Murine BM was fractionated using a series of hematopoietic markers to characterize its osteoclast progenitor populations. We found that the early osteoclastogenic activity in total BM was recapitulated by a population of cells contained within the CD11b^−/low CD45R⁻CD3⁻CD115^high fraction.

Introduction: Osteoclasts are of hematopoietic origin and they have been shown to share the same lineage as macrophages. We further characterized the phenotype of osteoclast progenitor populations in murine bone marrow (BM) by analyzing their cell surface markers.

Materials and Methods: We used fluorescence‐activated cell sorting (FACS) to identify the subsets of BM cells that contained osteoclast progenitors. We fractionated BM according to several markers and cultured the sorted populations for a period of 2–6 days with macrophage‐colony stimulating factor (M‐CSF) and RANKL. The numbers of multinucleated osteoclast‐like cells (OCLs) that formed in the cultures were counted.

Results: We found that the CD45R⁻CD11b^−/low population recapitulated the early osteoclastogenic activity of total BM. In addition, although previous experiments indicated that osteoclastogenic activity was enriched within the CD45R⁺ population, we found that highly purified CD45R⁺ BM was incapable of differentiating into osteoclasts in vitro. We also found that CD45R⁻CD11b^high BM cells were an inefficient source of osteoclast progenitors. However, CD11b was transiently upregulated by cells of the CD45R⁻CD11b^−/low fraction early (within 24 h) during culture with M‐CSF. Finally, further fractionation of BM using CD115 and CD117 showed that, as osteoclast precursor cells matured, they downregulate CD117 but remain CD115⁺. Curiously, pure populations of CD117⁻ (CD115^high) cells isolated fresh from BM have low osteoclastogenic activity in vitro.

Conclusions: We provided a refined analysis of the precise subpopulations of murine BM that are capable of differentiating into OCLs in vitro when treated with M‐CSF and RANKL.