Myeloid blasts are the mouse bone marrow cells prone to differentiate into osteoclasts.

Cells of the myeloid lineage at various stages of maturity can differentiate into multinucleated osteoclasts. Yet, it is unclear which developmental stages of this lineage are more prone to become osteoclasts than others. We investigated the osteoclastogenic potential of three successive stages of myeloid development isolated from mouse bone marrow. Early blasts (CD31hi/Ly-6C-), myeloid blasts (CD31+/Ly-6C+), and monocytes (CD31-/Ly-6Chi), as well as unfractionated marrow cells, were cultured in the presence of M-CSF and receptor activator of NF-B ligand (RANKL), and the differentiation toward multinucleated cells and their capacity to resorb bone was assessed. Myeloid blasts developed rapidly into multinucleated cells; in only 4 days, maximal numbers were reached, whereas the other fractions required 8 days to reach maximal numbers. Bone resorption was observed after 6 (myeloid blasts and monocyte-derived osteoclasts) and 8 (early blast-derived osteoclasts) days. This difference in kinetics in osteoclast-forming capacity was confirmed by the analysis of osteoclast-related genes. In addition, the myeloid blast fraction proved to be most sensitive to M-CSF and RANKL, as assessed with a colony-forming assay. Our results show that osteoclasts can develop from all stages of myeloid differentiation, but myeloid blasts are equipped to do so within a short period of time.