RANKL induces formation of avian osteoclasts from macrophages but not frommacrophage polykaryons

We have shown that chick macrophages express RANK at their surface and huma n RANKL (hRANKL) triggers the formation of osteoclasts able to degrade dent ine. As described for mammalian osteoclasts, hRANKL also stimulates the res orbing activity of chick bone-derived osteoclasts. In other hands, in cultu re, chick macrophages spontaneously form polykaryons sharing most of the os teoclast markers but unable to resorb bone. Since both bone-resorbing osteo clasts and macrophage polykaryons found in inflammatory tissues are multinu cleated cells deriving from the fusion of macrophages, we examined whether macrophage polykaryons could be induced toward bone-resorbing osteoclasts. Long-term exposure of macrophage polykaryons to hRANKL failed to activate a ny resorbing activity, indicating that although deriving from the same prec ursors macrophage polykaryons and osteoclasts are independent cell types an d polykaryons are not immature osteoclasts. (C) 2001 Academic Press.