A common lineage between monocytes and osteoclasts has been suggested but not yet proved, and a n osteoclast precursor might be an immature cell of the monocyte-macrophage family. We therefore compared the ability of cord blood and adult monocytes in long-term culture to differentiate toward osteoclasts. Both adult and cord monocytes were cultured for 3 weeks in the presence of 20% horse serum. The proportion of multinucleated cells formed was influenced by 1,25(OH)zD3 in cord, but not in adult monocyte cultures: 10-9M 1,25(OH)zD3 increased multinucleated cells from 13 +-2 to 26 & 1% of total cells in cord monocyte cultures. The formation of multinucleated cells in cord monocyte cultures, in the presence of lO-' M 1,25(OH)zD, was decreased by salmon calcitonin (dose dependently from 10-8 to 10p6 M) and increased by 1-34 parathormone (100 ng/ml). None of these hormones induced any modification of the proportion of multinucleated cells formed in adult monocytes culture. Specific antigens on the membrane of the cells obtained after 3 weeks culture in the presence of 10-9 M 1,25(OH)zD3 were assessed by immunocytochemistry. The respective proportion of adult and cord labeled cells was 64 * 11 vs. 63 * 6% with Leu M5 (specific for monocyte) and 68 ? 7 vs. 30 ? 10% (P<0.05) with the anti-HLA DR antibody. The monoclonal antibody 23C6 is specific to the vitronectin receptor, which is highly expressed by osteoclasts-41 * 2% of the cells in cord monocyte cultures-but none in the adult monocytes culture were labeled with 23C6 a t the end of the culture period. The ability of cells formed in cord monocyte culture to resorb devitalized bone on which they were cultured was investigated after 3 weeks culture. These cells did not release 45Ca from devitalized 45Ca-prelabeled rat calvaria; they did not form an acidic extracellular compartment, as assessed by acridine orange fluorescence, and there was no ultrastructural evidence of a brush border at the interface with bone. In conclusion, cord blood monocytes in long-term culture do not form mature os- teoclasts. However, in contrast to adult monocytes, some acquire, in culture, characteristics of osteoclast precursors: sensitivity to osteotropic hormones and expression of a n osteoclast membrane-specific antigen.
To study bone resorption, it is of interest to elaborate models allowing evaluation of local and systemic factors influencing osteoclast generation. It has been established that osteoclast precursor originates in hemopoietic tissue (Ash et al., 1980; Jotereau and Le Bouarin, 1978). Clinical and experimental studies in mice have shown that marrow transplantation restored defective osteoclastic bone resorption during osteopetrosis (Coccia et al., 1980;Walker, 1975); therefore, bone marrow cultures of different species have been used to study osteoclast differentiation (Testa et al., 1981;Roodman et al., 1985; MacDonald et al., 1987). This culture system gives rise to multinucleated tartrate-resistant, acid phosphatase positive cells responding appropriately to osteotropic hormones (Roodman et al., 1985; MacDonald et al., 1987); however, for studying the direct effect of factors regulating osteoclast precursor proliferation or differentiation, this 0 1990 ALAN R. LISS, INC.