[Imaging of bone and joint destruction]

Osteoclasts are bone-resorbing giant polykaryons that differentiate from mononuclear macrophage/monocyte-lineage hematopoietic precursors. We have originally established an advanced imaging system for visualizing in vivo behavior of osteoclasts and their precursors with intravital two-photon microscopy. By means of the system, we found that sphingosine-1-phosphate, a lipid mediator enriched in blood, controlled the migratory behavior of osteoclast precursors. We also developed pH-sensing chemical fluorescent probes to detect localized acidification by bone-resorbing osteoclasts on the bone surface in vivo, and identified two distinct functional states of differentiated osteoclasts, 'bone-resorptive' and 'non-resorptive'. In this review, we summarize our recent studies on the dynamics and functions of osteoclasts. Our intravital imaging techniques would be beneficial for studying the cellular dynamics in arthritic inflammation and bone destruction in vivo and would thus be useful for evaluating novel therapies targeting aspects of osteoclast dynamics in patients with bone-destructive diseases.