Search

Extracellular forces transmitted through the cytoskeleton can deform the cell nucleus. Large nuclear deformation increases the risk of disrupting the nuclear envelope's integrity and causing DNA damage. Mechanical stability of the nucleus defines its capability of maintaining nuclear shape by minimizing nuclear deformation and recovering strain when deformed. Understanding the deformation and recovery behavior of the nucleus requires characterization of nuclear viscoelastic properties. Here, we quantified the decoupled viscoelastic parameters of the cell membrane, cytoskeleton, and the nucleus. The results indicate that the cytoskeleton enhances nuclear mechanical stability by lowering the effective deformability of the nucleus while maintaining nuclear sensitivity to mechanical stimuli. Additionally, the cytoskeleton decreases the strain energy release rate of the nucleus and might thus prevent shape change-induced structural damage to chromatin. [ABSTRACT FROM AUTHOR]

Microscopic flow in fractured porous media is a typical problem for the exploitation of tight reservoirs. The rapid-flow in the fractured porous-media is of great significance to efficient and continuous oil/gas exploitation. The fractures are expected to enhance flow conductivity and mass transfer between matrix and fractures, and to improve oil displacement during water flooding. However, the fractures may also lead to water channeling under some conditions. The understanding on the mechanism of the microscopic flow in the fractured porous media has been insufficient until now. In this paper, a two-dimensional pore-fracture network model is presented to study the role of fractures in the flow. The effects of two main dimensionless parameters, fracture length to network length lf/l and fracture density Nf/N, on the absolute permeability and the oil displacement efficiency are investigated. The results show that the flow in the matrix plays a controlling role at a low fracture density. Once the fracture density exceeds a certain value, the flow is controlled by fractures. With the increase of the fracture density, the oil displacement efficiency develops into three typical stages: when Nf/N < 0.1, the oil displacement efficiency increases rapidly; when 0.1 < Nf/N < 0.5, the oil displacement efficiency changes slowly; and when Nf/N > 0.5, the oil displacement efficiency decreases rapidly. In the case lf/l > 0.8, the water mainly flows through a concentrated path connected by some fractures, resulting in it bypassing most oil regions, and thus the oil displacement efficiency decreases rapidly, similar to the water channeling. [ABSTRACT FROM AUTHOR]

Based on the survey records on netz-phytoplankton (>76 µm) collected in the South China Sea (SCS) during the cruise from the 21st August to the 18th October, 2012, the species composition, biodiversity and its spatial distribution were studied. The identified 326 phytoplankton species belonged to 75 genera and 4 phyla. Thalassionema nitzschioides was the most dominant species accounting for 33.24% of the total cell abundance, followed by Bacteriastrum furcatum - 8.69%. Whereas the phytoplankton cell abundance (cells l-1) was the highest in the Zhubi Reef (1106.45 ± 2475.38), the southern SCS (396.84 ± 969.87), the western SCS (90.82 ± 144.66) and the northern SCS (66.48 ± 70.89) in order. The distribution of phytoplankton appears to be affected by the Mekong river discharge and particles derived in Sumatra and Kalimantan from biomass burning, nutrient concentrations, the South China Sea Warm Current (SCSWC). The evaluation of biodiversity indices indicated the high level of species richness, evenness and biodiversity in the survey area. [ABSTRACT FROM AUTHOR]

