THE OSTEOCLAST GENERATION - AN IN-VITRO AND IN-VIVO STUDY WITH A GENETICALLY LABELED AVIAN MONOCYTIC CELL-LINE

Osteoclasts are multinucleate giant cells responsible for bone resorpt ion. Osteoclast precursors are hematopoietic mononucleate cells, which give rise to osteoclasts after fusion, Nevertheless, the precise stag e of differentiation where osteoclast precursors diverge from other he matopoietic lineages is still debated, We describe here both in vitro and in vivo approaches to the study of the osteoclast differentiation pathway. We used cells of the BM2 avian monocytic cell line, which are able to differentiate into macrophages both in vitro and in vivo, In order to follow the progeny of BM2 monocytes, we have derived a BM2 ce ll clone expressing the nlslacZ gene (BM2nlslacZ) which has still reta ined the main features of the parental cell line. In vitro, when BM2nl slacZ cells were triggered toward macrophages, they participated in th e formation of multi-nucleate osteoclast-like cells as seen by their b lue nuclei, Furthermore, when BM2nlslacZ cells were injected into the blood stream of chicken embryos, they could give rise to blue nucleate macrophages in the bone marrow, as well as to osteoclasts with blue n uclei in bone. Finally, we have shown that fusion of tagged mononuclea te precursor cells not only occurs with other mononucleate precursor c ells but also with mature multinucleate osteoclasts. This work shows t hat cells already engaged in the late stages of the monocytic differen tiation pathway are able to differentiate into osteoclasts and that os teoclast divergence takes place after the monocyte stage.