Asymmetric intercellular communication between bone cells: propagation of the calcium signaling.

Bone functional adaptation by remodeling is achieved by harmonized activities of bone cells in which osteocytes in the bone matrix are believed to play critical roles in sensing mechanical stimuli and transmitting signals to osteoclasts/osteoblasts on the bone surface in order to regulate their bone remodeling activities through the lacuno-canalicular network with many slender osteocytic processes. In this study, we investigated the intercellular communication between bone cells, particularly focusing on its directionality, through in vitro observations of the calcium signaling response to mechanical stimulus and its propagation to neighboring cells (NCs). Direct mechanical stimulus was applied to isolated bone cells from chick calvariae, osteocytes (Ocys) and bone surface cells (BSCs) mainly containing osteoblasts, and the percentage of calcium signaling propagation from the stimulated cell to NCs was analyzed.

Osteocyte calcium signaling response to bone matrix deformation.

Osteocytes embedded in calcified bone matrix have been widely believed to play important roles in mechanosensing to achieve adaptive bone remodeling in a changing mechanical environment. In vitro studies have clarified several types of mechanical stimuli such as hydrostatic pressure, fluid shear stress, and direct deformation influence osteocyte functions. However, osteocyte response to mechanical stimuli in the bone matrix has not been clearly understood.

Calcium response in single osteocytes to locally applied mechanical stimulus: differences in cell process and cell body.

It is proposed that osteocytes embedded in the bone matrix have the ability to sense deformation and/or damage to the matrix and to feed these mechanical signals back to the adaptive bone remodeling process. When osteoblasts differentiate into osteocytes during the bone formation process, they change their morphology to a stellate form with many slender processes. This characteristic cell shape may underlie the differences in mechanosensitivity between the cell processes and cell body.