Narrow Results

Thermal fluctuations of microtubules (MTs) and other cytoskeletal filaments govern to a great extent the complex rheological properties of the cytoskeleton in eukaryotic cells. In recent years, much effort has been put into capturing the dynamics of these fluctuations by means of analytical and numerical models. These attempts have been very successful for, but also remain limited to, isotropic polymers. To correctly interpret experimental work on (strongly) anisotropic semiflexible polymers, there is a need for a numerical modeling tool that accurately captures the dynamics of polymers with anisotropic material properties. In the current study, we present a finite element (FE) framework for simulating the thermal dynamics of a single anisotropic semiflexible polymer. First, we demonstrated the accuracy of our framework by comparison of the simulated mean square displacement (MSD) of the end-to-end distance with analytical predictions based on the worm-like chain model. Then, we implemented a transversely isotropic material model, characteristic for biopolymers such as MTs, and studied the persistence length for various ratios between the longitudinal shear modulus, G₁₂, and corresponding Young's modulus, E₁. Finally, we put our findings in context by addressing a recent experimental work on grafted transversely isotropic MTs. In that research, a simplified static mechanical model was used to deduce a very high level of MT anisotropy to explain the observation that the persistence length of grafted MTs increases as contour length increases. We showed, by means of our FE framework, that the anisotropic properties cannot account for the reported length-dependent persistence length.

We investigate the propagation of polarized light in fibrous tissues such as muscle and skin. The myofibrils and collagen fibers are approximated as long cylinders and the tissue phantom is composed of spherical and cylindrical structures. We apply Monte Carlo method based on this phantom to simulate and analyze polarization imaging process of muscle. The good agreement between the simulation results and the experimental results validate the assumption of the phantom composition. This paper also presents how to describe the fiber orientation distribution and tissue anisotropy according to three parameters derived from the polarization imaging.

The mechanism of action of clearing agents to improve optical imaging of mouse skin during reflectance-mode confocal microscopy was tested. The dermal side of excised dorsal mouse skin was exposed for one hour to saline, glycerin, or 80% DMSO, then the clearing agent was removed and the dermis placed against a glass cover slip through which a confocal microscope measured reflectance at 488 nm wavelength. An untreated control was also measured. The axial attenuation of reflectance signal, R(z_f) versus increasing depth of focus z_f behaved as R = ρexp(-μz_f2G), where ρ is tissue reflectivity and μ is attenuation [cm^-1]. The factor 2G accounts for the in/out path of photons, and the numerical aperture of the lens. The ρ, μ data were mapped to values of scattering coefficient (μ_s [cm^-1]) and anisotropy of scattering (g). Images showed that glycerin significantly increased the g of dermis from about 0.7 to about 0.99, with little change in the μ_s of dermis at about 300 cm^-1. DMSO and saline had only slight and inconsistent effects on g and μ_s.

We developed a model to describe polarized photon scattering in biological tissues. In this model, tissues are simplified to a mixture of scatterers and surrounding medium. There are two types of scatterers in the model: solid spheres and infinitely long solid cylinders. Variables related to the scatterers include: the densities and sizes of the spheres and cylinders, the orientation and angular distribution of cylinders. Variables related to the surrounding medium include: the refractive index, absorption coefficient and birefringence. In this paper, as a development we introduce an optical activity effect to the model. By comparing experiments and Monte Carlo simulations, we analyze the backscattering Mueller matrix patterns of several tissue-like media, and summarize the different effects coming from anisotropic scattering and optical properties. In addition, we propose a possible method to extract the optical activity values for tissues. Both the experimental and simulated results show that, by analyzing the Mueller matrix patterns, the microstructure and optical properties of the medium can be obtained. The characteristic features of Mueller matrix patterns are potentially powerful tools for studying the contrast mechanisms of polarization imaging for medical diagnosis.