Migration and phenotypic transformation of osteoclast precursors into mature osteoclasts: The effect of a bisphosphonate

Osteoclast-devoid bone explants were cultured together with embryonic liver as a source of osteoclast precursors, but separated from each other by a filter. Cells migrated through the filter toward the calcified matrix and acquired the characteristics of mature, tartrate-resistant acid phosphatase-positive (TRAP+) osteoclasts upon contact with the bone explant. Migration and attachment could be visualized separately. Progressive reduction of filter pore size resulted in progressive reduction of resorption because the use of smaller pores made it increasingly difficult for cells to pass. Indeed, the use of 0.22-micron filters, through which no cells can pass, but which still allow full passage of medium, completely blocked the resorption. When migrating cells from fetal liver were arrested for 10 days by using a combination of filters with different pore sizes, the arrested cells showed a tendency to fuse just opposite the mineralized matrix. Furthermore, a great number of the arrested cells expressed the macrophage-specific cell-surface antigen F4/80 and showed acid phosphatase activity, but none of these cells were tartrate resistant. The acquisition of tartrate-resistant acid phosphatase activity upon contact with the bone explant and subsequent resorption of this explant could be prevented by exposure of the system to the bisphosphonate dimethyl-APD (Me2-APD), whereas migration of cells through the filter was not affected. We suggest that the bisphosphonate interferes with a matrix factor that is essential for the attachment and subsequent transformation of the osteoclast precursor into the mature phenotype.