Fig. 1. Experimental setup: (A) configuration of chamber for galvanotactic assay. Electric fields were generated between salt bridges. Cell behaviors were observed by an inverted microscope and recorded by a CCD camera connected to a personal computer; (B) apparatus for electric field application. “A” denotes digital multimeter (ampere mode) (see Section 2).

Fig. 2. Cell motility analysis: (A) the geometric center of the cell is represented in x,y-coordinates by circles. The x-axis denotes the direction of the electric field. Net displacement and total path length of the cell are expressed by arrow and dashed line, respectively; (B) particles undergoing one-dimensional random movement with a bias. The x-axis denotes the direction of the electric field. Step size is l with a constant time interval τ. Probabilities towards X and the reverse direction are p_x and q_x for each step, respectively. The bias can be expressed by the difference between p_x and q_x.

Fig. 3. Directional migration toward the cathode: (A) typical cell shape of Dictyostelium cell under electric field (10 V/cm). Scale bar is 10 μm; (B) differential interference contrast image of the cells in the chamber applied by a 10 V/cm electric field. Red lines represent tracking of the cells. Scale bar is 100 μm; (C) tracks of cells with no electric field, showing random migration in all directions; (D) tracks of cells with 10 V/cm electric field, showing directional movements toward the cathode (left side). Both in (C) and (D), observation times were 30 min.

Fig. 4. Effects of reversing of electric field direction on cell migration: (A) cell tracks before and after reversal of electric field. Typical 10 cell tracks are presented. Each cell index is placed near the start position of respective cell tracks. Arrowheads represent the position where direction of electric field was reversed; (B) temporal changes of average directness upon the reversal. The data of 15 cells was analyzed. After reversal of electric field, cells turn their direction of locomotion within a few minutes.

Fig. 5. Dependence of cellular motile properties on electric field strength: trajectory speed (A), directness (B), asymmetric index (C), and path linearity (D) were analyzed to elucidate the effects of electric fields on cellular motile activities. See Section 2 for the calculation methods. Trajectory speeds were similar at different field strengths, while Directness, Asymmetric index, and Path linearity increased in a dose dependent manner. Data was obtained from at least three independent experiments.

Fig. 6. Input–output relationship to galvanotactic response of Dictyostelium cells: (A) temporal changes of mean cathodal displacements; (B) temporal changes of mean displacements in the direction perpendicular to the electric field; (C) dependence of mean cathodal displacement speed (MCS) on electric field strength. Solid line represents the fitting curve that was obtained by Eq. (5) with sigmoidal number n = 2. MCS reaches half-maximum at 2.6 V/cm, which represents sensitivity of the cells for the electric field.