Osteoclasts-key players in skeletal health and disease

Although bone is one of the hardest tissues in the body, necessary for its structural and protective roles, this organ is not static. Bone matrix must be renewed over time in order to maintain its mechanical properties, and myeloid lineage cells called osteoclasts (OCs) are the specialized cells that perform this critical function. Since bone is the major storage site for calcium, OCs play an important role in the regulation of this signaling ion by releasing it from bone. In this process, OCs respond indirectly to calcium-regulating hormones such as parathyroid hormone and 1,25(OH)₂vitamin D₃. Growth factors such as insulin-like growth factor-1 (IGF-1) and transforming growth factor β (TGF-β) are also incorporated into bone matrix and released by OCs, affecting the coupling of bone formation to bone resorption and potentially targeting other cells in the microenvironment, such as metastatic tumors. Lastly, OCs retain features of other myeloid cells, such as antigen presentation and cytokine production, which afford them the potential to affect immune responses. Thus, the OC plays many roles in health and disease.