp1 lineage cells to loading-induced lamellar bone formation, whereas recruitment of earlier osteoprogenitors and increased cell proliferation support robust woven bone formation. © 2021 The Authors.