Origin of Osteoclasts: Osteoclast Precursor Cells

Abstract

Osteoclasts are multinucleated bone-resorbing cells and a key player in bone remodeling for health and disease. Since the discovery of osteoclasts in 1873, the structure and function of osteoclasts and the molecular and cellular mechanisms of osteoclastogenesis have been extensively studied. Moreover, it has been well established that osteoclasts are differentiated in vitro from myeloid cells such as bone marrow macrophages or monocytes. The concept showing that osteoclasts are derived from a specific population (named osteoclast precursor cells [OCPs]) among myeloid cells has been long hypothesized. However, the specific precursor population of osteoclasts is not clearly defined yet. A growing body of work provides evidence of the developmental origin and lifespan of murine osteoclasts, particularly in vivo. Here, we review the emerging evidence that supports the existence of OCPs and discuss current insights into their identity.

Keywords: Osteoclast precursor cells; Osteoclast pregenitor; Osteoclasts.

Fig. 1

Emergence of yolk sac (YS) macrophage in mammalian embryos and their contributions to formation of neonatal osteoclasts. Mammalian embryos produce primitive erythro-myeloid progenitors (EMPs) at embryonic day 7 (E7). Primitive EMPs further give rise to colony stimulating factor 1 receptor+ (CSF1R⁺) YS macrophages at E8.5. Late EMPs are produced from the hemogenic endothelium at E8.25, inducing the emergence of CX₃C motif chemokine receptor 1+ (CX₃CR1⁺) YS macrophages at E8.5. Hematopoietic stem cell (HSC) precursors are also produced at E10.5 from the aorta-gonad-mesonephros (AGM) region, and they differentiate into HSCs at E12.5. CX₃CR1⁺ macrophages mainly contribute to neonatal osteoclastogenesis. CSF1R⁺ YS macrophages were present at postnatal day 0 (P0). At P0.5 months (mo), CSF1R⁺ YS macrophages were only present in small amounts in the diaphysis but none were found in the metaphysis. CX₃CR1⁺ macrophages that are differentiated in E8.5 and colonize the entire embryo at E9.5 were abundantly found in the P0 diaphysis and metaphysis. Unlike CSF1R⁺ macrophages, CX₃CR1⁺ macrophages contribute to both neonatal osteoclastogenesis and bone remodeling in adult long bones. CX₃CR1⁺ macrophages survive past 6 mo and reside in adult femurs. HSCs fuse with EMPs to produce multinucleated osteoclasts, which were found in P0 and adult femurs.

Fig. 2

Circulating osteoclast precursor cells (OCPs) and their engraftment in bone injury. Circulating CX₃C motif chemokine receptor 1+ (CX₃CR1⁺) macrophages infiltrate injury sites of the disturbed bone marrow to repair the fractured callus and produce new bone. Circulating macrophages specifically infiltrate the fractured callus as well as the bone around the injury site to promote ossification. Despite the consensus surrounding the contribution of circulating OCPs in disturbed bone marrows, the definitive method by which hematopoietic stem cells interact with erythro-myeloid progenitors to create long-lived osteoclasts in normal homeostasis remains unclear.

Fig. 3

Bone marrow (BM) and circulating osteoclast (OC) precursor cell (OCP) surface markers in murine and human models. (Left panel) CD11b^−loCD115⁺CD117⁺ BM cells have high frequency of OCP populations and can differentiate into OCs upon stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Further fractionation of BM cell populations has identified a population of B220⁻CD3⁻CD45R⁻CD11b^−/lowCD115⁺CD117⁺ cells that show early osteoclastogenic activity. CD11b^−/lowLy6C^hi cells in murine BM are enriched for osteoclast information upon co-culture with M-CSF and RANKL and are greatly increased following the induction of arthritis. In murine models, circulating CX₃C motif chemokine receptor 1+ cells have been shown to migrate to the bone and become mature OCs. Parabiosis experiments between Csf1r^cre;Rosa26LSL^−YFP and Csf1r^cre;Rosa26^LSL-tdTomato were able to demonstrate that tdTomato⁺ OCs are replenished by circulating YFP⁺ precursor cells. (Right panel) Human blood monocytes are segregated based on their cell surface expression of CD14 and CD16 into non-classical (CD14⁺CD16⁺), classical (CD14⁺CD16⁻), and intermediate (CD14⁺⁺CD16⁺) monocytes; all three subsets can differentiate into OCs. In human BM, the presence of CD117 and FLT3 and absence of CD11b are used to define human hematopoietic stem cells, which contains the common progenitor of GMODP in addition to MODPs.