This review describes the role of bone cells and their surrounding matrix in maintaining bone strength through the process of bone remodeling. Subsequently, this work focusses on how bone formation is guided by mechanical forces and fluid shear stress in particular. It has been demonstrated that mechanical stimulation is an important regulator of bone metabolism. Shear stress generated by interstitial fluid flow in the lacunar-canalicular network influences maintenance and healing of bone tissue. Fluid flow is primarily caused by compressive loading of bone as a result of physical activity. Changes in loading, e.g., due to extended periods of bed rest or microgravity in space are associated with altered bone remodeling and formation in vivo. In vitro, it has been reported that bone cells respond to fluid shear stress by releasing osteogenic signaling factors, such as nitric oxide, and prostaglandins. This work focusses on the application of in vitro models to study the effects of fluid flow on bone cell signaling, collagen deposition, and matrix mineralization. Particular attention is given to in vitro set-ups, which allow long-term cell culture and the application of low fluid shear stress. In addition, this review explores what mechanisms influence the orientation of collagen fibers, which determine the anisotropic properties of bone. A better understanding of these mechanisms could facilitate the design of improved tissue-engineered bone implants or more effective bone disease models.

Several computational models based on experimental techniques and theories have been proposed to describe cytoskeleton (CSK) mechanics. Tensegrity is a prominent model for force generation, but it cannot predict mechanics of individual CSK components, nor explain the discrepancies from the different single cell stimulating techniques studies combined with cytoskeleton-disruptors. A new numerical concept that defines a multi-structural 3D finite element (FE) model of a single-adherent cell is proposed to investigate the biophysical and biochemical differences of the mechanical role of each cytoskeleton component under loading. The model includes prestressed actin bundles and microtubule within cytoplasm and nucleus surrounded by the actin cortex. We performed numerical simulations of atomic force microscopy (AFM) experiments by subjecting the cell model to compressive loads. The numerical role of the CSK components was corroborated with AFM force measurements on U2OS-osteosarcoma cells and NIH-3T3 fibroblasts exposed to different cytoskeleton-disrupting drugs. Computational simulation showed that actin cortex and microtubules are the major components targeted in resisting compression. This is a new numerical tool that explains the specific role of the cortex and overcomes the difficulty of isolating this component from other networks in vitro. This illustrates that a combination of cytoskeletal structures with their own properties is necessary for a complete description of cellular mechanics.

Stromal-derived factor-1 (SDF-1) is a critical chemokine for endothelial progenitor cell (EPC) recruitment to areas of ischemia, allowing these cells to participate in compensatory angiogenesis. The SDF-1 receptor, CXCR4, is expressed in developing blood vessels as well as on CD34؉ EPCs. We describe that picomolar and nanomolar concentrations of SDF-1 differentially influence neovascularization, inducing CD34؉ cell migration and EPC tube formation. CD34؉ cells isolated from diabetic patients demonstrate a marked defect in migration to SDF-1. This defect is associated, in some but not all patients, with a cell surface activity of CD26/dipeptidyl peptidase IV, an enzyme that inactivates SDF-1. Diabetic CD34؉ cells also do not migrate in response to vascular endothelial growth factor and are structurally rigid. However, incubating CD34؉ cells with a nitric oxide (NO) donor corrects this migration defect and corrects the cell deformability. In addition, exogenous NO alters vasodilator-stimulated phosphoprotein and mammalian-enabled distribution in EPCs. These data support a common downstream cytoskeletal alteration in diabetic CD34؉ cells that is independent of growth factor receptor activation and is correctable with exogenous NO. This inability of diabetic EPCs to respond to SDF-1 may contribute to aberrant tissue vascularization and endothelial repair in diabetic patients. Diabetes 55: 102-109, 2006