Force measurements with optical tweezers inside living cells

The force exerted by optical tweezers can be measured by tracking the momentum changes of the trapping beam, a method which is more general and powerful than traditional calibration techniques as it is based on first principles, but which has not been brought to its full potential yet, probably due to practical difficulties when combined with high-NA optical traps, such as the necessity to capture a large fraction of the scattered light. We show that it is possible to measure forces on arbitrary biological objects inside cells without an in situ calibration, using this approach. The instrument can be calibrated by measuring three scaling parameters that are exclusively determined by the design of the system, thus obtaining a conversion factor from volts to piconewtons that is theoretically independent of the physical properties of the sample and its environment. We prove that this factor keeps valid inside cells as it shows good agreement with other calibration methods developed in recent years for viscoelastic media. Finally, we apply the method to measuring the stall forces of kinesin and dynein in living A549 cells.Publisher PD

Force measurements with optical tweezers inside living cells

The force exerted by optical tweezers can be measured by tracking the momentum changes of the trapping beam, a method which is more general and powerful than traditional calibration techniques as it is based on first principles, but which has not been brought to its full potential yet, probably due to practical difficulties when combined with high-NA optical traps, such as the necessity to capture a large fraction of the scattered light. We show that it is possible to measure forces on arbitrary biological objects inside cells without an in situ calibration, using this approach. The instrument can be calibrated by measuring three scaling parameters that are exclusively determined by the design of the system, thus obtaining a conversion factor from volts to piconewtons that is theoretically independent of the physical properties of the sample and its environment. We prove that this factor keeps valid inside cells as it shows good agreement with other calibration methods developed in recent years for viscoelastic media. Finally, we apply the method to measuring the stall forces of kinesin and dynein in living A549 cells

Assessment of bone mineral content from 3-D synchrotron radiation microtomography images

International audienceSynchrotron radiation /spl mu/CT (SR /spl mu/CT) may provide three-dimensional (3-D) images with spatial resolution as high as 1 /spl mu/m. The SR /spl mu/CT system developed at the European Synchrotron Radiation Facility was already used to quantify trabecular bone architecture in bone samples. Here, the authors propose for the first time to use SR /spl mu/CT to assess the mineral content in bones. This quantification is possible thanks to the monochromaticity and high photon flux of the X-ray beam extracted from synchrotron radiation. For this purpose, the authors relate the reconstructed gray levels of the images to a degree of mineralization in bone. They propose a calibration method to estimate the 3-D distribution of hydroxyapatite concentrations within the sample. This method is compared to a reference microradiography technique. Finally, the authors illustrate the possibilities of the method on 3-D images of human biopsies from osteoporotic patients before and after a treatment with bisphosphonates

Synchrotron Radiation Microtomography Allows the Analysis of Three-Dimensional Microarchitecture and Degree of Mineralization of Human Iliac Crest Biopsy Specimens: Effects of Etidronate Treatment

International audienceQuantitative microcomputed tomography using synchrotron radiation (SR μCT) was used to assess the effects of a sequential etidronate therapy on both three‐dimensional (3D) microarchitecture and degree of mineralization of bone (DMB) in postmenopausal osteoporosis. Thirty‐two iliac crest biopsy specimens were taken from 14 patients with osteoporosis (aged 64 ± 1.8 years) before (baseline) and after 1 year of etidronate treatment, and after 2 years of treatment for four of the patients. The samples were imaged at high spatial resolution (voxel size = 10 μm) using the microtomography system developed at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Three‐dimensional microarchitecture parameters were calculated and compared with those obtained from conventional histomorphometry. In addition, the DMB was evaluated also in 3D. No significant statistical changes regarding bone mass and structural parameters were observed in histomorphometry or 3D analyses. The distribution of the DMB in cortical and trabecular bone showed a trend to a shift toward highest mineralization values after 1 year of etidronate treatment (3.88% and 1.24% in cortical and trabecular bone, respectively). This trend was more evident after 2 years. The study also showed that SR μCT is an accurate technique and the only one for quantifying both the mineralization and the microarchitecture of bone samples at the same time in 3D

Fonament de l'arc de Sant Martí

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Predation and competition effects on the size diversity of aquatic communities

Body size has been widely recognised as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites with the aim to assess whether there is a general trend in the effect of predation and resource competition on body size distribution across a wide range of aquatic ecosystems. We used size diversity as a measure of the shape of size distribution. Size diversity was computed based on the Shannon-Wiener diversity expression, adapted to a continuous variable, i.e. as body size. Our results show that greater predation pressure was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depended on the trophic level considered. At upper trophic levels (zooplankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote diversification of aquatic communities by size. This pattern was not found for phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing and increasing nutrient availability. Relationships we found were weak, indicating that predation and competition are not the only determinants of size distribution. Our results suggest that predation pressure leads to accumulation of organisms in the less predated sizes, while resource competition tends to favour a wider size distribution

Subchondral Bone Micro-Architectural Changes In Osteoarthritis : a Synchrotron Micro-Computed Tomography Study

articleObjectivesWe evaluated the three-dimensional (3D) micro-architecture of subchondral trabecular (Tb) bone in osteoarthritis (OA). Due to high signal-to-noise ratio and high resolution, micro-computed tomography (micro-CT) by synchrotron radiation is considered as the gold standard for bone micro-architecture imaging.DesignSubchondral bone were extracted from femoral heads in OA cases in areas without cartilage (OAc−; n=6) and in adjacent areas with cartilage (OAc+; n=6) and compared to eight subchondral bone cores from osteoporosis cases (OP). The voxel size of images was 10.13 μm. We measured the bone volume fraction (BV/TV) and morphological parameters: Tb thickness (TbTh), Tb spacing (TbSp), Tb number (TbN), and bone surface/bone volume (BS/BV). The degree of anisotropy (DA), the connectivity by the Euler number and the degree of mineralization (DM) were equally assessed.ResultsBV/TV and morphological parameters showed significant differences between OAc− and OP samples (P<0.01 except TbTh: P<0.05) and between OAc− and OAc+ (0.05<P<0.01) but no difference between OAc+ and OP except TbN (P<0.01). The connectivity was higher in OAc− comparatively to OAc+ and OP. The DA were significantly different between OA and OP cases (P<0.01) but not between OAc− and OAc+ specimens. The DMs (mean±SD) were 0.817±0.142 g/cm3, 0.873±0.161 g/cm3, 0.906±0.156 g/cm3 for OAc−, OAc+, OP (P<0.01), respectively.ConclusionSubchondral bone changes were mainly observed in advanced OA, when cartilage has been deleted and preserved in adjacent area. These data suggest that subchondral bone changes would be rather secondary to the cartilage deterioration than a primitive mechanism of OA. Nevertheless, longitudinal data could bring more accurate conclusion