Microtubule coil diameters decrease in aging platelets and inhibition of microtubule dynamics prevents microtubule coil shrinkage. (A) Immunofluorescence image of platelets from mice treated with RAMPS (right) and control platelets (left) stained with antibodies to β1-tubulin show that young platelets have larger microtubule coil diameters than the microtubule coils of normal circulating platelets. (B) RAMPS treatment alters the average platelet diameter. RAMPS-treated platelets averaged 3.56 (± 0.5) μm (n = 31) in diameter, compared with 2.88 (± 0.3) μm (n = 31) for control platelets. Error bars indicate SD. (C) 5-FU treatment alters the average diameter of platelets. 5-FU–treated platelets averaged 3.54 (± 0.7) μm compared with 2.94 (± 0.4) μm (n = 85) in control platelets. After an additional 20 days, diameters of platelets from 5-FU–treated mice return to control levels. (D) Average microtubule coil diameters of human platelets treated with paclitaxel and nontreated control cells at day 0 and day 3 (n = 150 for each sample). Cells treated with paclitaxel for the 3-day incubation period maintain coil sizes similar to day-0 control cells, whereas day-3 cells incubated without paclitaxel possess smaller microtubule coils. Error bars indicate SD. (E) Immunofluorescence images of human platelets visualized with tubulin antibody labeling. Comparison of day-0 and day-3 platelets plus or minus paclitaxel, as shown, monitors the effect of paclitaxel on the reduction of microtubule coil size.