Conditioned medium of fetal mouse long bone rudiments stimulates the formation of osteoclast precursor-like cells from mouse bone marrow.

We studied the influence of skeletal tissue on expression of the osteoclastic phenotype in the mouse, in vivo and in vitro. In various soft and hard tissues of adult and fetal mice the distribution of mono- and multinucleate cells showing tartrate-resistant acid phosphatase was studied, using enzyme histochemistry on undecalcified plastic sections. Cells with strong TRAcP activity were only observed in mineralized tissues. Multinucleate TRAcP cells were exclusively found in close correlation with mineral resorption. In fetal bones mononuclear TRAcP cells appeared in the surrounding soft tissue prior to osteoclast formation. In addition, conditioned media of fetal long bone rudiments (FBCM) increased the number of mononuclear cells showing strong TRAcP activity in 7 d cultures of adult bone marrow. FBCM stimulated DNA synthesis in TRAcP cells and induced their multi-nuclearity. Pretreatment with FBCM increased the capacity of bone marrow cultures to form osteoclasts in coculture with fetal bone rudiments. However, FBCM did not change the number of cells with tartrate-sensitive acid phosphate activity (TSAcP cells). The activity of FBCM was heat labile and was not detectable in CM of killed bones. CM of embryonic mouse fibroblasts which contains M-CSF activity, strongly increased the number of TSAcP cells but reduced the number of TRAcP cells. These data suggest that fetal mouse bone tissue induces the differentiation of osteoclast precursors. In addition, fetal bone rudiments but not embryonic fibroblasts seem to produce a factor(s) which stimulate(s) the formation of cells showing characteristics of osteoclast precursor cells. Osteoclasts and macrophages seem to have different growth requirements, indicating that they represent separate cell lines which may nevertheless derive from a common progenitor.