Animal model and in vitro cultures suggest that osteoclasts and cells of the mononuclear phagocyte system share a common precursor. However, the human osteoclast precursor has not been positively identified. We attempted to identify the precursor in situ by using a number of osteoclast- and macrophage-selective markers, together with the expression of osteopontin mRNA, previously shown to be abundant in human osteoclasts. Sections of osteophytic bone and a panel of inflammatory connective tissues were processed for in situ hybridization; serial sections were analyzed for tartrate-resistant acid phosphatase (TRAP) and nonspecific esterase (NSE) activity, selective cytochemical markers for the osteoclast and cells of the macrophage/monocyte lineage, respectively. The murine anti-human osteoclast monoclonal antibodies 23C6 (vitronectin receptor) and C35 (osteoclast-selective) were used to further identify the osteoclast phenotype. We compared osteoclasts, giant cells, and their respective putative mononuclear precursors. At resorption sites within osteophytic bone, osteopontin mRNA was expressed in osteoclasts and a distinct population of TRAP(+), NSE(-) mononuclear cells. Adjacent clusters of mononuclear cells were TRAP(-) and NSE(+) or were active for both enzymes; these cells demonstrated variable expression of osteopontin mRNA. In the inflammatory connective tissues, abundant macrophage-like cells (NSE(+)/TRAP(-)) did not express osteopontin mRNA. However, TRAP(+) mononuclear cells observed among clusters of NSE(+) cells did express osteopontin mRNA. At these sites, dusters of putative macrophage polykaryons removing fragments of bone debris were observed. These giant cells and associated mononuclear cells were NSE(-) and distinctly TRAP(+), and expressed osteopontin mRNA, C35, and 23C6 (human osteoclast) reactivity. Therefore, cells involved in the remodeling (resorption) of bone or the removal of bone debris, together with their immediate precursors, switch from being NSE(+)/TRAP(-) to NSE(-)/TRAP(+) cells that express osteopontin mRNA. We propose that the dusters of NSE(+)/TRAP(-) mononuclear cells represent the immature osteoclast precursor. In support of this, TRAP(+)/NSE(+) cells were occasionally observed in both tissues, representing an intermediate stage in differentiation. These results further suggest that cells of the mononuclear phagocyte lineage within bone and inflammatory connective tissue have the potential to differentiate into osteoclasts.
