A fundamental paradox in bone mechanobiology is that tissuelevel strains caused by human locomotion are too small to initiate intracellular signaling in osteocytes. A cellular-level strainamplification model previously has been proposed to explain this paradox. However, the molecular mechanism for initiating signaling has eluded detection because none of the molecules in this previously proposed model are known mediators of intracellular signaling. In this paper, we explore a paradigm and quantitative model for the initiation of intracellular signaling, namely that the processes are attached directly at discrete locations along the canalicular wall by ␤3 integrins at the apex of infrequent, previously unrecognized canalicular projections. Unique rapid fixation techniques have identified these projections and have shown them to be consistent with other studies suggesting that the adhesion molecules are ␣v␤3 integrins. Our theoretical model predicts that the tensile forces acting on the integrins are <15 pN and thus provide stable attachment for the range of physiological loadings. The model also predicts that axial strains caused by the sliding of actin microfilaments about the fixed integrin attachments are an order of magnitude larger than the radial strains in the previously proposed strain-amplification theory and two orders of magnitude greater than whole-tissue strains. In vitro experiments indicated that membrane strains of this order are large enough to open stretch-activated cation channels.bone mechanotransduction ͉ integrin attachments ͉ osteocyte cell process ͉ strain amplification ͉ bone fluid flow A fundamental paradox in bone biology is that tissue-level strains, which rarely exceed 0.1% in vivo (1, 2), are too small to initiate intracellular signaling in bone cells in vitro (3,4), where the necessary strains (typically 1.0%) would cause bone fracture. Osteocytes, the most abundant cells in adult bone, are widely believed to be the primary sensory cells for mechanical loading because of their ubiquitous distribution throughout the bone tissue and their dendritic interconnections with both neighboring osteocytes and osteoblasts (5, 6), but osteocytes also require high local strains for mechanical stimulation. You et al. (7) developed an intuitive strain-amplification model to explain this paradox wherein osteocyte processes are attached to the canalicular wall by transverse tethering elements in the pericellular matrix. According to this model, the drag generated by load-induced fluid flow through the pericellular matrix would create tensile forces along the transverse elements supporting the pericellular matrix. These resulting tensions then were transmitted by transmembrane proteins to the central actin filament bundle in the osteocyte cell process leading to circumferential expansion of the cell process. The basic structural features in this model, the transverse tethering elements, and the organization of the actin filament bundle in the dendritic cell process were shown experimentally by You et...

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