Measurement of Modes of Vibration of a Cultured Cell for its Mechanosensing Mechanisms

[1. Introduction] It is widely known that each individual cell has the abilities to adapt to surrounding mechanical environments. This is because a single cell senses mechanical stimuli by its sensors and transduces them into the changes of intracellular biochemical responses. Although many researchers have made efforts to clarify cellular mechanosensing mechanisms by affording static or dynamic stimuli, many of the mechanisms are still unclear. Mechanical vibration is one of the dynamic stimuli to cells. At specific frequencies, some of the intracellular biochemical responses reach peaks like a resonance curve in the mechanical engineering field. The modes of vibration of a cell as mechanical response may be related to its intracellular biochemical responses while cell deformation in static stimuli is related to them. It is important to experimentally measure the modes of vibration of a cell for its mechanosensing mechanisms because it is very difficult to model the complex mechanical system of a cell and to realize the numerical simulation. Here we show an improved experimental system to measure the modes of vibration of a single cell by tracking its organelles under mechanical vibration and the experimental results obtained using the system. This study may contribute to elucidating the mechanosensing mechanisms of a single cell under mechanical vibration and developing medical devices for the heeling of bone fracture and the regenerative medicine for articular cartilage. [2. Materials and Methods] HeLa cells, which are isolated from human cervical carcinoma, were used as sample cultured cells because its shape has so high aspect ratio that it can be easier to induce the vibration response and measure the mode of vibration of the cells than the other types of cells under mechanical vibration in the horizontal direction. The experimental system was basically composed of a fluorescent microscope and electromagnetic exciter set on the stage. To realize the observation of the mode of vibration and the excitation of a single living cell with the resolution of submicron order, the points challenged in this study are as follows; First, the experimental system was designed and fabricated to have high natural frequencies with high stiffness and low mass, precisely excite a cell and measure the modes of vibration in the high frequency range under mechanical vibration. Second, the mode shape of a cell was measured by fluorescently labelling organelles in a cell and microbeads adhering on the bottom of a culture dish excited in the horizontal direction and measuring the deformation of the local parts of a cell. Third, a high-sensitivity and high-speed camera was used to capture the weak fluorescent signals emitted by a cell and the microbeads under mechanical vibration. [3. Preliminary Results and Discussion] The experimental system realized the 0.5 G excitation of a single cell in the high frequency range up to 100 Hz and the high resolution of submicron order. Using this system, the deformation of the local parts of a cell was measured and the resonance curve of a cell was obtained. On the basis of the resonance curve, the natural frequencies and the mode shapes of a cell can be identified.