Rat osteoclast precursors in vivo express a vitronectin receptor and a chloride-bicarbonate exchanger

In vivo osteoclast precursors, which are mononuclear, were previously found to express TRAP (tartrate-resistant acid phosphatase) and CTR (calcitonin receptor), like multinucleated osteoclasts. In vitro, they were found to express, in addition, VNR (vitronectin receptor) and CBE (chloride-bicarbonate exchanger). In order to ascertain that osteoclast precursors in vivo express VNR and CBE like their in vitro counterparts, we used immunohistochemistry to localize these molecules in developing long bones of neonatal rats. Frozen sections of metatarsals and phalanges of 1-2 day-old rats were stained for TRAP and mineralization using histochemistry or were reacted with polyclonal antibodies specific for either the beta3 chain of the VNR or synthetic sequences of the CBE. Both mature, multinucleated osteoclasts within the forming marrow cavity of metatarsals (as shown previously) and mononuclear osteoclast precursors located outside the bony collar of the phalangeal calcified rudiment (as shown here for the first time) expressed both TRAP, VNR and CBE. These findings suggest that mononuclear osteoclast precursors express many of the phenotypical markers of multinucleated osteoclasts prior to their fusion and multinucleation which may allow them to resorb bone, as suggested by in vitro observations of pit formation by preosteoclasts cultured on resorbable substances.