Macrophage polarization and bone formation: A review

The contribution of inflammation to bone loss is well documented in arthritis and other diseases with an emphasis on how inflammatory cytokines promote osteoclastogenesis. Macrophages are the major producers of cytokines in inflammation, and the factors they produce depend upon their activation state or polarization. In recent years, it has become apparent that macrophages are also capable of interacting with osteoblasts and their mesenchymal precursors. This interaction provides growth and differentiation factors from one cell that act on the other and visa versa-a concept akin to the requirement for a feeder layer to grow hemopoietic cells or the coupling that occurs between osteoblasts and osteoclasts to maintain bone homeostasis. Alternatively, activated macrophages are the most likely candidates to promote bone formation and have also been implicated in the tissue repair process in other tissues. In bone, a number of factors, including oncostatin M, have been shown to promote osteoblast formation both in vitro and in vivo. This review discusses the different cell types involved, cellular mediators, and how this can be used to direct new bone anabolic approaches.