Role of vitamin D in bone resorption

AbstractThe idea that vitamin D must function at the bone site to promote bone mineralization has long existed since its discovery as an anti‐rachitic agent. However, the definite evidence for this is still lacking. In contrast, much evidence has accumulated that 1α,25(OH)2D3 is involved in bone resorption. 1α,25(OH)2D3 tightly regulates differentiation of osteoclast progenitors into osteoclasts. Osteoclast progenitors have been thought to belong to the monocyte‐macrophage lineage. 1α,25(OH)2D3 greatly stimulates differentiation and activation of mononuclear phagocytes. Recent reports have indicated that differentiation of mononuclear phagocytes into osteoclasts is strictly regulated by osteoblastic cells, the process of which is also stimulated by 1α,25(OH)2D3. In the differentiation of mononuclear phagocytes into osteoclasts, the target cells for 1α,25(OH)2D3 appear to be osteoblastic stromal cells. Osteoblastic cells produce several proteins such as BGP, MGP, osteopontin and the third component of complement (C3) in response to the vitamin. They appear to be somehow involved in osteoclast differentiation and functions. Thus, 1α,25(OH)2D3 seems to be involved in the differentiati