Figure 2. LRO ultrastructure.

Shown are images from electron microscopy analyses or three-dimensional (3d) reconstructions of electron tomograms for four model LROs discussed in this review. A, B, melanosomes from MNT-1 human melanoma cells fixed by high pressure freezing and embedded in plastic by freeze substitution. A, a thin section emphasizing stage I and II premelanosomes and stage III and IV mature melanosomes, as indicated. Note the striated appearance in stages II and III. M, mitochondrion. B, 3d reconstruction emphasizing fibrillar structure (yellow) emanating from intralumenal vesicles (green) in stage I/ II melanosomes. Note the accumulation of melanin (brown) on the fibrils in a stage III melanosome. Melanosome limiting membranes are indicated in red; surrounding endosomal tubules are indicated in blue. C, D, cytolytic granules from primary human CTLs. C, an ultrathin cryosection was immunogold labeled for perforin. Note labeling over dense cores (arrowheads), with surrounding ILVs (arrows). D, 3d reconstruction emphasizing dense core (green) surrounded by multilamellar membranes (violet) and ILVs (yellow). The limiting membrane is pseudocolored blue. E, WPBs within an ultrathin cryosection of a human umbilical vein endothelial cell are shown, immunogold labeled for CD63. Note the tubular vWF polymers captured in profile (arrow) or cross-section (*). F, G, H, α granules within human platelets. F, G, ultrathin cryosections were immunogold labeled for vWF or fibrinogen (FG). Note that fibrinogen is present throughout the tubular α granules but vWF is polarized to one side, adjacent to ILVs (arrows in G). H, 3d reconstruction emphasizing vWF tubules (blue) polarized to one side of a spherical α granule domain, shown in two orientations; the limiting membrane is pseudocolored red. Bars: A–D, 200 nm; E–H, 100 nm.