Activation of elongation in a Triton X-100–permeabilized proplatelet by ATP. Changes in proplatelet length after the addition of ATP were monitored by microscopy. (A) A proplatelet viewed with DIC optics just before detergent permeabilization. Two proplatelets can be observed extending from the cell body (CB) of a megakaryocyte. (B) Treatment with 0.5% Triton X-100, followed by washing in a microtubule-stabilizing buffer, preserves the general structure of the proplatelet. (C) Time-lapse sequence after the addition of 1 mM ATP. ATP causes the proplatelet residue to increase its contour length and individual microtubules to splay apart from the bundle. Note the increase in distance between the cell body (right arrow) and the swelling that was attached to the substrate (left arrow). The rate of elongation in this example is approximately 0.7 μm/min. The lengthening of the proplatelet slows after 125 seconds. Scale bar, 5 μm. See Movie S6.