Video-enhanced differential-interference-contrast microscopy showing a representative proplatelet elongating from a mouse megakaryocyte. By 3 minutes (arrow), the initial broad pseudopodia has converted into a proplatelet process that continues to lengthen and reduce its diameter at the proplatelet tip (arrow at 9 minutes). Proplatelets elongate at an average rate of 0.85 ± 0.24 μm/min (n = 77). Bar, 5 μm. See Movie S1.