Biomedical subjects

Yasuji Sawada

Publications and source records attributed to Yasuji Sawada.

3 recordsLinked to original sources

PubMed2006

An iterative method to calculate forces exerted by single cells and multicellular assemblies from the detection of deformations of flexible substrates.

We present a new method for quantification of traction forces exerted by migrating single cells and multicellular assemblies from deformations of flexible substrate. It is based on an iterative biconjugate gradient inversion method. We show how the iteration and the solution are influenced by experimental parameters such as the noise on deformations sigma ( XY ), and the mean depth of recorded deformations Z (M). In order to find the validity range of our computational method, we simulated two different patterns of force. The first artificial force pattern mimics the forces exerted by a migrating Dictyostelium slug at a spatial resolution of Delta=20 mum (Rieu et al. in Biophys J 89:3563-3576, 2005) and corresponds to a large and spread force field. The second simulated force pattern mimics forces exerted by a polarized fibroblast at discrete focal adhesion sites separated by Delta=4 microm. Our iterative method allows, without using explicit regularization, the detailed reconstruction of the two investigated patterns when noise is not too high (sigma ( XY )/u (max)< or =6%, where u (max )is the maximal deformation), and when the plane of recorded deformations is close to the surface (Delta/Z (M)> or =4). The method and the required range of parameters are particularly suitable to study forces over large fields such as those observed in multicellular assemblies.

Algorithms↗

PubMed2005

Direct mechanical force measurements during the migration of Dictyostelium slugs using flexible substrata.

We use the flexible substrate method to study how and where mechanical forces are exerted during the migration of Dictyostelium slugs. This old and contentious issue has been left poorly understood so far. We are able to identify clearly separate friction forces in the tip and in the tail of the slug, traction forces mostly localized in the inner slug/surface contact area in the prespore region and large perpendicular forces directed in the outward direction at the outline of contact area. Surprisingly, the magnitude of friction and traction forces is decreasing with slug velocity indicating that these quantities are probably related to the dynamics of cell/substrate adhesion complexes. Contrary to what is always assumed in models and simulations, friction is not of fluid type (viscous drag) but rather close to solid friction. We suggest that the slime sheath confining laterally the cell mass of the slug experiences a tension that in turn is pulling out the elastic substrate in the direction tangential to the slug profile where sheath is anchored. In addition, we show in the appendix that the iterative method we developed is well adapted to study forces over large and continuous fields when the experimental error is sufficiently low and when the plane of recorded bead deformations is close enough to the elastomer surface, requirements fulfilled in this experimental study of Dictyostelium slugs.

Acrylic Resins↗

PubMed2002

Rapid patterning and zonal differentiation in a two-dimensional Dictyostelium cell mass: the role of pH and ammonia.

Recently it was demonstrated that a rapidly forming, self-organizing pattern that emerges within two-dimensional Dictyostelium discoideum cell cultures could later give rise to stripes of distinct zones, each comprising different cell types. Here we report physiological aspects of the initial rapid patterning and its relationship to cell differentiation. We found that as the temperature is lowered the characteristic length of the pattern increases. From this we estimated the activation energy of the patterning kinetics. Fluorescence of fluorescein-conjugated dextran revealed that the cytosolic pH of cells in the inside zone becomes lower than that in the outer zone facing the air. The patterning could be inhibited by addition of the plasma-membrane proton pump inhibitors diethystilbestrol (DES) or miconazole. Preincubation of cells with weak acid delayed the timing of the patterning, whereas weak base hastened it. A pH-indicating dye revealed localized accumulation of ammonia in the extracellular space. These results suggest that gradients of secreted metabolites may be directly responsible for the rapid patterning and its consequence on cell differentiation in a confined geometrical situation. Possible diffusible candidate molecules and a reaction scheme coupled to the imposed oxygen gradient are discussed.

Ammonia↗