An individual trabecula in a cancellous bone is a porous material consisting of a lamellar bone matrix and interstitial fluid in a lacuno-canalicular porosity. The flow of interstitial fluid created by the application of mechanical load to a bone is considered to stimulate osteocytes for regulating bone remodeling, as well as enhance the transport of signaling molecules. The purpose of this study is to investigate, based on poroelastic theory, the flow-induced stimuli given to the osteocytes embedded in an individual lamellar trabecula. A single trabecula was modeled as a two-dimensional poroelastic slab composed of multiple layers subjected to cyclic uniaxial and bending loading. To consider the spatial variations in material properties due to lamellar structure of the trabecula, each layer was assumed to have a different value of permeability. By analytically solving the diffusion equation obtained from poroelasticity, we developed a solution for the interstitial fluid pressure in the lacuno-canalicular porosity within the single trabecula. Based on the solution obtained, we demonstrated the distribution of seepage velocity across the trabecula and qualitatively assessed the mechanical stimuli given to the osteocytes. The results suggested that osteocytes close to the trabecular surfaces are normally exposed to larger flow stimuli than those located around the center of the trabecula, regardless of the loading conditions and the spatial variations in permeability. On the other hand, osteocytes around the center of the trabecula, particularly with relatively large inner permeability, are stimulated by the fluid flow when the bending load is more dominant than the uniaxial load. Our theoretical approach might provide a better understanding of the effect of the spatial variations in bone material properties on the flow-mediated cellular mechanotransduction and signal transport for bone remodeling.
In order to determine how cells change their morphology during adhesion process to a substrate, we focused on the actin cytoskeleton and investigated its morphological change along with that of the whole cell during adhesion process. An osteoblastic cell line MC3T3-E1 was used as the test model. We plated cells whose cell cycle had been synchronized by serum starvation on fibronectin-coated glass plate and cultured them for 10 min to 24 h. We then stained their F-actin and nucleus and observed them with a fluorescent microscope for cell area and shape index and 2D parameters for actin morphology, and with a laser scanning microscope for 3D morphology of actin and nucleus. In the beginning of adhesion, the trypsinized cells were round and their nuclei were surrounded uniformly by thick layer of actin. The actin layer in the upper side became actin aggregate (AA) and lower side dense peripheral band (DPB) in 30 min. The upper AA then became smaller and finally to actin filaments (AFs) spanning the cell top. The DPB expanded and finally became AFs on cell bottom by 1 h. The nucleus becomes flattened possibly due to compression by the cell membrane caused by the expansion of the DPB in the early stage of adhesion. In the later stage of adhesion, the number of AFs continuously increased and nucleus became flattened more and more until 12 h. This may be caused by the increase in the top AFs that may compress the nucleus. Cells become more elongated in response to further alignment of AFs until 12 h. These results indicate that change in AFs during adhesion process is complicated not only temporally but also spatially.
For cyclist fatalities in 2014 in Japan, the head was the most frequently injured body region. In the present study, the authors analyzed the features of cyclist head injuries in real-world traffic accidents using the data of patients who were taken to the emergency room in Dokkyo Medical University Koshigaya Hospital in Japan, from 2011 to 2013. The results indicated that the percentage of skull fractures was the highest among cyclist head injuries. Assuming that a helmet can prevent head injuries sustained by cyclists in traffic accidents, the effect of wearing a helmet was investigated in impact tests against a vehicle and road pavement. In the tests, the severity of potential head injuries was determined from the Head Injury Criterion (HIC) obtained in an adult pedestrian head-form impactor with and without a helmet. The impact location selected for a vehicle was the A-pillar because the pillar had much higher stiffness than the vehicle bonnet or windshield. It was found that the HIC values for the head-form impactor wearing a helmet were much lower than the HIC values for the head-form impactor not wearing a helmet in both the head-versus-A-pillar impacts and head-versus-pavement impacts. The results suggest that wearing a helmet could reduce the possibility of skull fracture in cyclists.
Previous studies provided evidence that the mechanical forces of blood flow in embryos and fetuses may play a role in causing congenital cardiovascular. It is thus important to understand the fluid mechanical forces in the human fetuses. In the current study, we present a new technique for performing computational fluid dynamics of the cardiac chambers, based on patient-specific clinical ultrasound scans of human fetuses. Ultrasound images were acquired using the Spatio-Temporal Image Correlation (STIC) mode. The images were segmented for the right ventricle blood space at various time points. A mathematical model of ventricular wall motion was developed and used to define mesh motion for computational fluid dynamics simulation of fluid within the ventricle. The ventricular mesh models created by the mathematical model was shown to satisfactorily agree with the ventricular geometries segmented from ultrasound images. Fluid dynamics simulations successfully provided details of spatial gradients of pressures, ventricular wall shear stresses, and vorticity dynamics in the ventricle. Results showed that the right ventricle diastolic flows featured two prominent vortex rings, which were sustained until systole, when part of the vorticity structures were ejected through the pulmonary outflow tract. Diastolic wall shear stress was in the range of 0.4-1.2 Pa, while systolic shear stress elevated near to the outflow tract at 1.5-3.9 Pa. In conclusion, We have established methodologies for performing patient-specific simulations of the fluid mechanics in the heart chambers of human fetuses, based on clinical ultrasound scans, and demonstrated its feasibility on a 20 weeks human fetus right ventricle.
Most bacteria are motile, moving toward suitable environments for subsistence and reproduction. In isotropic chemical environments, they move randomly, changing direction at regular time intervals. In the presence of chemical gradients, they modulate the frequency of the direction change. Collectively, this modulation constitutes the chemotactic response toward a desirable chemical. This study investigates a one-dimensional discrete biased random walk model based on bacterial chemotaxis; a modified version of the classical random walk model. Each cell in the group moves along a uniformly spaced number line at the rate of one interval per time step. A chemical attractant is placed at the origin of the number line. When a cell has receded from the origin in the previous time step, it changes its direction with a probability of 1/2 in the current time step. On the other hand, when a cell has approached the origin in the previous time step, its direction changes with probability (1－α)/ 2 , where α denotes the intensity of the bias toward the origin. In numerical simulations, the cells establish a steady distribution from the origin. This distribution is expressed using a geometric progression whose common ratio depends on α. We provide an analytical explanation of this distribution, which actually constitutes two steady distributions alternating at odd and even positions. Next, the results of the discrete model are compared with those of the corresponding continuum model, namely, a diffusion-advection equation wherein α determines the advection speed. The theoretical solution of the diffusion-advection equation is an exponential decay function, consistent with the distribution obtained by the discrete model.
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