The origin of osteoclasts: an immunohistochemical study on macrophages and osteoclasts in embryonic rat bone.

The origin of osteoclasts was studied in embryonic rat bone primordia using a set of monoclonal antibodies (ED1, ED2, and ED3) that exclusively recognize monocytes and macrophage. ED1 recognizes monocytes and macrophages. Mononuclear phagocytes which were ED1 positive were found in the perichondrium/periosteum of developing bone. These cells started to infiltrate the primordia when the cartilage became hypertrophic. During bone formation, multinucleated ED1-positive cells with the morphological characteristics of osteoclasts were found in the developing bone marrow cavity and against the bone collar. The present findings support the notion that osteoclasts arise by fusion of mononuclear phagocytes derived from blood monocytes.