Detection of microspheres in vivo using multispectral optoacoustic tomography

We introduce a new approach to detect individual microparticles that contain NIR fluorescent dye by multispectral optoacoustic tomography in the context of the hemoglobin-rich environment within murine liver. We encapsulated a near infrared (NIR) fluorescent dye within polystyrene microspheres, then injected them into the ileocolic vein, which drains to the liver. NIR absorption was determined using multispectral optoacoustic tomography. To quantitate the minimum diameter of microspheres, we used both colorimetric and spatial information to segment the regions in which the microspheres appear. Regional diameter was estimated by doubling the maximum regional distance. We found that the minimum microsphere size threshold for detection by multispectral optoacoustic tomography images is 78.9 µm.     

A theoretical model to study the effects of cellular stiffening on the damage evolution in deep tissue injury

Pressure induced deep tissue injury (DTI) is a severe form of pressure ulcers that is hard to detect in early stages and difficult to prevent and treat. High prevalence figures are partly due to a lack of understanding of pathological pathways involved in DTI. The aim of this study was to investigate, whether changes in material properties of damaged tissue can play a role in DTI aetiology. A numerical model was developed based on muscle microstructure and tissue engineering experiments. A time dependent damage law was proposed and stiffening of dead cells incorporated. The results obtained in the microstructural investigations were used to include the stiffening information in a pre-existing macroscopic model based on animal experiments, which correlated strains to tissue damage measured in the tibialis anterior muscle in rat limbs. With the modelling approach employed in this paper, the damaged area in the rat limb models increased up to 1.65-fold and the rate of damage progression was up to 2.1 times higher in microstructural simulations when stiffening was included.     

Linear shear response of the upper skin layers

This study presents an in vitro experimental method to determine shear properties of the epidermis. Shear tests were performed with a parallel plate rheometer on samples of stratum corneum and the viable epidermis. The method was validated on very thin silicon sheets. Preliminary test were performed to determine the linear viscoelastic range, the effect of normal loading on the sample and the time to reach equilibrium after changes of temperature and relative humidity. The study shows that reproducible results can be obtained for the shear properties of epidermis in an in vitro set up. The dynamic shear modulus for stratum corneum ranges from about 4-12 kPa, decreasing with increasing relative humidity. The values are considerably lower than the shear modulus value based on tensile Young's moduli in the literature, indicating a considerable anisotropic material behavior. Results for the epidermis were of the same order of magnitude, but were less consistent possibly due to a less well-defined tissue composition.     

Isoform switching facilitates period control in the Neurospora crassa circadian clock

A striking and defining feature of circadian clocks is the small variation in period over a physiological range of temperatures. This is referred to as temperature compensation, although recent work has suggested that the variation observed is a specific, adaptive control of period. Moreover, given that many biological rate constants have a Q₁₀ of around 2, it is remarkable that such clocks remain rhythmic under significant temperature changes. We introduce a new mathematical model for the Neurospora crassa circadian network incorporating experimental work showing that temperature alters the balance of translation between a short and long form of the FREQUENCY (FRQ) protein. This is used to discuss period control and functionality for the Neurospora system. The model reproduces a broad range of key experimental data on temperature dependence and rhythmicity, both in wild‐type and mutant strains. We present a simple mechanism utilising the presence of the FRQ isoforms (isoform switching) by which period control could have evolved, and argue that this regulatory structure may also increase the temperature range where the clock is robustly rhythmic.     

The effects of deformation, ischemia, and reperfusion on the development of muscle damage during prolonged loading

Deep tissue injury (DTI) is a severe form of pressure ulcer where tissue damage starts in deep tissues underneath intact skin. In the present study, the contributions of deformation, ischemia, and reperfusion to skeletal muscle damage development were examined in a rat model during a 6-h period. Magnetic resonance imaging (MRI) was used to study perfusion (contrast-enhanced MRI) and tissue integrity (T2-weighted MRI). The levels of tissue deformation were estimated using finite element models. Complete ischemia caused a gradual homogeneous increase in T2 (～20% during the 6-h period). The effect of reperfusion on T2 was highly variable, depending on the anatomical location. In experiments involving deformation, inevitably associated with partial ischemia, a variable T2 increase (17-66% during the 6-h period) was observed reflecting the significant variation in deformation (with two-dimensional strain energies of 0.60-1.51 J/mm) and ischemia (50.8-99.8% of the leg) between experiments. These results imply that deformation, ischemia, and reperfusion all contribute to the damage process during prolonged loading, although their importance varies with time. The critical deformation threshold and period of ischemia that cause muscle damage will certainly vary between individuals. These variations are related to intrinsic factors, such as pathological state, which partly explain the individual susceptibility to the development of DTI and highlight the need for regular assessments of individual subjects.     

A reaction-diffusion model to predict the influence of neo-matrix on the subsequent development of tissue-engineered cartilage

Extracellular matrix (ECM) in chondrocytes-seeded agarose aggregates to form islands of matrix. These islands need to coalesce to develop functional cartilage. Hence, macroscopic properties are determined by transport and aggregation of macromolecules at the microscale, which varies temporally and spatially. This study evaluates the importance of the mutual interaction between matrix components and matrix development. Fluorescence recovery after photobleaching measurements demonstrates that diffusivity depends on the presence and density of ECM. A reaction-diffusion model describing synthesis, transport and immobilisation of ECM predicts steep gradients in ECM around chondrocytes, resembling histology. Steric hindrance of diffusion by ECM is essential for the formation of these gradients. Finally, microscopic ECM concentration is linked with macroscopic mechanical properties. Construct softening is predicted when temporal and spatial variations in diffusivity are considered. In conclusion, non-constant diffusion renders significant effects on both the microscopic ECM development and the macroscopic mechanical properties of developing tissue-engineered cartilage.