Oscillatory intracellular Ca²⁺ responses in living bone

Intracellular calcium (Ca²⁺) is an important secondary messenger that modulates many cellular processes. Its oscillatory signaling is considered to participate in the regulation of many different cell functions, including bone metabolism. However, it is not entirely clear whether Ca²⁺ oscillations occur between osteoblasts and osteocytes in integrated bone tissues because of the complex mineralized matrices surrounding bone cells. To address this issue, we have recently developed a novel ex vivo realtime imaging system, which made it possible to observe repetitive and autonomous oscillations in the intracellular Ca²⁺ concentration ([Ca²⁺]i) in intact chick calvarial explants. This system revealed that Ca²⁺ release from intracellular stores plays a key role in Ca²⁺ oscillations in bone cells. Additionally, gap junctions are important for the maintenance of these oscillations in osteocytes but not in osteoblasts. In this review, we describe the dynamic oscillatory elevations of Ca²⁺ levels that occur in osteoblasts and osteocytes in living bone.