Fig. 7. Population analysis of MTs. (A) Sequential subtraction analysis for identification of growth and shortening. Image of Cy3-labeled MTs from a time-lapse series and corresponding differential image (I_n — I_n+1) obtained by subtracting from an image (I_n) the next image (I_n+1) in the series. Black and white segments represent MT growth and shortening, respectively, during the time interval. Five trapezoidal zones, each one-fifth of the cell radius, indicate the regions where growth and shortening events were scored. In the two right-hand panels, the fifth trapezoid is enlarged and growth and shortening excursions are shown in colors, — green and red, respectively. Parallel black and white segments arise from lateral shifts of MTs and were not scored. (B) Frequency distribution of growth and shortening velocities in region of 0.6-0.9 of cell radius fraction obtained by subtraction analysis. (C) Drift coefficient as calculated from histogram of growth and shortening velocities. Drift is positive and fairly uniform within the cell interior, dropping to negative values only near the cell boundary. (D) Distribution of MT ends. The position of active MT ends (growing or shortening) was scored by subtraction analysis and assigned to one of five trapezoidal zones shown in panel A (n=7421 episodes; 4244 growing; 3177 shortening; 4 cells). Data were fit to a single exponential function. The exponential functions taken separately for growth or shortening episodes were essentially identical.