The strong correlation between a bone's architectural properties and the mechanical forces that it experiences has long been attributed to the existence of a cell that not only detects mechanical load but also structurally adapts the bone matrix to counter it. One of the most likely cellular candidates for such a "mechanostat" is the osteocyte, which resides within the mineralized bone matrix and is perfectly situated to detect mechanically induced signals. However, as osteocytes can neither form nor resorb bone, it has been hypothesized that they orchestrate mechanically induced bone remodeling by coordinating the actions of cells residing on the bone surface, such as osteoblasts. To investigate this hypothesis, we developed a novel osteocyte-osteoblast coculture model that mimics in vivo systems by permitting us to expose osteocytes to physiological levels of fluid shear while shielding osteoblasts from it. Our results show that osteocytes exposed to a fluid shear rate of 4.4 dyn/cm 2 rapidly increase the alkaline phosphatase activity of the shielded osteoblasts and that osteocytic-osteoblastic physical contact is a prerequisite. Furthermore, both functional gap junctional intercellular communication and the mitogen-activated protein kinase, extracellular signal-regulated kinase 1/2 signaling pathway are essential components in the osteoblastic response to osteocyte communicated mechanical signals. By utilizing other nonosteocytic coculture models, we also show that the ability to mediate osteoblastic alkaline phosphatase levels in response to the application of fluid shear is a phenomena unique to osteocytes and is not reproduced by other mesenchymal cell types.osteocyte; osteoblast; fluid-flow; coculture; mechanical stimulation; gap junction; intercellular communication ONE OF THE major tenets of bone biology is that mechanical load perturbs interstitial fluid and that, upon detecting these perturbations, osteocytes embedded deep within mineralized bone remotely coordinate the adaptive response by directing the actions of effector cells such as bone-forming osteoblasts and bone-resorbing osteoclasts (4, 12). Although this theory is widely accepted, there is little experimental evidence that, in response to fluid shear, embedded osteocytes directly alter the cellular behavior of surface-residing osteoblasts or osteoclasts. In vivo, osteocytes pass dendritic-like cellular processes through channels in the mineralized matrix (canaliculi) and form physical connections, specifically gap junctions, with both neighboring osteocytes and surface-residing bone cells (11,23). We hypothesize that these intercellular gap junctions provide the means by which mechanically induced fluid shear signals may be communicated from osteocytes to effector cells such as osteoblasts and osteoclasts.There is abundant evidence suggesting that gap junctional intercellular communication (GJIC) contributes to mechanotransduction in the musculoskeletal system. Banes et al. (3) demonstrated that equibiaxial strain upregulates connexin 43 (C...

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