Fracture healing: micro-computed tomography tells more
Noninvasive characterization of fracture callus structure and composition may facilitate development of surrogate measures of the regain of mechanical function. As such, quantitative computed tomography- (CT-) based analyses of fracture calluses could enable more reliable clinical assessments of bone healing. Although previous studies have used CT to quantify and predict fracture healing, it is unclear which of the many CT-derived metrics of callus structure and composition are the most predictive of callus mechanical properties.
The goal of this study [1] was to identify the changes in fracture callus structure and composition that occur over time and that are most closely related to the regain of mechanical function. Micro-computed tomography (μCT) imaging and torsion testing were performed on murine fracture calluses (n=188) at multiple postfracture timepoints and under different experimental conditions that alter fracture healing. Total callus volume (TV), mineralized callus volume (BV), callus mineralized volume fraction (BV/TV), bone mineral content (BMC), tissue mineral density (TMD), standard deviation of mineral density (σTMD), effective polar moment of inertia (Jeff), torsional strength, and torsional rigidity were quantified. Multivariate statistical analyses were used to identify differences in callus structure and composition among experimental groups, and to determine which of the μCT outcome measures were the strongest predictors of mechanical properties.
Fibroblast growth factor FGF23: a new hormonal inhibitor of bone formation and mineralization
Fibroblast growth factor (FGF)23 is a circulating peptide produced primarily in bone which acts on kidney as a systemic phosphaturic factor; high levels result in rickets and osteomalacia. However, it remains unclear whether FGF23 acts locally and directly on bone formation. In order to address this question, the authors of a recent study [1] overexpressed human FGF23 in a stage-specific manner during osteoblast development in fetal rat calvaria cell cultures by using the adenoviral overexpression system and analyzed its effects on osteoprogenitor proliferation, osteoid nodule formation, and mineralization. Bone formation was also measured by calcein labeling in parietal bone organ cultures. Finally, the role of tyrosine phosphorylation of FGF receptor in mineralized nodule formation was also addressed.
Osterix regulates adult bone formation
Osterix (Osx) is essential for osteoblast differentiation and bone formation, because mice lacking Osx die within 1 hour of birth with a complete absence of intramembranous and endochondral bone formation. Perinatal lethality caused by the disruption of the Osx gene prevents studies of the role of Osx in bones that are growing or already formed.
Here [1], the function of Osx was examined in adult bones using the time- and site-specific inactivation of this gene only in osteoblasts. Even though no bone defects were observed in newborn mice, Osx inactivation induced osteopenia in growing mice. BMD and bone-forming rate were decreased in lumbar vertebra, and the cortical bone of the long bones was thinner and more porous with reduced bone length. The trabecular bones were increased, but they were immature or premature. The expression of early marker genes for osteoblast differentiation such as Runx2, osteopontin, and alkaline phosphatase was markedly increased, but the late marker gene, osteocalcin, was decreased. However, no functional defects were found in osteoclasts.
Nuclear factor-kB inhibits osteoblastic bone formation
An imbalance in bone formation relative to bone resorption results in the net bone loss that occurs in osteoporosis and inflammatory bone diseases. Although it is well known that RANKL/RANK stimulate bone resorption by activating nuclear factor-kB (NF-kB) in osteoclasts, the molecular mechanisms that mediate impaired bone formation are poorly understood.
 Download here the slide set presented by Prof. Friedlander, on Thursday, November 10th. |
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