Recombinant MIP-1α induces osteoclast formation and bone resorption in vivo via RANK ligand (RANKL)/RANK signaling pathway. (A) Injection of 10 ng MIP-1α daily for 5 days into supracalvarial subcutaneous tissues in normal Swiss ICR mice resulted in a significantly increased number of osteoclasts (TRAP+; pink/red stained) and marrow spaces (arrows) compared with saline controls. Representative sections of calvariae from 3 separate mice are shown (each of the top 3 panels on either side represents the midsagittal suture area of calvaria of one mouse; bottom panels represent the region of calvariae away from the midline). (B) Although injection of MIP-1α induced osteoclast formation and bone resorption, represented by increased marrow spaces (arrows) in calvariae of wild-type litter-mates, it had no effect in RANK null mutant mice. Note lack of marrow spaces and bowing of calvariae of RANK-/- mice due to absence of osteoclasts.