Microtubule dynamics during the conversion of preplatelets to barbell-proplatelets. (A) Fluorescent time-lapse microscopy of a released preplatelet isolated from megakaryocytes retrovirally directed to express GFP–β1-tubulin. The oval preplatelet microtubule marginal band is observed to twist about its center several times in a clockwise fashion over the course of 4 min to yield a barbell-shaped proplatelet with well-defined microtubule loops at each end. (B) Rapid-freeze and thin-section electron micrographs showing the cytoskeleton and ultrastructure of microtubules in proplatelets. (C) High magnification electron micrograph of the boxed region in B shows microtubules twisting about the proplatelet center to yield a “figure 8” structure.