To elucidate the histological events that follow administration of eldecalcitol, a second-generation of vitamin D analog currently awaiting approval as a drug for treatment of osteoporosis, we employed the ovariectomy (OVX) rat model. OVX rats received vehicle or 30ng/kg of eldecalcitol, and sham-operated animals received vehicle only. Rats were sacrificed after 12weeks and had their femora and tibiae removed and processed for histochemical and histomorphometrical analyses. When compared with OVX group, osteoclastic number and bone resorption parameters were significantly reduced in eldecalcitol-treated rats, accompanied by decreased bone formation parameters. The preosteoblastic layer, with which osteoclastic precursors interact for mutual differentiation, was poorly developed in the eldecalcitol group, indicating less cell-to-cell contact between preosteoblasts and osteoclast precursors. Interestingly, eldecalcitol did promote a type of focal bone formation that is independent of bone resorption, a process known as bone minimodeling. While the number of ED-1-positive macrophages was higher in the bone marrow of treated rats, though osteoclastic number was deceased. Taken together, our findings suggest that eldecalcitol stimulates preosteoblastic differentiation rather than their proliferation, which in turn may prevent or diminish cell-to-cell contact between preosteoblasts and osteoclastic precursors, and therefore, lead to lower osteoclast numbers and decreased bone resorption.
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