Osteocytes as dynamic multifunctional cells.

The target of bone systemic factors and therapeutics has been assumed to be primarily osteoblasts and/or osteoclasts and their precursors. All the action with regard to bone modeling or remodeling has been assumed to take place on the bone surface. In this scenario, cells below the bone surface, that is, osteocyte, are considered to be inactive placeholders in the bone matrix. New data show osteocytes are involved. In addition to the function of osteocytes translating mechanical strain into biochemical signals between osteocytes and cells on the bone surface to affect (re)modeling, new functions are emerging. Osteocytes are exquisitely sensitive to mechanical strain in the form of shear stress compared to osteoblasts or osteoclasts and communicate with each other, with cells on the bone surface, and with marrow cells. Osteocytes are able to move their cell body and their dendritic processes and appear to be able to modify their local microenvironment. A novel function now attributed to osteocytes includes regulation of phosphate metabolism. Therefore, in addition to osteoblasts and osteoclasts, osteocytes are also important for bone health.