Mast cells accumulate in rat bone marrow after ovariectomy

We have previously shown that mast cells (MC) may act as accessory cells of bone resorption in a synchronized model of resorption. The aim of this stu dy was to verify whether bone marrow MC accumulated during the phase of rap id osteopenia development in estrogen-deprived rats, Sprague-Dawley rats, 9 0 days old, were subjected to bilateral ovariectomy (OVX) or sham surgery ( controls). Bone histomorphometry was performed in the proximal tibia metaph ysis. Thirty days after surgery, serum estradiol had dropped (p < 0.002), a nd osteopenia was well established in the OVX rats (trabecular bone volume decreased by 69%, p<0.0001). The indices of bone resorption were strongly m odified: the number (p < 0.0001) and individual activity (p < 0.001) of the trabecular osteoclasts were prominently increased in the OVX rats. Ovariec tomy resulted in a dramatic increase in the number of bone marrow MC (p < 0 .0001), 97% of which were of the mucosal subtype. These results show that M C accumulate concomitantly with osteoclast generation in estrogen-deficient rats. Although the biological significance of this MC increase is unknown from these results, we hypothesize that MC might play a role in ovariectomy -induced bone loss similar to that they we previously proposed in a mechani cally activated model of resorption.