Bone Cells

The molecular events involved in the differentiation and function of bone cells have not been clarified for a long time because of the lack of suitable in vitro models to investigate bone cell biology. Over the past two decades, several novel approaches have been developed and adopted to investigate the biological bases of the differentiation processes of bone cells: osteoblasts, osteocytes, and osteoclasts. In the present chapter, we would like to update recent progress in the elucidation of the molecular mechanism of bone cells. The differentiation of osteoblasts from mesenchymal progenitors requires the activity of specific transcription factors such as Runx2 and Osterix, expressions of which are modulated by secreted molecules belonging to the Wnt family of glycoproteins. Differentiated osteoblasts, at the end of the bone formation phase, can become embedded in bone as osteocytes, cells enriched in dendritic processes that express specific markers as E11/gp38, SOST, DMP-1, PHEX, MEPE, and FGF-23. Osteocytes are important modulators of the bone remodeling process, as demonstrated by their ability to influence mineral homeostasis and control matrix mineralization as well as bone resorption through the secretion of the osteoclastogenic cytokine RANKL. Moreover, osteoclast differentiation from monocyte-macrophage lineage is tightly controlled by the RANK/RANKL/OPG system.