Growth of osteoclast precursor-like cells from whole mouse bone marrow: inhibitory effect of CSF-1

We studied the growth of mononuclear phagocytes (MPs) and mononuclear osteoclast precursor-resembling (OCP-like) cells from freshly isolated whole mouse bone marrow. Expression of tartrate-resistant acid phosphatase (TRAcP) served as a general marker for the identification of OCP-like cells. Bone resorbing capacity of the cultured cells was studied in a coculture assay with periost-free fetal bone rudiments. Freshly isolated mouse bone marrow contained approximately 30 OCP-like cells and 1100 MPs per 10(6) nucleated bone marrow cells. OCP-like cell numbers did not increase in suspension cultures containing macrophage-colony stimulating factor (CSF-1), in contrast to the number of MPs which increased strongly. OCP-like cell numbers however did increase in monolayer cultures, which also allowed anchorage-dependent growth of bone marrow fibroblasts. Strongest increase of OCP-like cells occurred in monolayer cultures in the absence of CSF-1. Dermal fibroblasts of fetal mice did not enhance OCP-like cell growth. OCP-like cell density was strongly correlated with the number of osteoclast nuclei formed in cocultures with periost-free bone rudiments. These data indicate that mononuclear cells, cytochemically and functionally resembling osteoclast precursors, may be grown from mouse bone marrow. CSF-1 inhibited growth of OCP-like cells, indicating that osteoclast precursors differ from mononuclear phagocytes in growth requirements.