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 study was to verify whether bone marrow MC accumulated during the phase of rapid osteopenia development in estrogen-deprived rats. Sprague-Dawley rats, 90 days old, were subjected to bilateral ovariectomy (OVX) or sham surgery (controls). Bone histomorphometry was performed in the proximal tibia metaphysis. Thirty days after surgery, serum estradiol had dropped (p < 0.002), and osteopenia was well established in the OVX rats (trabecular bone volume decreased by 69%, p < 0.0001). The indices of bone resorption were strongly modified: the number (p < 0.0001) and individual activity (p < 0.001) of the trabecular osteoclasts were prominently increased in the OVX rats. Ovariectomy 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 MC 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 mechanically activated model of resorption.