[Frontiers in Live Bone Imaging Researches. Intravital imaging of osteoclast dynamics]

Osteoclasts are bone-resorbing giant polykaryons that differentiate from mononuclear macrophage/monocyte-lineage hematopoietic precursors. Upon the stimulation of essential factors such as M-CSF and RANKL, osteoclast precursor monocytes attach to the bone surface ( "migration" ), fuse with each other to form giant cells ( "differentiation" ) and mediate bone resorption ( "function" ). To reveal the regulatory mechanism of these three dynamic steps of osteoclastic activity, we have originally established an advanced imaging system for visualizing living bone tissues with intravital multiphoton microscopy. By means of the system, we have recently succeeded in visualization of osteoclast migration, differentiation, and function in living bone tissues in vivo. In this review we summarize the latest data of intravital imaging of osteoclast dynamics, and discuss novel lines of osteoclast-targeted therapies that will impact future treatment of bone destructive diseases.