Bone is a complex highly structured mechanically active 3D tissue composed of cellular and matrix elements. The true biological environment of a bone cell is thus derived from a dynamic interaction between responsively active cells experiencing mechanical forces and a continuously changing 3D matrix architecture. To investigate this phenomenon in vitro, marrow stromal osteoblasts were cultured on 3D scaffolds under flow perfusion with different rates of flow for an extended period to permit osteoblast differentiation and significant matrix production and mineralization. With all flow conditions, mineralized matrix production was dramatically increased over statically cultured constructs with the total calcium content of the cultured scaffolds increasing with increasing flow rate. Flow perfusion induced de novo tissue modeling with the formation of pore-like structures in the scaffolds and enhanced the distribution of cells and matrix throughout the scaffolds. These results represent reporting of the long-term effects of fluid flow on primary differentiating osteoblasts and indicate that fluid flow has far-reaching effects on osteoblast differentiation and phenotypic expression in vitro. Flow perfusion culture permits the generation and study of a 3D, actively modeled, mineralized matrix and can therefore be a valuable tool for both bone biology and tissue engineering. Bone is a complex, highly organized tissue consisting of a structured extracellular matrix composed of inorganic and organic elements containing a conglomeration of cell types responsible for its metabolism and upkeep that are responsive to a variety of signals (1). As would be expected from the skeleton's central role in support and structural integrity, bone cells cultured in vitro respond to a variety of different mechanical signals including fluid flow, hydrostatic pressure, and substrate deformation.Current studies indicate that fluid flow is a potentially stronger stimulus for bone cell behavior than either hydrostatic compression (2) or substrate deformation (3, 4). The in vitro mechanical stimulation of bone cells by fluid flow has been reported to impact the levels of many biochemical factors including intracellular calcium (5, 6), nitric oxide (4, 7-9), prostaglandin E 2 (3, 4, 7, 8), the expression of the genes for osteopontin, cyclooxygenase-2, and c-Fos (6, 10-12) as well as other intracellular messengers and transcription factors (6,(13)(14)(15). This mechanostimulation of bone cells in vitro by fluid flow mimics the physiological response of bone cells in vivo where pressure gradients from mechanical loading of locomotion and other stressors deform the mineralized bone matrix and move extracellular fluid radially outward toward the cortex through the lacunocanalicular network (16)(17)(18)(19). Mechanical loading of bone plays an important role because it can both increase bone formation and decrease bone resorption (1). Indeed, its absence can lead to lower bone matrix protein production, mineral content, and bone formation plus incr...

Osteogenesis Imperfecta (OI) is a heritable disorder of connective tissue characterized by brittle bones, fractures and extraskeletal manifestations1. How structural mutations of type I collagen (dominant OI) or of its post-translational modification machinery (recessive OI) can cause abnormal quality and quantity of bone is poorly understood. Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms2. Here, we show that excessive transforming growth factor-beta (TGFβ) signaling is a mechanism of OI in both recessive (Crtap−/−) and dominant (Col1a2tm1.1Mcbr) OI mouse models. In the skeleton, we find higher expression of TGFβ target genes, ratio of pSmad2/Smad2 protein, and in vivo Smad2 reporter activity. Anti-TGFβ treatment using the neutralizing antibody 1D11 corrects the bone phenotype in both forms of OI, and improves the lung abnormalities in Crtap−/− mice. Moreover, type I collagen of Crtap−/− mice shows reduced binding to the small leucine rich proteoglycan decorin, a known regulator of TGFβ activity3–4. Hence, altered TGFβ matrix-cell signaling is a primary mechanism in the pathogenesis of OI, and could be a promising target for the treatment of OI.

The burden of hip fracture is illustrated by the high incidence of postfracture disability and the high mortality rate in both men and women. Nevertheless, few men receive antiresorptive treatment.

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