Megakaryocyte organelles associate with microtubules. Double immunofluorescence microscopy of a proplatelet using antibodies against (A) serotonin to stain dense granules and (B) tubulin to highlight microtubules and (C) an overlay. Note the teardrop-shaped pattern of dense granules at the tip of the proplatelet. Scale bar, 5 μm. (D) Proplatelet of a living megakaryocyte stained with the fluorescent probe MitoTracker Red to highlight the distribution of mitochondria. Note the circular pattern of mitochondria at the proplatelet tip (arrow in D). (E) Thin-section electron micrograph showing the ultrastructure of a maturing proplatelet. (F-H) High magnification electron micrographs of the boxed regions in panel E show organelles/granules in close proximity to microtubules. Scale bar, 500 nm.