Figure 2. Possible signaling cascades that may promote the fusion of the mononuclear cells recruited to the DF. Both CSF-1 and ODF are known to promote osteoclast formation, whereas OPG inhibits this by preventing cell-to-cell signaling. The downstream products of the transcription factors c-fos and NFB that promote osteoclast formation are not yet known. Arrows with crosses ( ) reflect inhibition of gene expression, whereas other arrows reflect enhancement of gene expression or stimulation of cell fusion.