The figure shows RANK/RANKL system. Multinucleated osteoclasts are formed by cell–cell fusion of mononuclear preosteoclasts. Osteoclast precursors express DC-STAMP (Dendritic cells-specific transmembrane protein) which is the protein responsible of the fusion process. Moreover, they express on their membrane RANK and c-Fms, which is the receptor of M-CSF (Macrophage colony stimulating factor). In presence of M-CSF, osteoblasts regulate the fusion process by RANKL expression. As shown in the figure, the active form of vitamin D is between the stronger inducers of RANKL expression .
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