Endosomal tubules are continuous with the melanosome limiting membrane. (a) Low magnification EM analysis of thin sections of MNT-1 cells fixed by HPF. Several smooth tubules are close to and surround melanosomes (arrows). Note the distinction of these smooth tubules from the rough ribosome-studded ER. (b–f) Successive slices through the tomographic volume highlight continuities between the vesicular/tubular membranes and the melanosomal membrane (arrows). The ER, studded with ribosomes, is close to the melanosome but does not show continuities. Insets show magnified views of the boxed area. (g and h) Membranes in consecutive tomogram slices are manually contoured; one slice is shown in g, and the deduced 3D model is shown in h. Tubular membranes (green) that are continuous with the melanosome limiting membrane (red) are indicated by arrows. See Video 2 for tilt images, Video 3 for tomographic reconstruction, Video 4 for manual contouring, and Video 5 for a 3D model. M, melanosome; PM, plasma membrane. Bars, 200 nm.