The elasticity of the extracellular matrix profoundly affects biological responses of cells, but also their mechanical properties. Single cell mechanical properties are often measured by atomic force microscopy (AFM), but technical guidelines for AFM measurement of cells grown on soft substrates are not well established. In this study, the local and global elastic moduli of aortic valve interstitial cells (VICs) cultured on soft polyacrylamide substrates (3-144 kPa) were characterized via AFM force mapping using pyramidal and spherical tips, respectively. Local and global VIC modulus values both increased with substrate stiffness (p < 0.05), with the average local cell modulus being consistently two to three times greater than the global modulus (p < 0.05). For local measurements, a minimum of four measurements was required to observe the trend of increasing cell modulus with substrate stiffness, but there was no advantage to testing additional spots. The Hertz model was relatively accurate in estimating the global cell elastic modulus ( < 12% error, based on validated finite element analyses), despite the cells being of finite thickness and grown on deformable substrates, neither of which are accounted for in the Hertz model. The results of this study provide practical guidelines for efficient AFM-based measurement of the mechanical properties of cells on gels. They also provide new physiologically-relevant data on VIC mechanical properties and their correlation with substrate stiffness-dependent cytoskeletal changes, with relevance to heart valve mechanobiology and disease. [ABSTRACT FROM AUTHOR]

Longitudinal data on patient-reported outcomes (PROs), such as quality of life of patients, are frequently collected in clinical trials and other medical studies. Joint analysis of these data with survival times may improve the accuracy of statistical inferences, especially when PRO measurements may be missing after the death of patients. Classical linear mixed models are often used as the models for the longitudinal measurements in a joint analysis, but it may not be suitable for longitudinal PRO measurements with potential ceiling and floor effects caused by a large portion of patients who report either a maximum or minimum score. In this paper, we introduce a new joint model that uses a longitudinal Tobit model for the longitudinal outcomes with potential ceiling and floor effects and a Cox proportional hazard model for survival time with a random effect connecting these two models. An estimation procedure based on the partial likelihood and Laplace approximation is developed to estimate the parameters in both models, and a random weighting method is proposed to calculate the variances of these parameter estimators. Performances of the proposed procedures are evaluated through simulation studies and an application to the analysis of quality of life data from a clinical trial. [ABSTRACT FROM AUTHOR]

Extracellular forces transmitted through the cytoskeleton can deformthe cell nucleus. Large nuclear deformations increase the risk of disrupting the integrity of the nuclear envelope and causing DNA damage. The mechanical stability of the nucleus defines its capability to maintain nuclear shape by minimizing nuclear deformation and allowing strain to be minimized when deformed. Understanding the deformation and recovery behavior of the nucleus requires characterization of nuclear viscoelastic properties. Here, we quantified the decoupled viscoelastic parameters of the cell membrane, cytoskeleton, and the nucleus. The results indicate that the cytoskeleton enhances nuclear mechanical stability by lowering the effective deformability of the nucleus while maintaining nuclear sensitivity to mechanical stimuli. Additionally, the cytoskeleton decreases the strain energy release rate of the nucleus and might thus prevent shape change-induced structural damage to chromatin. [ABSTRACT FROM AUTHOR]

Protoporphyrin IX (PPIX) produced following the administration of exogenous 5d-aminolevulinic acid is clinically approved for photodynamic therapy and fluorescence-guided resection in various jurisdictions around the world. For both applications, quantification of PPIX forms the basis for accurate therapeutic dose calculation and identification of malignant tissues for resection. While it is well established that the PPIX synthesis and accumulation rates are subject to the cell's biochemical microenvironment, the effect of the physical microenvironment, such as matrix stiffness, has received little attention to date. Here we studied the proliferation rate and PPIX synthesis and accumulation in two glioma cell lines U373 and U118 cultured under five different substrate conditions, including the conventional tissue culture plastic and polyacrylamide gels that simulated tissue stiffness of normal brain (1 kPa) and glioblastoma tumors (12 kPa). We found that the proliferation rate increased with substrate stiffness for both cell lines, but not in a linear fashion. PPIX concentration was significantly higher in cells cultured on tissue-simulating gels than on the much stiffer tissue culture plastic for both cell lines. These findings, albeit preliminary, suggest that the physical microenvironment might be an important determinant of tumor aggressiveness and PPIX synthesis in glioma cells. [ABSTRACT FROM AUTHOR]

An abstract of the article "Perfusable branching microvessel bed for vascularization of engineered tissues," by Loraine L.V. Chiu is presented.