Resorbing bone is chemotactic for monocytes [16]

MONOCYTES are often found adjacent to bone-resorbing surfaces. Their function at these sites is unknown, although they have been shown in vitro to have the ability to resorb bone^1,2. There is also strong circumstantial evidence that circulating monocytes are precursors for the major bone-resorbing cell, the osteoclast^3-7. The mechanism by which circulating monocytes are attracted to sites of bone remodelling are unknown. We have examined the products of resorbing bone for chemotactic effects on circulating human monocytes and found that organ cultures of resorbing fetal rat long bones produced factors which were chemotactic for human peripheral blood monocytes, but not for neutrophils or lymphocytes. The chemotactic effect of resorbing bones occurred independent of the humoral mediator of bone resorption. The mechanisms by which monocytes are attracted to bone surfaces may be of fundamental importance to our understanding of bone cell dynamics and bone resorption.