DYNAMIC PROPERTIES OF A SINGLE LIVING CELL Kazushige Kawabata Div Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan, 　　　　　Cell migration is widely common in biological phenomena at tissue level. 　　　　　For example, this plays a central role in healing of wounded tissue and self- 　　　　　organizing of the early embryo. Many researchers have studied intensively 　　　　　molecular processes on mechanisms of the cell migration. However, the cell 　　　　　migration is considered one of difficult phenomena to explain at microscopic 　　　　　level, because it results from complex interactions among numerous proteins. 　　　　　Recently, it is suggested that macroscopic mechanical properties like elasticity 　　　　　relate to cell migration. 　　　　　We have studied temporal change of local stiffness of a cell to clarify the 　　　　　mechanism of cell migration by using Scanning Probe Microscopy (SPM). In 　　　　　the present study, we present spatio-temporal structure of the stiffness of a 　　　　　single living fibroblast. The local stiffness on the cell surface is not uniform, 　　　　　ranging from 1 to 100 kPa and stiffer regions correspond with stress fiber 　　　　　network. Since the stiffness decreases drastically corresponding to the cell 　　　　　movements, the stiffness reflects to both the density of the actin filaments 　　　　　and intracellular stress acting along the stress fibers. These results suggest 　　　　　that intracellular mechanical network consisting of actin filaments is able to 　　　　　make cooperation in motion of a single living cell. Furthermore, we have 　　　　　investigated mechanical response of a living cell against external forces. We 　　　　　measured temporal changes of the stiffness using SPM when fibroblast cells 　　　　　were shortened or elongated using deformable elastic sheets. On shortening, 　　　　　the stiffness decreases drastically and then recovers gradually to a constant 　　　　　value. On elongating, it increases drastically and then decreases gradually to 　　　　　a constant value. We confirmed preservation of the actin filament network 　　　　　stained by GFP even under the stress. Therefore, there are two types of 　　　　　response against step-like external stress. These results imply that cells 　　　　　inherently make the tension via actin network constant. We also show that 　　　　　epithelial cells (MDCK cell) migrated collectively as a massive stream along 　　　　　one direction on a soft collagen gel surface as an example for mechanical 　　　　　cooperation among cell movements. 　　　　　references 　　　　　1. T. Mizutani, H. Haga, and K. Kawabata, Cell Motil. Cytoskeleton (2004) in press. 　　　　　2. M. Nagayama, H. Haga, M. Takahashi, T. Saitoh, and K. Kawabata, Exp. Cell Res. 　　　　　　 300 (2004) 396-405. 　　　　　3. H. Haga, C. Irahara, R. Kobayashi, T. Nakagaki　and K. Kawabata, Biophysical J. 　　　　　　 (2005) in press